Exporting to Images

To export data from the MATLAB^® workspace using one of the standard graphics file formats, use the imwrite function. Using this function, you can export data in formats such as the Tagged Image File Format (TIFF), Joint Photographic Experts Group (JPEG), and Portable Network Graphics (PNG). For a complete list of supported formats, see the imwrite reference page.

The following example writes a multidimensional array of uint8 data I from the MATLAB workspace into a file in TIFF format. The class of the output image written to the file depends on the format specified. For most formats, if the input array is of class uint8, imwrite outputs the data as 8-bit values. See the imwrite reference page for details.

whos I
  Name      Size                           Bytes  Class

  I       650x600x3                      1170000  uint8 array

Grand total is 1170000 elements using 1170000 bytes
imwrite(I, 'my_graphics_file.tif','tif');

Note   imwrite supports different syntaxes for several of the standard formats. For example, with TIFF file format, you can specify the type of compression MATLAB uses to store the image. See the imwrite reference page for details.

For more control writing data to a TIFF file, use the Tiff object—see Exporting Image Data and Metadata to TIFF Files for more information.

Exporting Image Data and Metadata to TIFF Files

While you can use imwrite to export image data and metadata (tags) to Tagged Image File Format (TIFF) files, the function does have some limitations. For example, when you want to modify image data or metadata in the file, you must write the all the data to the file. You cannot write only the updated portion. Using the Tiff object, you can write portions of the image data and modify or add individual tags to a TIFF file. When you construct a Tiff object, it represents your connection with a TIFF file and provides access to many of the routines in the LibTIFF library.

The following sections provide step-by-step examples of using Tiff object methods and properties to perform some common tasks with TIFF files. To get the most out of the Tiff object, you must be familiar with the TIFF specification and technical notes. View this documentation at LibTIFF - TIFF Library and Utilities

Creating a New TIFF File

1. Create some image data. This example reads image data from a JPEG file included with MATLAB:

   imgdata = imread('ngc6543a.jpg');

2. Create a new TIFF file by constructing a Tiff object, specifying the name of the new file as an argument. To create a file you must specify either write mode ('w') or append mode ('a'):

   t = Tiff('myfile.tif','w');

   When you create a new TIFF file, the Tiff constructor creates a file containing an image file directory (IFD). A TIFF file uses this IFD to organize all the data and metadata associated with a particular image. A TIFF file can contain multiple IFDs. The Tiff object makes the IFD it creates the current IFD. Tiff object methods operate on the current IFD. You can navigate among IFDs in a TIFF file and specify which IFD is the current IFD using Tiff object methods.

3. Set required TIFF tags using the setTag method of the Tiff object. These required tags specify information about the image, such as its length and width. To break the image data into strips, specify a value for the RowsPerStrip tag. To break the image data into tiles, specify values for the TileWidth and TileLength tags. The example creates a structure that contains tag names and values and passes that to setTag. You also can set each tag individually.

   tagstruct.ImageLength = size(imgdata,1)
   tagstruct.ImageWidth = size(imgdata,2)
   tagstruct.Photometric = Tiff.Photometric.RGB
   tagstruct.BitsPerSample = 8
   tagstruct.SamplesPerPixel = 3
   tagstruct.RowsPerStrip = 16
   tagstruct.PlanarConfiguration = Tiff.PlanarConfiguration.Chunky
   tagstruct.Software = 'MATLAB'
   t.setTag(tagstruct)

   For information about supported TIFF tags and how to set their values, see Setting Tag Values. For example, the Tiff object supports properties that you can use to set the values of certain properties. This example uses the Tiff object PlanarConfigurationproperty to specify the correct value for the chunky configuration: Tiff.PlanarConfiguration.Chunky.

4. Write the image data and metadata to the current directory using the write method of the Tiff object.

   t.write(imgdata);
   

   If you wanted to put multiple images into your file, call the writeDirectory method right after performing this write operation. ThewriteDirectory method sets up a new image file directory in the file and makes this new directory the current directory.

5. Close your connection to the file by closing the Tiff object:

   t.close();
   

6. Test that you created a valid TIFF file by using the imread function to read the file, and then display the image:

   imagesc(imread('myfile.tif'));

Writing a Strip or Tile of Image Data

Note:   You can only modify a strip or a tile of image data if the data is not compressed.

