PyGobject（九十三）Cairo系列——简介&简单示例

Cairo

Cario是一个2D图形库，支持多种输出设备。
目前支持的输出目标包括X Window，Quartz，Win32，image.buffers，PostScript，PDF和SVG文件输出。
后续支持OpenGL，XCB，BeOS，OS/2操作系统和DirectFB。
Cario的目的是同一输出在不同的输出媒体上，同时能利用硬件加速器。

主要优点：在X、Win32、Quartz的基础上统一了图形库的操作方式，同时支持PS、PDF、SVG、PNG/JPEG等图像格式的输出，方便页面的再次利用，在glitz的支持下支持部分3D效果。——摘自百度百科

安装

参见PyGobject（一）安装篇

Cairo在PyGobject中的使用

在PyGobject中，通常使用Gtk.DrawingArea部件，并连接到Gtk.Widget ::draw信号，使用回调函数中的第二个参数cairo.Context，进行绘制操作

cairo.Context介绍

Context is the main object used when drawing with cairo. To draw with cairo, you create a Context, set the target surface, and drawing options for the Context, create shapes with functions like Context.move_to() and Context.line_to(), and then draw shapes with Context.stroke() or Context.fill().

Contexts can be pushed to a stack via Context.save(). They may then safely be changed, without loosing the current state. Use Context.restore() to restore to the saved state.

class cairo.Context(target)

Parameters: target – target Surface for the context
Returns:    a newly allocated Context

Raises : MemoryError in case of no memory
Creates a new Context with all graphics state parameters set to default values and with target as a target surface. The target surface should be constructed with a backend-specific function such as ImageSurface (or any other cairo backend surface create variant).

append_path(path)

Parameters: path – Path to be appended

Append the path onto the current path. The path may be either the return value from one of Context.copy_path() or Context.copy_path_flat() or it may be constructed manually (in C).

arc(xc, yc, radius, angle1, angle2)

Parameters: 
    xc (float) – X position of the center of the arc
    yc (float) – Y position of the center of the arc
    radius (float) – the radius of the arc
    angle1 (float) – the start angle, in radians
    angle2 (float) – the end angle, in radians

Adds a circular arc of the given radius to the current path. The arc is centered at (xc, yc), begins at angle1 and proceeds in the direction of increasing angles to end at angle2. If angle2 is less than angle1 it will be progressively increased by 2*PI until it is greater than angle1.

If there is a current point, an initial line segment will be added to the path to connect the current point to the beginning of the arc. If this initial line is undesired, it can be avoided by calling Context.new_sub_path() before calling Context.arc().

Angles are measured in radians. An angle of 0.0 is in the direction of the positive X axis (in user space). An angle of PI/2.0 radians (90 degrees) is in the direction of the positive Y axis (in user space). Angles increase in the direction from the positive X axis toward the positive Y axis. So with the default transformation matrix, angles increase in a clockwise direction.

To convert from degrees to radians, use degrees * (math.pi / 180).

This function gives the arc in the direction of increasing angles; see Context.arc_negative() to get the arc in the direction of decreasing angles.

The arc is circular in user space. To achieve an elliptical arc, you can scale the current transformation matrix by different amounts in the X and Y directions. For example, to draw an ellipse in the box given by x, y, width, height:

ctx.save()
ctx.translate(x + width / 2., y + height / 2.)
ctx.scale(width / 2., height / 2.)
ctx.arc(0., 0., 1., 0., 2 * math.pi)
ctx.restore()

arc_negative(xc, yc, radius, angle1, angle2)

Parameters: 
    xc (float) – X position of the center of the arc
    yc (float) – Y position of the center of the arc
    radius (float) – the radius of the arc
    angle1 (float) – the start angle, in radians
    angle2 (float) – the end angle, in radians

Adds a circular arc of the given radius to the current path. The arc is centered at (xc, yc), begins at angle1 and proceeds in the direction of decreasing angles to end at angle2. If angle2 is greater than angle1 it will be progressively decreased by 2*PI until it is less than angle1.

See Context.arc() for more details. This function differs only in the direction of the arc between the two angles.

clip()

Establishes a new clip region by intersecting the current clip region with the current path as it would be filled by Context.fill() and according to the current FILL RULE (see Context.set_fill_rule()).

After clip(), the current path will be cleared from the Context.

The current clip region affects all drawing operations by effectively masking out any changes to the surface that are outside the current clip region.

Calling clip() can only make the clip region smaller, never larger. But the current clip is part of the graphics state, so a temporary restriction of the clip region can be achieved by calling clip() within a Context.save()/Context.restore() pair. The only other means of increasing the size of the clip region is Context.reset_clip().

clip_extents()

Returns:    (x1, y1, x2, y2)
Return type:    (float, float, float, float)
    x1: left of the resulting extents
    y1: top of the resulting extents
    x2: right of the resulting extents
    y2: bottom of the resulting extents

Computes a bounding box in user coordinates covering the area inside the current clip.

New in version 1.4.

clip_preserve()

Establishes a new clip region by intersecting the current clip region with the current path as it would be filled by Context.fill() and according to the current FILL RULE (see Context.set_fill_rule()).

Unlike Context.clip(), clip_preserve() preserves the path within the Context.

The current clip region affects all drawing operations by effectively masking out any changes to the surface that are outside the current clip region.

