How do I convert a 24 bit depth image file to an 8 bit depth image file?

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.

Answer 1

[MathWorks Support Team]

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