1. Open an existing TIFF file for modification by creating a Tiff object. This example uses the file created in Creating a New TIFF File. The Tiff constructor returns a handle to a Tiff object.

   t = Tiff('myfile.tif','r+');

2. Generate some data to write to a strip in the image. This example creates a three-dimensional array of zeros that is the size of a strip. The code uses the number of rows in a strip, the width of the image, and the number of samples per pixel as dimensions. The array is an array of uint8 values.

   width = t.getTag('ImageWidth');
   height = t.getTag('RowsPerStrip');
   numSamples = t.getTag('SamplesPerPixel');
   stripData = zeros(height,width,numSamples,'uint8');
   

   If the image data had a tiled layout, you would use the TileWidth and TileLength tags to specify the dimensions.

3. Write the data to a strip in the file using the writeEncodedStrip method. Specify the index number that identifies the strip you want to modify. The example picks strip 18 because it is easier to see the change in the image.

   t.writeEncodedStrip(18, stripData);
   

   If the image had a tiled layout, you would use the writeEncodedTile method to modify the tile.

4. Close your connection to the file by closing the Tiff object.

   t.close();
   

5. Test that you modified a strip of the image in the TIFF file by using the imread function to read the file, and then display the image.

   modified_imgdata = imread('myfile.tif');
   imagesc(modified_imgdata)

   Note the black strip across the middle of the image.

Modifying TIFF File Metadata (Tags)

1. Open an existing TIFF file for modification using the Tiff object. This example uses the file created in Creating a New TIFF File. TheTiff constructor returns a handle to a Tiff object.

   t = Tiff('myfile.tif','r+');

2. Verify that the file does not contain the Artist tag, using the getTag method. This code should issue an error message saying that it was unable to retrieve the tag.

   artist_value = t.getTag('Artist');

3. Add the Artist tag using the setTag method.

   t.setTag('Artist','Pablo Picasso');

4. Write the new tag data to the TIFF file using the rewriteDirectory method. Use the rewriteDirectory method when modifying existing metadata in a file or adding new metadata to a file.

   t.rewriteDirectory();

5. Close your connection to the file by closing the Tiff object.

   t.close();
   

6. Test your work by reopening the TIFF file and getting the value of the Artist tag, using the getTag method.

   t = Tiff('myfile.tif', 'r');
   
   t.getTag('Artist')
   
   ans =
   
   Pablo Picasso
   
   t.close();

Creating Subdirectories in a TIFF File

1. Create some image data. This example reads image data from a JPEG file included with MATLAB. The example then creates two reduced-resolution (thumbnail) versions of the image data.

   imgdata = imread('ngc6543a.jpg');
   %
   % Reduce number of pixels by a half.
   img_half = imgdata(1:2:end,1:2:end,:);
   %
   % Reduce number of pixels by a third.
   img_third = imgdata(1:3:end,1:3:end,:);

2. Create a new TIFF file by constructing a Tiff object and specifying the name of the new file as an argument. To create a file you must specify either write mode ('w') or append mode ('a'). The Tiff constructor returns a handle to a Tiff object.

   t = Tiff('my_subimage_file.tif','w');

3. Set required TIFF tags using the setTag method of the Tiff object. These required tags specify information about the image, such as its length and width. To break the image data into strips, specify a value for the RowsPerStrip tag. To break the image data into tiles, use the TileWidth and TileLength tags. The example creates a structure that contains tag names and values and passes that tosetTag. You can also set each tag individually.

   To create subdirectories, you must set the SubIFD tag, specifying the number of subdirectories you want to create. Note that the number you specify isn't the value of the SubIFD tag. The number tells the Tiff software to create a SubIFD that points to two subdirectories. The actual value of the SubIFD tag will be the byte offsets of the two subdirectories.

   tagstruct.ImageLength = size(imgdata,1)
   tagstruct.ImageWidth = size(imgdata,2)
   tagstruct.Photometric = Tiff.Photometric.RGB
   tagstruct.BitsPerSample = 8
   tagstruct.SamplesPerPixel = 3
   tagstruct.RowsPerStrip = 16
   tagstruct.PlanarConfiguration = Tiff.PlanarConfiguration.Chunky
   tagstruct.Software = 'MATLAB'
   tagstruct.SubIFD = 2  % required to create subdirectories
   t.setTag(tagstruct)

   For information about supported TIFF tags and how to set their values, see Setting Tag Values. For example, the Tiff object supports properties that you can use to set the values of certain properties. This example uses the Tiff object PlanarConfigurationproperty to specify the correct value for the chunky configuration: Tiff.PlanarConfiguration.Chunky.