Calling clip_preserve() can only make the clip region smaller, never larger. But the current clip is part of the graphics state, so a temporary restriction of the clip region can be achieved by calling clip_preserve() within a Context.save()/Context.restore() pair. The only other means of increasing the size of the clip region is Context.reset_clip().

close_path()

Adds a line segment to the path from the current point to the beginning of the current sub-path, (the most recent point passed to Context.move_to()), and closes this sub-path. After this call the current point will be at the joined endpoint of the sub-path.

The behavior of close_path() is distinct from simply calling Context.line_to() with the equivalent coordinate in the case of stroking. When a closed sub-path is stroked, there are no caps on the ends of the sub-path. Instead, there is a line join connecting the final and initial segments of the sub-path.

If there is no current point before the call to close_path(), this function will have no effect.

Note: As of cairo version 1.2.4 any call to close_path() will place an explicit MOVE_TO element into the path immediately after the CLOSE_PATH element, (which can be seen in Context.copy_path() for example). This can simplify path processing in some cases as it may not be necessary to save the “last move_to point” during processing as the MOVE_TO immediately after the CLOSE_PATH will provide that point.

copy_clip_rectangle_list()

Returns:    the current clip region as a list of rectangles in user coordinates
Return type:    list of 4-tuples of float

(The status in the list may be %CAIRO_STATUS_CLIP_NOT_REPRESENTABLE to indicate that the clip region cannot be represented as a list of user-space rectangles. The status may have other values to indicate other errors. - not implemented in pycairo)

New in version 1.4.

copy_page()

Emits the current page for backends that support multiple pages, but doesn’t clear it, so, the contents of the current page will be retained for the next page too. Use Context.show_page() if you want to get an empty page after the emission.

This is a convenience function that simply calls Surface.copy_page() on Context’s target.

copy_path()

Returns:    Path

Raises : MemoryError in case of no memory
Creates a copy of the current path and returns it to the user as a Path.

copy_path_flat()

Returns:    Path

Raises : MemoryError in case of no memory
Gets a flattened copy of the current path and returns it to the user as a Path.

This function is like Context.copy_path() except that any curves in the path will be approximated with piecewise-linear approximations, (accurate to within the current tolerance value). That is, the result is guaranteed to not have any elements of type CAIRO_PATH_CURVE_TO which will instead be replaced by a series of CAIRO_PATH_LINE_TO elements.

curve_to(x1, y1, x2, y2, x3, y3)

Parameters: 
    x1 (float) – the X coordinate of the first control point
    y1 (float) – the Y coordinate of the first control point
    x2 (float) – the X coordinate of the second control point
    y2 (float) – the Y coordinate of the second control point
    x3 (float) – the X coordinate of the end of the curve
    y3 (float) – the Y coordinate of the end of the curve

Adds a cubic Bézier spline to the path from the current point to position (x3, y3) in user-space coordinates, using (x1, y1) and (x2, y2) as the control points. After this call the current point will be (x3, y3).

If there is no current point before the call to curve_to() this function will behave as if preceded by a call to ctx.move_to(x1, y1).

device_to_user(x, y)

Parameters: 
    x (float) – X value of coordinate
    y (float) – Y value of coordinate
Returns:    
    (x, y)
Return type:    
    (float, float)

Transform a coordinate from device space to user space by multiplying the given point by the inverse of the current transformation matrix (CTM).

device_to_user_distance(dx, dy)

Parameters: 
    dx (float) – X component of a distance vector
    dy (float) – Y component of a distance vector
Returns:    
    (dx, dy)
Return type:    
    (float, float)

Transform a distance vector from device space to user space. This function is similar to Context.device_to_user() except that the translation components of the inverse CTM will be ignored when transforming (dx,dy).

fill()

A drawing operator that fills the current path according to the current FILL RULE, (each sub-path is implicitly closed before being filled). After fill(), the current path will be cleared from the Context. See Context.set_fill_rule() and Context.fill_preserve().

fill_extents()

Returns:    (x1, y1, x2, y2)
Return type:    (float, float, float, float)
    x1: left of the resulting extents
    y1: top of the resulting extents
    x2: right of the resulting extents
    y2: bottom of the resulting extents

Computes a bounding box in user coordinates covering the area that would be affected, (the “inked” area), by a Context.fill() operation given the current path and fill parameters. If the current path is empty, returns an empty rectangle (0,0,0,0). Surface dimensions and clipping are not taken into account.

Contrast with Context.path_extents(), which is similar, but returns non-zero extents for some paths with no inked area, (such as a simple line segment).

Note that fill_extents() must necessarily do more work to compute the precise inked areas in light of the fill rule, so Context.path_extents() may be more desirable for sake of performance if the non-inked path extents are desired.

See Context.fill(), Context.set_fill_rule() and Context.fill_preserve().

fill_preserve()

A drawing operator that fills the current path according to the current FILL RULE, (each sub-path is implicitly closed before being filled). Unlike Context.fill(), fill_preserve() preserves the path within the Context.

See Context.set_fill_rule() and Context.fill().

font_extents()

Returns:    (ascent, descent, height, max_x_advance, max_y_advance)
Return type:    (float, float, float, float, float)

Gets the font extents for the currently selected font.

get_antialias()

Returns:    the current ANTIALIAS mode, as set by Context.set_antialias().

get_current_point()

Returns:    (x, y)
Return type:    (float, float)
    x: X coordinate of the current point
    y: Y coordinate of the current point

Gets the current point of the current path, which is conceptually the final point reached by the path so far.

The current point is returned in the user-space coordinate system. If there is no defined current point or if Context is in an error status, x and y will both be set to 0.0. It is possible to check this in advance with Context.has_current_point().

