MATLAB: 16 BitDepth vs 24 BitDepth BMP image

bitdepthbmp imageimage processing

I have some BMP images from an optical CT scan that I need to process in matlab. Using the function imfinfo I know that my image before being read by matlab is at 16 BitDepth and is transformed to a 24 BitDepth after.

Before
FileSize: 1572918
Format: 'bmp'
FormatVersion: 'Version 3 (Microsoft Windows 3.x)'
Width: 1024
Height: 768
BitDepth: 16
ColorType: 'truecolor'
FormatSignature: 'BM'
NumColormapEntries: 0
Colormap: []
RedMask: []
GreenMask: []
BlueMask: []
ImageDataOffset: 54
BitmapHeaderSize: 40
NumPlanes: 1
CompressionType: 'none'
BitmapSize: 1572864
HorzResolution: 0
VertResolution: 0
NumColorsUsed: 0
NumImportantColors: 0
After
FileSize: 2359350
Format: 'bmp'
FormatVersion: 'Version 3 (Microsoft Windows 3.x)'
Width: 1024
Height: 768
BitDepth: 24
ColorType: 'truecolor'
FormatSignature: 'BM'
NumColormapEntries: 0
Colormap: []
RedMask: []
GreenMask: []
BlueMask: []
ImageDataOffset: 54
BitmapHeaderSize: 40
NumPlanes: 1
CompressionType: 'none'
BitmapSize: 2359296
HorzResolution: 0
VertResolution: 0
NumColorsUsed: 0
NumImportantColors: 0

I need the image to be a 16 BitDepth BMP when I save it to use it in a reconstruction software. Cheers Pierre

Best Answer

imwrite() in MATLAB cannot write 16 bits per pixel for BMP files.

Note: 16 bits per pixel for BMP files is 4 bits each for R, G, B, and Alpha (possibly not in that order.)

imwrite() for BMP can use logical (total 1 bit per pixel) or uint8 grayscale (total 8 bits per pixel) or uint8 RGB (total 24 bits per pixel.)

Related Solutions

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

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

MATLAB: How to change colortype truecolor to grayscale

You have to display the gray scale image with imshow() after you do the conversion.

grayImage = rgb2gray(rgbImage);
imshow(grayImage, []);  % You probably forgot to do this step.

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