4. Write the image data and metadata to the current directory using the write method of the Tiff object.

   t.write(imgdata);
   

5. Set up the first subdirectory by calling the writeDirectory method. The writeDirectory method sets up the subdirectory and make the new directory the current directory. Because you specified that you wanted to create two subdirectories, writeDirectorysets up a subdirectory.

   t.writeDirectory();
   

6. Set required tags, just as you did for the regular directory. According to the LibTIFF API, a subdirectory cannot contain a SubIFD tag.

   tagstruct2.ImageLength = size(img_half,1)
   tagstruct2.ImageWidth = size(img_half,2)
   tagstruct2.Photometric = Tiff.Photometric.RGB
   tagstruct2.BitsPerSample = 8
   tagstruct2.SamplesPerPixel = 3
   tagstruct2.RowsPerStrip = 16
   tagstruct2.PlanarConfiguration = Tiff.PlanarConfiguration.Chunky
   tagstruct2.Software = 'MATLAB'
   t.setTag(tagstruct2)

7. Write the image data and metadata to the subdirectory using the write method of the Tiff object.

   t.write(img_half);
   

8. Set up the second subdirectory by calling the writeDirectory method. The writeDirectory method sets up the subdirectory and makes it the current directory.

   t.writeDirectory();
   

9. Set required tags, just as you would for any directory. According to the LibTIFF API, a subdirectory cannot contain a SubIFD tag.

   tagstruct3.ImageLength = size(img_third,1)
   tagstruct3.ImageWidth = size(img_third,2)
   tagstruct3.Photometric = Tiff.Photometric.RGB
   tagstruct3.BitsPerSample = 8
   tagstruct3.SamplesPerPixel = 3
   tagstruct3.RowsPerStrip = 16
   tagstruct3.PlanarConfiguration = Tiff.PlanarConfiguration.Chunky
   tagstruct3.Software = 'MATLAB'
   t.setTag(tagstruct3)

10. Write the image data and metadata to the subdirectory using the write method of the Tiff object:

    t.write(img_third);
    

11. Close your connection to the file by closing the Tiff object:

    t.close();
    

Setting Tag Values

The following table lists all the TIFF tags that the Tiff object supports and includes information about their MATLAB class and size. For certain tags, the table also indicates the set of values that the Tiff object supports, which is a subset of all the possible values defined by the TIFF specification. You can use Tiff object properties to specify the supported values for these tags. For example, useTiff.Compression.JPEG to specify JPEG compression. See the Tiff class reference page for a full list of properties.

Table 1: Supported TIFF Tags

TIFF Tag	Class	Size	Supported Values	Notes
Artist	char	1xN
BitsPerSample	double	1x1	1,8,16,32,64	See Table 2
ColorMap	double	256x3	Values should be normalized between 0–1. Stored internally asuint16 values.	Photometric must bePalette
Compression	double	1x1	None: 1 CCITTRLE: 2 CCITTFax3: 3 CCITTFax4: 4 LZW: 5 JPEG: 7 CCITTRLEW: 32771 PackBits: 32773 Deflate: 32946 AdobeDeflate: 8	See Table 3.
Copyright	char	1xN
DateTime	char	1x19	Return value is padded to 19 chars if required.
DocumentName	char	1xN
DotRange	double	1x2		Photometric must beSeparated
ExtraSamples	double	1xN	Unspecified: 0 AssociatedAlpha: 1 UnassociatedAlpha: 2	See Table 4.
FillOrder	double	1x1
GeoAsciiParamsTag	char	1xN
GeoDoubleParamsTag	double	1xN
GeoKeyDirectoryTag	double	Nx4
Group3Options	double	1x1		Compression must beCCITTFax3
Group4Options	double	1x1		Compression must beCCITTFax4
HalfToneHints	double	1x2
HostComputer	char	1xn
ICCProfile	uint8	1xn
ImageDescription	char	1xn
ImageLength	double	1x1
ImageWidth	double	1x1
InkNames	char cell array	1x NumInks		Photometric must beSeparated
InkSet	double	1x1	CMYK: 1 MultiInk: 2	Photometric must beSeparated
JPEGQuality	double	1x1	A value between 1 and 100
Make	char	1xn
MaxSampleValue	double	1x1	0–65,535
MinSampleValue	double	1x1	0–65,535
Model	char	1xN
ModelPixelScaleTag	double	1x3
ModelTiepointTag	double	Nx6
ModelTransformationMatrixTag	double	1x16
NumberOfInks	double	1x1		Must be equal toSamplesPerPixel
Orientation	double	1x1	TopLeft: 1 TopRight: 2 BottomRight: 3 BottomLeft: 4 LeftTop: 5 RightTop: 6 RightBottom: 7 LeftBottom: 8
PageName	char	1xN
PageNumber	double	1x2
Photometric	double	1x1	MinIsWhite: 0 MinIsBlack: 1 RGB: 2 Palette: 3 Mask: 4 Separated: 5 YCbCr: 6 CIELab: 8 ICCLab: 9 ITULab: 10	See Table 2.
Photoshop	uint8	1xN
PlanarConfiguration	double	1x1	Chunky: 1  Separate: 2
PrimaryChromaticities	double	1x6
ReferenceBlackWhite	double	1x6
ResolutionUnit	double	1x1
RICHTIFFIPTC	uint8	1xN
RowsPerStrip	double	1x1
SampleFormat	double	1x1	Uint: 1 Int: 2 IEEEFP: 3	See Table 2
SamplesPerPixel	double	1x1
SMaxSampleValue	double	1x1	Range of MATLAB data type specified for Image data
SMinSampleValue	double	1x1	Range of MATLAB data type specified for Image data
Software	char	1xN
StripByteCounts	double	1xN		Read-only
StripOffsets	double	1xN		Read-only
SubFileType	double	1x1	Default : 0 ReducedImage: 1 Page: 2 Mask: 4
SubIFD	double	1x1
TargetPrinter	char	1xN
Thresholding	double	1x1	BiLevel: 1  HalfTone: 2  ErrorDiffuse: 3	Photometric can be either:MinIsWhite MinIsBlack
TileByteCounts	double	1xN		Read-only
TileLength	double	1x1	Must be a multiple of 16
TileOffsets	double	1xN		Read-only
TileWidth	double	1x1	Must be a multiple of 16
TransferFunction	double	See note1	Each value should be within 0–2^16-1	SamplePerPixel can be either 1 or 3
WhitePoint	double	1x2		Photometric can be: RGB Palette  YCbCr CIELab ICCLab ITULab
XMP	char	1xn		N>5
XPostion	double	1x1
XResolution	double	1x1
YCbCrCoefficents	double	1x3		Photometric must beYCbCr
YCbCrPositioning	double	1x1	Centered: 1  Cosited: 2	Photometric must beYCbCr
YCbCrSubSampling	double	1x2		Photometric must beYCbCr
YPosition	double	1x1
YResolution	double	1x1
ZipQuality	double	1x1	Value between 1 and 9