Most path construction functions alter the current point. See the following for details on how they affect the current point: Context.new_path(), Context.new_sub_path(), Context.append_path(), Context.close_path(), Context.move_to(), Context.line_to(), Context.curve_to(), Context.rel_move_to(), Context.rel_line_to(), Context.rel_curve_to(), Context.arc(), Context.arc_negative(), Context.rectangle(), Context.text_path(), Context.glyph_path(), Context.stroke_to_path().

Some functions use and alter the current point but do not otherwise change current path: Context.show_text().

Some functions unset the current path and as a result, current point: Context.fill(), Context.stroke().

get_dash()

Returns:    (dashes, offset)
Return type:    (tuple, float)
    dashes: return value for the dash array
    offset: return value for the current dash offset

Gets the current dash array.

New in version 1.4.

get_dash_count()

Returns:    the length of the dash array, or 0 if no dash array set.
Return type:    int

See also Context.set_dash() and Context.get_dash().

New in version 1.4.

get_fill_rule()

Returns:    the current FILL RULE, as set by Context.set_fill_rule().

get_font_face()

Returns:    the current FontFace for the Context.

get_font_matrix()

Returns:    the current Matrix for the Context.

See Context.set_font_matrix().

get_font_options()

Returns:    the current FontOptions for the Context.

Retrieves font rendering options set via Context.set_font_options(). Note that the returned options do not include any options derived from the underlying surface; they are literally the options passed to Context.set_font_options().

get_group_target()

Returns:    the target Surface.

Gets the current destination Surface for the Context. This is either the original target surface as passed to Context or the target surface for the current group as started by the most recent call to Context.push_group() or Context.push_group_with_content().

New in version 1.2.

get_line_cap()

Returns:    the current LINE_CAP style, as set by Context.set_line_cap().

get_line_join()

Returns:    the current LINE_JOIN style, as set by Context.set_line_join().

get_line_width()

Returns:    the current line width
Return type:    float

This function returns the current line width value exactly as set by Context.set_line_width(). Note that the value is unchanged even if the CTM has changed between the calls to Context.set_line_width() and get_line_width().

get_matrix()

Returns:    the current transformation Matrix (CTM)

get_miter_limit()

Returns:    the current miter limit, as set by Context.set_miter_limit().
Return type:    float

get_operator()

Returns:    the current compositing OPERATOR for a Context.

get_scaled_font()

Returns:    the current ScaledFont for a Context.

New in version 1.4.

get_source()

Returns:    the current source Pattern for a Context.

get_target()

Returns:    the target Surface for the Context

get_tolerance()

Returns:    the current tolerance value, as set by Context.set_tolerance()
Return type:    float

glyph_extents(glyphs[, num_glyphs])

Parameters: 
    glyphs (a sequence of (int, float, float)) – glyphs
    num_glyphs (int) – number of glyphs to measure, defaults to using all
Returns:    
    x_bearing, y_bearing, width, height, x_advance, y_advance
Return type:    
    6-tuple of float

Gets the extents for an array of glyphs. The extents describe a user-space rectangle that encloses the “inked” portion of the glyphs, (as they would be drawn by Context.show_glyphs()). Additionally, the x_advance and y_advance values indicate the amount by which the current point would be advanced by Context.show_glyphs().

Note that whitespace glyphs do not contribute to the size of the rectangle (extents.width and extents.height).

glyph_path(glyphs[, num_glyphs])

Parameters: 
    glyphs (a sequence of (int, float, float)) – glyphs to show
    num_glyphs (int) – number of glyphs to show, defaults to showing all

Adds closed paths for the glyphs to the current path. The generated path if filled, achieves an effect similar to that of Context.show_glyphs().

has_current_point()

returns: True iff a current point is defined on the current path.

See Context.get_current_point() for details on the current point.
New in version 1.6.

identity_matrix()

Resets the current transformation Matrix (CTM) by setting it equal to the identity matrix. That is, the user-space and device-space axes will be aligned and one user-space unit will transform to one device-space unit.

in_fill(x, y)

Parameters: 
    x (float) – X coordinate of the point to test
    y (float) – Y coordinate of the point to test
Returns:    
    True iff the point is inside the area that would be affected by a Context.fill() operation given the current path and filling parameters. Surface dimensions and clipping are not taken into account.

See Context.fill(), Context.set_fill_rule() and Context.fill_preserve().

in_stroke(x, y)

Parameters: 
    x (float) – X coordinate of the point to test
    y (float) – Y coordinate of the point to test
Returns:    
    True iff the point is inside the area that would be affected by a Context.stroke() operation given the current path and stroking parameters. Surface dimensions and clipping are not taken into account.

See Context.stroke(), Context.set_line_width(), Context.set_line_join(), Context.set_line_cap(), Context.set_dash(), and Context.stroke_preserve().

line_to(x, y)

Parameters: 
    x (float) – the X coordinate of the end of the new line
    y (float) – the Y coordinate of the end of the new line

Adds a line to the path from the current point to position (x, y) in user-space coordinates. After this call the current point will be (x, y).

