HTML5 Canvas实现平移/放缩/旋转deom示例(附截图)

 

HTML5 Canvas中提供了实现图形平移，旋转，放缩的API。 
平移(translate) 
平移坐标translate(x, y)意思是把(0,0)坐标平移到(x, y)，原来的(0,0)坐标则变成(-x, -y) 
图示如下： 
 
任何原来的坐标点p(ox, oy)在translate之后的坐标点为p(ox-x, oy-y),其中点(x, y)为平移 
点坐标translate(x, y)。 
代码演示： 

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代码如下:

// translate is move the startpoint to centera and back to top left corner 
function renderText(width, height, context) { 
context.translate(width/ 2, height / 2); // 中心点坐标为(0, 0) 
context.font="18px Arial"; 
context.fillStyle="blue"; 
context.fillText("Please Press to Exit Game",5,50); 
context.translate(-width/2, -height/2); // 平移恢复(0,0)坐标为左上角 
context.fillText("I'm Back to Top",5,50); 
} 

放缩(Scale) 
Scale(a, b)意思是将对象沿着XY轴分别放缩至a*x, b*y大小。效果如图示： 
 

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代码如下:

// translation the rectangle. 
function drawPath(context) { 
context.translate(200,200); 
context.scale(2,2);// Scale twice size of original shape 
context.strokeStyle= "green"; 
context.beginPath(); 
context.moveTo(0,40); 
context.lineTo(80,40); 
context.lineTo(40,80); 
context.closePath(); 
context.stroke(); 
} 

旋转(rotate) 
旋转角度rotate(Math.PI/8) 
 
旋转前的坐标p(x, y)在对应的旋转后的坐标P(rx, ry)为 
Rx = x * cos(-angle) - y * sin(-angle); 
Ry = y * cos(-angle) + x * sin(-angle); 
旋转90度可以简化为： 
Rx = y; 
Ry = -x; 
Canvas中旋转默认的方向为顺时针方向。演示代码如下： 

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代码如下:

// new point.x = x * cos(-angle) -y * sin(-angle), 
// new point.y = y * cos(-angle) +x * sin(-angle) 
function renderRotateText(context) { 
context.font="24px Arial"; 
context.fillStyle="red"; 
context.fillText("i'm here!!!",5,50); 
// rotate -90 degreee 
// context.rotate(-Math.PI/2); 
// context.fillStyle="blue"; 
// context.fillText("i'm here!!!", -400,30); 
// rotate 90 degreee 
context.rotate(Math.PI/2); 
context.fillStyle="blue"; 
context.fillText("i'm here!!!",350,-420); 
console.log(Math.sin(Math.PI/2)); 
// rotae 90 degree and draw 10 lines 
context.fillStyle="green"; 
for(var i=0; i<4; i++) { 
var x = (i+1)*20; 
var y = (i+1)*60; 
var newX = y; 
var newY = -x; 
context.fillRect(newX,newY, 200, 6); 
} 
} 

通常做法是旋转与平移一起使用，先将坐标(0,0)平移到中心位置 
translate (width/2, height/2)然后再使用rotate(Math.PI/2)完成旋转 
代码示例如下： 

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代码如下:

function saveAndRestoreContext(context) { 
context.save(); 
context.translate(200,200); 
context.rotate(Math.PI/2); 
context.fillStyle="black"; 
context.fillText("2D Context Rotate And Translate", 10, 10); 
context.restore(); 
context.fillText("2D Context Rotate And Translate", 10, 10); 
} 

全部JavaScript代码： 

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代码如下:

var tempContext = null; // global variable 2d context 
window.onload = function() { 
var canvas = document.getElementById("target"); 
canvas.width = 450; 
canvas.height = 450; 
if (!canvas.getContext) { 
console.log("Canvas not supported. Please install a HTML5 compatible browser."); 
return; 
} 
// get 2D context of canvas and draw image 
tempContext = canvas.getContext("2d"); 
// renderText(canvas.width, canvas.height, tempContext); 
saveAndRestoreContext(tempContext); 
// drawPath(tempContext); 
} 
// translate is move the start point to centera and back to top left corner 
function renderText(width, height, context) { 
context.translate(width / 2, height / 2); 
context.font="18px Arial"; 
context.fillStyle="blue"; 
context.fillText("Please Press to Exit Game",5,50); 
context.translate(-width / 2, -height / 2); 
context.fillText("I'm Back to Top",5,50); 
} 
// translation the rectangle. 
function drawPath(context) { 
context.translate(200, 200); 
context.scale(2,2); // Scale twice size of original shape 
context.strokeStyle = "green"; 
context.beginPath(); 
context.moveTo(0, 40); 
context.lineTo(80, 40); 
context.lineTo(40, 80); 
context.closePath(); 
context.stroke(); 
} 
// new point.x = x * cos(-angle) - y * sin(-angle), 
// new point.y = y * cos(-angle) + x * sin(-angle) 
function renderRotateText(context) { 
context.font="24px Arial"; 
context.fillStyle="red"; 
context.fillText("i'm here!!!",5,50); 
// rotate -90 degreee 
// context.rotate(-Math.PI/2); 
// context.fillStyle="blue"; 
// context.fillText("i'm here!!!", -400,30); 
// rotate 90 degreee 
context.rotate(Math.PI/2); 
context.fillStyle="blue"; 
context.fillText("i'm here!!!", 350,-420); 
console.log(Math.sin(Math.PI/2)); 
// rotae 90 degree and draw 10 lines 
context.fillStyle="green"; 
for(var i=0; i<4; i++) { 
var x = (i+1)*20; 
var y = (i+1)*60; 
var newX = y; 
var newY = -x; 
context.fillRect(newX, newY, 200, 6); 
} 
} 
function saveAndRestoreContext(context) { 
context.save(); 
context.translate(200,200); 
context.rotate(Math.PI/2); 
context.fillStyle="black"; 
context.fillText("2D Context Rotate And Translate", 10, 10); 
context.restore(); 
context.fillText("2D Context Rotate And Translate", 10, 10); 
}