MATLAB: How to convert a 24 bit depth image file to an 8 bit depth image file

248bitconvertdepthimageImage Processing Toolboxprocessingtiff

I have a 24 bit depth image that needs to be used in a different program. This program however needs only an image of 8 bit depth. I want to be able to change the bit depth at will when generating a TIFF file using MATLAB.

Best Answer

To achieve the bit depth conversion, use the RGB2IND function. The following code sample illustrates this:
RGB = imread('myimage.tif')
[X,map] = rgb2ind(RGB, 256);
imwrite(X,map,'myindexedimage.tif');
Now myindexedimage.tif has a bit depth of 8.
Note IM2UINT8 and the IM2UINT16 functions, are also useful when attempting to convert between different bit depths. The following code sample illustrates the conversion from a 8 bit depth image to a 24 bit depth image. 'trees.tif' is an image shipped with the Image processing toolbox that has a bit depth of 8.
[Xtrees,maptrees] = imread('trees.tif');
XtreesRGB = ind2rgb(Xtrees,maptrees);
XtreesRGB = im2uint8(XtreesRGB);
imwrite(Xtreesind,'myindexedtrees.tiff')
imfinfo('myindexedtrees.tiff')
 ans = 
                      Filename: 'myindexedtrees.tiff'
                   FileModDate: '08-May-2008 09:48:27'
                      FileSize: 259354
                        Format: 'tif'
                 FormatVersion: []
                         Width: 350
                        Height: 258
                      BitDepth: 24
                     ColorType: 'truecolor'
               FormatSignature: [73 73 42 0]
                     ByteOrder: 'little-endian'
                NewSubFileType: 0
                 BitsPerSample: [8 8 8]
                   Compression: 'PackBits'
     PhotometricInterpretation: 'RGB'
                  StripOffsets: [37x1 double]
               SamplesPerPixel: 3
                  RowsPerStrip: 7
               StripByteCounts: [37x1 double]
                   XResolution: 72
                   YResolution: 72
                ResolutionUnit: 'Inch'
                      Colormap: []
           PlanarConfiguration: 'Chunky'
                     TileWidth: []
                    TileLength: []
                   TileOffsets: []
                TileByteCounts: []
                   Orientation: 1
                     FillOrder: 1
              GrayResponseUnit: 0.0100
                MaxSampleValue: [255 255 255]
                MinSampleValue: 0
                  Thresholding: 1
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