If there is no current point before the call to line_to() this function will behave as ctx.move_to(x, y).

mask(pattern)

Parameters: pattern – a Pattern

A drawing operator that paints the current source using the alpha channel of pattern as a mask. (Opaque areas of pattern are painted with the source, transparent areas are not painted.)

mask_surface(surface, x=0.0, y=0.0)

Parameters: 
    surface – a Surface
    x (float) – X coordinate at which to place the origin of surface
    y (float) – Y coordinate at which to place the origin of surface

A drawing operator that paints the current source using the alpha channel of surface as a mask. (Opaque areas of surface are painted with the source, transparent areas are not painted.)

move_to(x, y)

Parameters: 
    x (float) – the X coordinate of the new position
    y (float) – the Y coordinate of the new position

Begin a new sub-path. After this call the current point will be (x, y).

new_path()

Clears the current path. After this call there will be no path and no current point.

new_sub_path()

Begin a new sub-path. Note that the existing path is not affected. After this call there will be no current point.

In many cases, this call is not needed since new sub-paths are frequently started with Context.move_to().

A call to new_sub_path() is particularly useful when beginning a new sub-path with one of the Context.arc() calls. This makes things easier as it is no longer necessary to manually compute the arc’s initial coordinates for a call to Context.move_to().

New in version 1.6.

paint()

A drawing operator that paints the current source everywhere within the current clip region.

paint_with_alpha(alpha)

Parameters: alpha (float) – alpha value, between 0 (transparent) and 1 (opaque)

A drawing operator that paints the current source everywhere within the current clip region using a mask of constant alpha value alpha. The effect is similar to Context.paint(), but the drawing is faded out using the alpha value.

path_extents()

Returns:    (x1, y1, x2, y2)
Return type:    (float, float, float, float)
    x1: left of the resulting extents
    y1: top of the resulting extents
    x2: right of the resulting extents
    y2: bottom of the resulting extents

Computes a bounding box in user-space coordinates covering the points on the current path. If the current path is empty, returns an empty rectangle (0, 0, 0, 0). Stroke parameters, fill rule, surface dimensions and clipping are not taken into account.

Contrast with Context.fill_extents() and Context.stroke_extents() which return the extents of only the area that would be “inked” by the corresponding drawing operations.

The result of path_extents() is defined as equivalent to the limit of Context.stroke_extents() with cairo.LINE_CAP_ROUND as the line width approaches 0.0, (but never reaching the empty-rectangle returned by Context.stroke_extents() for a line width of 0.0).

Specifically, this means that zero-area sub-paths such as Context.move_to(); Context.line_to() segments, (even degenerate cases where the coordinates to both calls are identical), will be considered as contributing to the extents. However, a lone Context.move_to() will not contribute to the results of Context.path_extents().

New in version 1.6.

pop_group()

Returns:    a newly created SurfacePattern containing the results of all drawing operations performed to the group.

Terminates the redirection begun by a call to Context.push_group() or Context.push_group_with_content() and returns a new pattern containing the results of all drawing operations performed to the group.

The pop_group() function calls Context.restore(), (balancing a call to Context.save() by the Context.push_group() function), so that any changes to the graphics state will not be visible outside the group.

New in version 1.2.

pop_group_to_source()

Terminates the redirection begun by a call to Context.push_group() or Context.push_group_with_content() and installs the resulting pattern as the source Pattern in the given Context.

The behavior of this function is equivalent to the sequence of operations:

group = cairo_pop_group()
ctx.set_source(group)

but is more convenient as their is no need for a variable to store the short-lived pointer to the pattern.

The Context.pop_group() function calls Context.restore(), (balancing a call to Context.save() by the Context.push_group() function), so that any changes to the graphics state will not be visible outside the group.

New in version 1.2.

push_group()

Temporarily redirects drawing to an intermediate surface known as a group. The redirection lasts until the group is completed by a call to Context.pop_group() or Context.pop_group_to_source(). These calls provide the result of any drawing to the group as a pattern, (either as an explicit object, or set as the source pattern).

This group functionality can be convenient for performing intermediate compositing. One common use of a group is to render objects as opaque within the group, (so that they occlude each other), and then blend the result with translucence onto the destination.

Groups can be nested arbitrarily deep by making balanced calls to Context.push_group()/Context.pop_group(). Each call pushes/pops the new target group onto/from a stack.

The push_group() function calls Context.save() so that any changes to the graphics state will not be visible outside the group, (the pop_group functions call Context.restore()).

By default the intermediate group will have a CONTENT type of cairo.CONTENT_COLOR_ALPHA. Other content types can be chosen for the group by using Context.push_group_with_content() instead.

As an example, here is how one might fill and stroke a path with translucence, but without any portion of the fill being visible under the stroke:

ctx.push_group()
ctx.set_source(fill_pattern)
ctx.fill_preserve()
ctx.set_source(stroke_pattern)
ctx.stroke()
ctx.pop_group_to_source()
ctx.paint_with_alpha(alpha)

New in version 1.2.

push_group_with_content(content)

Parameters: content – a CONTENT indicating the type of group that will be created

Temporarily redirects drawing to an intermediate surface known as a group. The redirection lasts until the group is completed by a call to Context.pop_group() or Context.pop_group_to_source(). These calls provide the result of any drawing to the group as a pattern, (either as an explicit object, or set as the source pattern).

The group will have a content type of content. The ability to control this content type is the only distinction between this function and Context.push_group() which you should see for a more detailed description of group rendering.

New in version 1.2.

rectangle(x, y, width, height)

Parameters: 
    x (float) – the X coordinate of the top left corner of the rectangle
    y (float) – the Y coordinate to the top left corner of the rectangle
    width (float) – the width of the rectangle
    height (float) – the height of the rectangle

Adds a closed sub-path rectangle of the given size to the current path at position (x, y) in user-space coordinates.

