Is it possible to save a 32bit BMP image (with alpha)

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Slimane 2011-5-10

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评论： DGM 2024-7-15，5:12

Is it possible to save a 32bit BMP image. i.e. image size (imheight x imwidth x 4) where the 4.th channel consist of alpha values?

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Answer 1

Walter Roberson 2011-5-10

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imwrite does not support alpha in BMP. See http://www.mathworks.com/matlabcentral/answers/6335-writing-32-bit-images for information on what can be used.

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Slimane 2011-5-11

移动：DGM 2024-7-15，5:01

在 MATLAB Online 中打开

Yes i can see that bitdepth is 32, but when i open the saved image in e.g. 'irfanView' it says the image size is 240x376x24BPP. Here is the imfinfo i receive:

ans =

                     Filename: 'New.tiff'
                  FileModDate: '11-May-2011 21:52:55'
                     FileSize: 364374
                       Format: 'tif'
                FormatVersion: []
                        Width: 376
                       Height: 240
                     BitDepth: 32
                    ColorType: 'unknown'
              FormatSignature: [73 73 42 0]
                    ByteOrder: 'little-endian'
               NewSubFileType: 0
                BitsPerSample: [8 8 8 8]
                  Compression: 'PackBits'
    PhotometricInterpretation: 'CMYK'
                 StripOffsets: [48x1 double]
              SamplesPerPixel: 4
                 RowsPerStrip: 5
              StripByteCounts: [48x1 double]
                  XResolution: 72
                  YResolution: 72
               ResolutionUnit: 'Inch'
                     Colormap: []
          PlanarConfiguration: 'Chunky'
                    TileWidth: []
                   TileLength: []
                  TileOffsets: []
               TileByteCounts: []
                  Orientation: 1
                    FillOrder: 1
             GrayResponseUnit: 0.0100
               MaxSampleValue: [255 255 255 255]
               MinSampleValue: 0
                 Thresholding: 1

Slimane 2011-5-12

移动：DGM 2024-7-15，5:01

thanks a lot Walter, i guess i will just use c++ it is much easier :)

DGM 2024-7-15，5:12

This problem is occurring because imwrite() interprets a 4-channel RGBA image as CMYK (regardless of any asserted colorspace parameter). So anything that decodes the file will likely wind up treating it as CMYK and converting it back into an opaque 24b RGB equivalent.

As far as I know, writing an RGBA TIFF this way isn't an option with imwrite(). It should be possible with the tiff class.

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Answer 2

John 2011-5-12

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https://ww2.mathworks.cn/matlabcentral/answers/7169-is-it-possible-to-save-a-32bit-bmp-image-with-alpha#answer_10031

在 MATLAB Online 中打开

Here's a demo I did a while back on how to create a TIFF with an alpha layer.

% This image is included with MATLAB
D = imread('street1.jpg');
image(D);  title('RGB image without alpha layer.');
h = size(D,1);
w = size(D,2);
% Create a gradually increasing transparency mask 
% from left-to-right across the image.  
T = zeros(h,w,'uint8');
for j=1:w
  T(:,j) = min(255,j) * ones(h,1); 
end
% The dimensions of our image are h x w x 3 (remember, it's RGB).  If 
% we wish to add an alpha component, we need to give the image data a 4th
% component.  
rgbaData = cat(3,D,T);
% Let's create our new image.
t = Tiff('alpha.tif','w');
% The photometric interpretation remains RGB, even though we will be
% adding an extra sample.
t.setTag('Photometric',Tiff.Photometric.RGB);
% These three tags define the dimensions of the image in the file. 
t.setTag('ImageWidth',w);
t.setTag('ImageLength',h);
t.setTag('BitsPerSample',8);
t.setTag('SamplesPerPixel',4);
% We will make the image consist of 64x64 tiles.
t.setTag('TileWidth',64);
t.setTag('TileLength',64);
t.setTag('PlanarConfiguration',Tiff.PlanarConfiguration.Chunky);
% A photometric interpretation of RGB means that the TIFF file is expecting
% three components.  We have to tell it that there will really be four
% components (the alpha layer is the 4th).
t.setTag('ExtraSamples',Tiff.ExtraSamples.UnassociatedAlpha);
t.write(rgbaData);
t.close();