¹Size is 1x2^BitsPerSample or3x2^BitsPerSample.

Table 2: Valid SampleFormat Values for BitsPerSample Settings

BitsPerSample	SampleFormat	MATLAB Data Type
1	Uint	logical
8	Uint, Int	uint8, int8
16	Uint, Int	uint16, int16
32	Uint, Int, IEEEFP	uint32, int32, single
64	IEEEFP	double

Table 3: Valid SampleFormat Values for BitsPerSample and Photometric Combinations

	BitsPerSample Values
Photometric Values	1	8	16	32	64
MinIsWhite	Uint	Uint/Int	Uint Int	Uint Int IEEEFP	IEEEFP
MinIsBlack	Uint	Uint/Int	Uint Int	Uint Int IEEEFP	IEEEFP
RGB		Uint	Uint	Uint IEEEFP	IEEEFP
Pallette		Uint	Uint
Mask	Uint
Separated		Uint	Uint	Uint IEEEFP	IEEEFP
YCbCr		Uint	Uint	Uint IEEEFP	IEEEFP
CIELab		Uint	Uint
ICCLab		Uint	Uint
ITULab		Uint	Uint

Table 4: Valid SampleFormat Values for BitsPerSample and Compression Combinations

	BitsPerSample Values
Compression Values	1	8	16	32	64
None	Uint	Uint Int	Uint Int	Uint Int IEEEFP	IEEEFP
CCITTRLE	Uint
CCITTFax3	Uint
CCITTFax4	Uint
LZW	Uint	Uint Int	Uint Int	Uint Int IEEEFP	IEEEFP
JPEG		Uint Int
CCITTRLEW	Uint
PackBits	Uint	Uint Int	Uint Int	Uint Int IEEEFP	IEEEFP
Deflate	Uint	Uint Int	Uint Int	Uint Int IEEEFP	IEEEFP
AdobeDeflate	Uint	Uint Int	Uint Int	Uint Int IEEEFP	IEEEFP

Table 5: Valid SamplesPerPixel Values for Photometric Settings

Photometric Values	SamplesPerPixel1
MinIsWhite	1+
MinIsBlack	1+
RGB	3+
Pallette	1
Mask	1
Separated	1+
YCbCr	3
CIELab	3+
ICCLab	3+
ITULab	3+

¹ When you specify more than the expected number of samples per pixel (n+), you must set the ExtraSamples tag accordingly.