This function is logically equivalent to:

ctx.move_to(x, y)
ctx.rel_line_to(width, 0)
ctx.rel_line_to(0, height)
ctx.rel_line_to(-width, 0)
ctx.close_path()

rel_curve_to(dx1, dy1, dx2, dy2, dx3, dy3)

Parameters: 
    dx1 (float) – the X offset to the first control point
    dy1 (float) – the Y offset to the first control point
    dx2 (float) – the X offset to the second control point
    dy2 (float) – the Y offset to the second control point
    dx3 (float) – the X offset to the end of the curve
    dy3 (float) – the Y offset to the end of the curve

Raises :
cairo.Error if called with no current point.
Relative-coordinate version of Context.curve_to(). All offsets are relative to the current point. Adds a cubic Bézier spline to the path from the current point to a point offset from the current point by (dx3, dy3), using points offset by (dx1, dy1) and (dx2, dy2) as the control points. After this call the current point will be offset by (dx3, dy3).

Given a current point of (x, y), ctx.rel_curve_to(dx1, dy1, dx2, dy2, dx3, dy3) is logically equivalent to ctx.curve_to(x+dx1, y+dy1, x+dx2, y+dy2, x+dx3, y+dy3).

rel_line_to(dx, dy)

Parameters: 
    dx (float) – the X offset to the end of the new line
    dy (float) – the Y offset to the end of the new line

Raises :
cairo.Error if called with no current point.
Relative-coordinate version of Context.line_to(). Adds a line to the path from the current point to a point that is offset from the current point by (dx, dy) in user space. After this call the current point will be offset by (dx, dy).

Given a current point of (x, y), ctx.rel_line_to(dx, dy) is logically equivalent to ctx.line_to(x + dx, y + dy).

rel_move_to(dx, dy)

Parameters: 
    dx (float) – the X offset
    dy (float) – the Y offset

Raises :
cairo.Error if called with no current point.
Begin a new sub-path. After this call the current point will offset by (dx, dy).

Given a current point of (x, y), ctx.rel_move_to(dx, dy) is logically equivalent to ctx.(x + dx, y + dy).

reset_clip()

Reset the current clip region to its original, unrestricted state. That is, set the clip region to an infinitely large shape containing the target surface. Equivalently, if infinity is too hard to grasp, one can imagine the clip region being reset to the exact bounds of the target surface.

Note that code meant to be reusable should not call reset_clip() as it will cause results unexpected by higher-level code which calls clip(). Consider using save() and restore() around clip() as a more robust means of temporarily restricting the clip region.

restore()

Restores Context to the state saved by a preceding call to save() and removes that state from the stack of saved states.

rotate(angle)

Parameters: angle (float) – angle (in radians) by which the user-space axes will be rotated

Modifies the current transformation matrix (CTM) by rotating the user-space axes by angle radians. The rotation of the axes takes places after any existing transformation of user space. The rotation direction for positive angles is from the positive X axis toward the positive Y axis.

save()

Makes a copy of the current state of Context and saves it on an internal stack of saved states. When restore() is called, Context will be restored to the saved state. Multiple calls to save() and restore() can be nested; each call to restore() restores the state from the matching paired save().

scale(sx, sy)

Parameters: 
    sx (float) – scale factor for the X dimension
    sy (float) – scale factor for the Y dimension

Modifies the current transformation matrix (CTM) by scaling the X and Y user-space axes by sx and sy respectively. The scaling of the axes takes place after any existing transformation of user space.

select_font_face(family[, slant[, weight]])

Parameters: 
    family (str) – a font family name
    slant – the FONT_SLANT of the font, defaults to cairo.FONT_SLANT_NORMAL.
    weight – the FONT_WEIGHT of the font, defaults to cairo.FONT_WEIGHT_NORMAL.

Note: The select_font_face() function call is part of what the cairo designers call the “toy” text API. It is convenient for short demos and simple programs, but it is not expected to be adequate for serious text-using applications.

Selects a family and style of font from a simplified description as a family name, slant and weight. Cairo provides no operation to list available family names on the system (this is a “toy”, remember), but the standard CSS2 generic family names, (“serif”, “sans-serif”, “cursive”, “fantasy”, “monospace”), are likely to work as expected.

For “real” font selection, see the font-backend-specific font_face_create functions for the font backend you are using. (For example, if you are using the freetype-based cairo-ft font backend, see cairo_ft_font_face_create_for_ft_face() or cairo_ft_font_face_create_for_pattern().) The resulting font face could then be used with cairo_scaled_font_create() and cairo_set_scaled_font().

Similarly, when using the “real” font support, you can call directly into the underlying font system, (such as fontconfig or freetype), for operations such as listing available fonts, etc.

It is expected that most applications will need to use a more comprehensive font handling and text layout library, (for example, pango), in conjunction with cairo.

If text is drawn without a call to select_font_face(), (nor set_font_face() nor set_scaled_font()), the default family is platform-specific, but is essentially “sans-serif”. Default slant is cairo.FONT_SLANT_NORMAL, and default weight is cairo.FONT_WEIGHT_NORMAL.

This function is equivalent to a call to ToyFontFace followed by set_font_face().

set_antialias(antialias)

Parameters: antialias – the new ANTIALIAS mode

Set the antialiasing mode of the rasterizer used for drawing shapes. This value is a hint, and a particular backend may or may not support a particular value. At the current time, no backend supports cairo.ANTIALIAS_SUBPIXEL when drawing shapes.

