[Octave-bug-tracker] [bug #45779] Writing and 8-bit image to BMP produces a 24-bit bmp

[Carnë Draug]

Update of bug #45779 (project octave):

                  Status:               Confirmed => Need Info              

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Follow-up Comment #2:

Matlab documentation on this is very much sparse.  My interpretation is that
the 1, 8, and 24 bit format that they support are for boolean, indexed, and
RGB images (8-bit per color channel) respectively.

Octave does check the type of the matrix being saved and supports saving
boolean matrices as 1-bit images. I just tested it and GM seems to respect
it:


$ octave
octave> imwrite (rand(100) > 0.6, "bool.bmp")
octave> exit
$ gm identify -verbose bool.bmp 
Image: bool.bmp
  Format: BMP (Microsoft Windows bitmap image)
[...]
  Type: bilevel
  Depth: 1 bits-per-pixel component
  Channel Depths:
    Gray:     1 bits
[...]


An image with a colormap should be saved as an 8 bit indexed image.  In the
following, it seems like both are saved as indexed images, the difference is
that GM seems to identify that the colormap is gray but the image itself is
indexed and 8 bit in both cases:


$ octave
octave> imwrite (randi ([0 255], 100, 100, "uint8"), gray (256),
"with-gray-map.bmp")
octave> imwrite (randi ([0 255], 100, 100, "uint8"), cubehelix (256),
"with-color-map.bmp")
octave> exit
$ gm identify -verbose with-gray-map.bmp 
Image: with-gray-map.bmp
  Format: BMP (Microsoft Windows bitmap image)
  Geometry: 100x100
  Class: PseudoClass
  Type: grayscale
  Depth: 8 bits-per-pixel component
  Channel Depths:
    Gray:     8 bits
[...]
$ gm identify -verbose with-color-map.bmp 
Image: with-color-map.bmp
  Format: BMP (Microsoft Windows bitmap image)
  Geometry: 100x100
  Class: PseudoClass
  Type: palette
  Depth: 8 bits-per-pixel component
  Channel Depths:
    Red:      8 bits
    Green:    8 bits
    Blue:     8 bits


Finally, without a colormap, I'm unsure what should happen for Matlab
compatibility. I think the logical thing would be to save an 8-bit image for
grayscale image and 24-bit (8-bit per channel) for RGB images which seems to
be happening:


$ octave
octave> imwrite (randi ([0 255], 100, 100, 3, "uint8"),
"without-map-rgb.bmp")
octave> imwrite (randi ([0 255], 100, 100, "uint8"), "without-map-gray.bmp")
octave> exit
$ gm identify -verbose without-map-rgb.bmp
Image: without-map-rgb.bmp
  Format: BMP (Microsoft Windows bitmap image)
  Geometry: 100x100
  Class: DirectClass
  Type: true color
  Depth: 8 bits-per-pixel component
  Channel Depths:
    Red:      8 bits
    Green:    8 bits
    Blue:     8 bits
[...]
$ gm identify -verbose without-map-gray.bmp
Image: without-map-gray.bmp
  Format: BMP (Microsoft Windows bitmap image)
  Geometry: 100x100
  Class: DirectClass
  Type: grayscale
  Depth: 8 bits-per-pixel component
  Channel Depths:
    Gray:     8 bits
[...]


Unfortunately, `gm identify` is not 100% reliable and I'm not very familiar
with the BMP format either.

So, could you please be a bit more verbose on what exactly should be
happening? 8-bit image means very little as you can see from above.

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