Note that this option does not affect text rendering, instead see FontOptions.set_antialias().

set_dash(dashes[, offset=0])

Parameters: 
    dashes (sequence of float) – a sequence specifying alternate lengths of on and off stroke portions.
    offset (int) – an offset into the dash pattern at which the stroke should start, defaults to 0.

Raises :
cairo.Error if any value in dashes is negative, or if all values are 0.
Sets the dash pattern to be used by stroke(). A dash pattern is specified by dashes - a sequence of positive values. Each value provides the length of alternate “on” and “off” portions of the stroke. The offset specifies an offset into the pattern at which the stroke begins.

Each “on” segment will have caps applied as if the segment were a separate sub-path. In particular, it is valid to use an “on” length of 0.0 with cairo.LINE_CAP_ROUND or cairo.LINE_CAP_SQUARE in order to distributed dots or squares along a path.

Note: The length values are in user-space units as evaluated at the time of stroking. This is not necessarily the same as the user space at the time of set_dash().

If the number of dashes is 0 dashing is disabled.

If the number of dashes is 1 a symmetric pattern is assumed with alternating on and off portions of the size specified by the single value in dashes.

set_fill_rule(fill_rule)

Parameters: fill_rule – a FILL RULE to set the within the cairo context. The fill rule is used to determine which regions are inside or outside a complex (potentially self-intersecting) path. The current fill rule affects both fill() and clip().

The default fill rule is cairo.FILL_RULE_WINDING.

set_font_face(font_face)

Parameters: font_face – a FontFace, or None to restore to the default FontFace

Replaces the current FontFace object in the Context with font_face.

set_font_matrix(matrix)

Parameters: matrix – a Matrix describing a transform to be applied to the current font.

Sets the current font matrix to matrix. The font matrix gives a transformation from the design space of the font (in this space, the em-square is 1 unit by 1 unit) to user space. Normally, a simple scale is used (see set_font_size()), but a more complex font matrix can be used to shear the font or stretch it unequally along the two axes

set_font_options(options)

Parameters: options – FontOptions to use

Sets a set of custom font rendering options for the Context. Rendering options are derived by merging these options with the options derived from underlying surface; if the value in options has a default value (like cairo.ANTIALIAS_DEFAULT), then the value from the surface is used.

set_font_size(size)

Parameters: size (float) – the new font size, in user space units

Sets the current font matrix to a scale by a factor of size, replacing any font matrix previously set with set_font_size() or set_font_matrix(). This results in a font size of size user space units. (More precisely, this matrix will result in the font’s em-square being a size by size square in user space.)

If text is drawn without a call to set_font_size(), (nor set_font_matrix() nor set_scaled_font()), the default font size is 10.0.

set_line_cap(line_cap)

Parameters: line_cap – a LINE_CAP style

Sets the current line cap style within the Context.

As with the other stroke parameters, the current line cap style is examined by stroke(), stroke_extents(), and stroke_to_path(), but does not have any effect during path construction.

The default line cap style is cairo.LINE_CAP_BUTT.

set_line_join(line_join)

Parameters: line_join – a LINE_JOIN style

Sets the current line join style within the Context.

As with the other stroke parameters, the current line join style is examined by stroke(), stroke_extents(), and stroke_to_path(), but does not have any effect during path construction.

The default line join style is cairo.LINE_JOIN_MITER.

set_line_width(width)

Parameters: width (float) – a line width

Sets the current line width within the Context. The line width value specifies the diameter of a pen that is circular in user space, (though device-space pen may be an ellipse in general due to scaling/shear/rotation of the CTM).

Note: When the description above refers to user space and CTM it refers to the user space and CTM in effect at the time of the stroking operation, not the user space and CTM in effect at the time of the call to set_line_width(). The simplest usage makes both of these spaces identical. That is, if there is no change to the CTM between a call to set_line_width() and the stroking operation, then one can just pass user-space values to set_line_width() and ignore this note.

As with the other stroke parameters, the current line width is examined by stroke(), stroke_extents(), and stroke_to_path(), but does not have any effect during path construction.

The default line width value is 2.0.

set_matrix(matrix)

Parameters: matrix – a transformation Matrix from user space to device space.

Modifies the current transformation matrix (CTM) by setting it equal to matrix.

set_miter_limit(limit)

Parameters: limit – miter limit to set

Sets the current miter limit within the Context.

If the current line join style is set to cairo.LINE_JOIN_MITER (see set_line_join()), the miter limit is used to determine whether the lines should be joined with a bevel instead of a miter. Cairo divides the length of the miter by the line width. If the result is greater than the miter limit, the style is converted to a bevel.

As with the other stroke parameters, the current line miter limit is examined by stroke(), stroke_extents(), and stroke_to_path(), but does not have any effect during path construction.

The default miter limit value is 10.0, which will convert joins with interior angles less than 11 degrees to bevels instead of miters. For reference, a miter limit of 2.0 makes the miter cutoff at 60 degrees, and a miter limit of 1.414 makes the cutoff at 90 degrees.

A miter limit for a desired angle can be computed as:

miter limit = 1/math.sin(angle/2)

set_operator(op)

Parameters: op – the compositing OPERATOR to set for use in all drawing operations.

The default operator is cairo.OPERATOR_OVER.

set_scaled_font(scaled_font)

Parameters: scaled_font – a ScaledFont

Replaces the current font face, font matrix, and font options in the Context with those of the ScaledFont. Except for some translation, the current CTM of the Context should be the same as that of the ScaledFont, which can be accessed using ScaledFont.get_ctm().

New in version 1.2.

set_source(source)

Parameters: source – a Pattern to be used as the source for subsequent drawing operations.

Sets the source pattern within Context to source. This pattern will then be used for any subsequent drawing operation until a new source pattern is set.

Note: The pattern’s transformation matrix will be locked to the user space in effect at the time of set_source(). This means that further modifications of the current transformation matrix will not affect the source pattern. See Pattern.set_matrix().

The default source pattern is a solid pattern that is opaque black, (that is, it is equivalent to set_source_rgb(0.0, 0.0, 0.0).

set_source_rgb(red, green, blue)

Parameters: 
    red (float) – red component of color
    green (float) – green component of color
    blue (float) – blue component of color

Sets the source pattern within Context to an opaque color. This opaque color will then be used for any subsequent drawing operation until a new source pattern is set.

The color components are floating point numbers in the range 0 to 1. If the values passed in are outside that range, they will be clamped.

The default source pattern is opaque black, (that is, it is equivalent to set_source_rgb(0.0, 0.0, 0.0).

set_source_rgba(red, green, blue[, alpha=1.0])

Parameters: 
    red (float) – red component of color
    green (float) – green component of color
    blue (float) – blue component of color
    alpha (float) – alpha component of color

Sets the source pattern within Context to a translucent color. This color will then be used for any subsequent drawing operation until a new source pattern is set.

The color and alpha components are floating point numbers in the range 0 to 1. If the values passed in are outside that range, they will be clamped.

The default source pattern is opaque black, (that is, it is equivalent to set_source_rgba(0.0, 0.0, 0.0, 1.0).

set_source_surface(surface[, x=0.0[, y=0.0]])

Parameters: 
    surface – a Surface to be used to set the source pattern
    x (float) – User-space X coordinate for surface origin
    y (float) – User-space Y coordinate for surface origin

This is a convenience function for creating a pattern from a Surface and setting it as the source in Context with set_source().

The x and y parameters give the user-space coordinate at which the surface origin should appear. (The surface origin is its upper-left corner before any transformation has been applied.) The x and y patterns are negated and then set as translation values in the pattern matrix.

Other than the initial translation pattern matrix, as described above, all other pattern attributes, (such as its extend mode), are set to the default values as in SurfacePattern. The resulting pattern can be queried with get_source() so that these attributes can be modified if desired, (eg. to create a repeating pattern with Pattern.set_extend()).

set_tolerance(tolerance)

Parameters: tolerance (float) – the tolerance, in device units (typically pixels)

Sets the tolerance used when converting paths into trapezoids. Curved segments of the path will be subdivided until the maximum deviation between the original path and the polygonal approximation is less than tolerance. The default value is 0.1. A larger value will give better performance, a smaller value, better appearance. (Reducing the value from the default value of 0.1 is unlikely to improve appearance significantly.) The accuracy of paths within Cairo is limited by the precision of its internal arithmetic, and the prescribed tolerance is restricted to the smallest representable internal value.

show_glyphs(glyphs[, num_glyphs])

Parameters: 
    glyphs (a sequence of (int, float, float)) – glyphs to show
    num_glyphs (int) – number of glyphs to show, defaults to showing all glyphs

A drawing operator that generates the shape from an array of glyphs, rendered according to the current font face, font size (font matrix), and font options.

show_page()

Emits and clears the current page for backends that support multiple pages. Use copy_page() if you don’t want to clear the page.

This is a convenience function that simply calls ctx.get_target() . show_page()

show_text(text)

Parameters: text (str) – text

A drawing operator that generates the shape from a string of text, rendered according to the current font_face, font_size (font_matrix), and font_options.

This function first computes a set of glyphs for the string of text. The first glyph is placed so that its origin is at the current point. The origin of each subsequent glyph is offset from that of the previous glyph by the advance values of the previous glyph.

After this call the current point is moved to the origin of where the next glyph would be placed in this same progression. That is, the current point will be at the origin of the final glyph offset by its advance values. This allows for easy display of a single logical string with multiple calls to show_text().

Note: The show_text() function call is part of what the cairo designers call the “toy” text API. It is convenient for short demos and simple programs, but it is not expected to be adequate for serious text-using applications. See show_glyphs() for the “real” text display API in cairo.

stroke()

A drawing operator that strokes the current path according to the current line width, line join, line cap, and dash settings. After stroke(), the current path will be cleared from the cairo context. See set_line_width(), set_line_join(), set_line_cap(), set_dash(), and stroke_preserve().

Note: Degenerate segments and sub-paths are treated specially and provide a useful result. These can result in two different situations:

1. Zero-length “on” segments set in set_dash(). If the cap style is cairo.LINE_CAP_ROUND or cairo.LINE_CAP_SQUARE then these segments will be drawn as circular dots or squares respectively. In the case of cairo.LINE_CAP_SQUARE, the orientation of the squares is determined by the direction of the underlying path.

2. A sub-path created by move_to() followed by either a close_path() or one or more calls to line_to() to the same coordinate as the move_to(). If the cap style is cairo.LINE_CAP_ROUND then these sub-paths will be drawn as circular dots. Note that in the case of cairo.LINE_CAP_SQUARE a degenerate sub-path will not be drawn at all, (since the correct orientation is indeterminate).

In no case will a cap style of cairo.LINE_CAP_BUTT cause anything to be drawn in the case of either degenerate segments or sub-paths.

stroke_extents()

Returns:    (x1, y1, x2, y2)
Return type:    (float, float, float, float)
    x1: left of the resulting extents
    y1: top of the resulting extents
    x2: right of the resulting extents
    y2: bottom of the resulting extents

Computes a bounding box in user coordinates covering the area that would be affected, (the “inked” area), by a stroke() operation given the current path and stroke parameters. If the current path is empty, returns an empty rectangle (0, 0, 0, 0). Surface dimensions and clipping are not taken into account.

Note that if the line width is set to exactly zero, then stroke_extents() will return an empty rectangle. Contrast with path_extents() which can be used to compute the non-empty bounds as the line width approaches zero.

Note that stroke_extents() must necessarily do more work to compute the precise inked areas in light of the stroke parameters, so path_extents() may be more desirable for sake of performance if non-inked path extents are desired.

See stroke(), set_line_width(), set_line_join(), set_line_cap(), set_dash(), and stroke_preserve().

stroke_preserve()

A drawing operator that strokes the current path according to the current line width, line join, line cap, and dash settings. Unlike stroke(), stroke_preserve() preserves the path within the cairo context.

See set_line_width(), set_line_join(), set_line_cap(), set_dash(), and stroke_preserve().

text_extents(text)

Parameters: text (str) – text to get extents for
Returns:    x_bearing, y_bearing, width, height, x_advance, y_advance
Return type:    6-tuple of float

Gets the extents for a string of text. The extents describe a user-space rectangle that encloses the “inked” portion of the text, (as it would be drawn by Context.show_text()). Additionally, the x_advance and y_advance values indicate the amount by which the current point would be advanced by Context.show_text().

Note that whitespace characters do not directly contribute to the size of the rectangle (extents.width and extents.height). They do contribute indirectly by changing the position of non-whitespace characters. In particular, trailing whitespace characters are likely to not affect the size of the rectangle, though they will affect the x_advance and y_advance values.

text_path(text)

Parameters: text (str) – text

Adds closed paths for text to the current path. The generated path if filled, achieves an effect similar to that of Context.show_text().

Text conversion and positioning is done similar to Context.show_text().

Like Context.show_text(), After this call the current point is moved to the origin of where the next glyph would be placed in this same progression. That is, the current point will be at the origin of the final glyph offset by its advance values. This allows for chaining multiple calls to to Context.text_path() without having to set current point in between.

Note: The text_path() function call is part of what the cairo designers call the “toy” text API. It is convenient for short demos and simple programs, but it is not expected to be adequate for serious text-using applications. See Context.glyph_path() for the “real” text path API in cairo.

transform(matrix)

Parameters: matrix – a transformation Matrix to be applied to the user-space axes

Modifies the current transformation matrix (CTM) by applying matrix as an additional transformation. The new transformation of user space takes place after any existing transformation.

translate(tx, ty)

Parameters: 
    tx (float) – amount to translate in the X direction
    ty (float) – amount to translate in the Y direction

Modifies the current transformation matrix (CTM) by translating the user-space origin by (tx, ty). This offset is interpreted as a user-space coordinate according to the CTM in place before the new call to translate(). In other words, the translation of the user-space origin takes place after any existing transformation.

user_to_device(x, y)

Parameters: 
    x (float) – X value of coordinate
    y (float) – Y value of coordinate
Returns:(x, y)
Return type:(float, float)
    x: X value of coordinate
    y: Y value of coordinate

Transform a coordinate from user space to device space by multiplying the given point by the current transformation matrix (CTM).

user_to_device_distance(dx, dy)

Parameters: 
    dx (float) – X value of a distance vector
    dy (float) – Y value of a distance vector

Returns:(dx, dy)
Return type:(float, float)
    dx: X value of a distance vector
    dy: Y value of a distance vector

Transform a distance vector from user space to device space. This function is similar to Context.user_to_device() except that the translation components of the CTM will be ignored when transforming (dx,dy).

简单示例

代码：

#!/usr/bin/env python3
# Created by xiaosanyu at 16/7/6
# section 143
TITLE = "Simple drawing"
DESCRIPTION = """
The stroke operation draws the outlines of shapes and the fill operation
fills the insides of shapes. Next we will demonstrate these two operations
"""

import gi

gi.require_version("Gtk", "3.0")
from gi.repository import Gtk

import math


class PyApp(Gtk.Window):
    def __init__(self):
        super(PyApp, self).__init__()
        self.set_title("Simple drawing")
        self.resize(230, 150)
        self.connect("destroy", Gtk.main_quit)
        darea = Gtk.DrawingArea()
        darea.connect("draw", self.draw)
        self.add(darea)
        self.show_all()

    @staticmethod
    def draw(drawingArea, cr):
        # 获取宽和高
        w = drawingArea.get_allocated_width()
        h = drawingArea.get_allocated_height()
        # 设置线条宽度
        cr.set_line_width(9)
        # 设置颜色为暗红色
        cr.set_source_rgb(0.7, 0.2, 0.0)
        # 移动到区域中心位置
        cr.translate(w / 2, h / 2)
        # 已半径为50 画一个圆
        cr.arc(0, 0, 50, 0, 2 * math.pi)
        # 描边,stroke()方法会清除路径信息.后续再填充其它颜色无效
        cr.stroke_preserve()
        # cr.stroke()
        # 设置颜色为蓝色
        cr.set_source_rgb(0.3, 0.4, 0.6)
        # 填充
        cr.fill()


def main():
    PyApp()
    Gtk.main()


if __name__ == "__main__":
    main()

代码下载地址：http://download.csdn.net/detail/a87b01c14/9594728