[Octave-bug-tracker] [bug #41673] imwrite creates bad grayscale images

[Carnë Draug]

Update of bug #41673 (project octave):

              Item Group:        Incorrect Result => Regression             
        Operating System:       Microsoft Windows => Any                    
                 Summary: imwrite problems with jpg and bmp => imwrite creates
bad grayscale images

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Follow-up Comment #1:

I can replicate this problem with the more simple case:


a = repmat (uint8 (0:255), 100, 1);
imshow (a)
imwrite (a, "test.jpg");
b = imread ("test.jpg");
figure;
imshow (b)


The example above shows that the problem happens during imwrite since the jpg
file created is already wrong.

We can also see that the problem happens only with grayscale images since
writing an RGB image creates the image file correctly:


a = repmat (uint8 (0:255), 100, 1, 3);
imwrite (a, "test.jpg");


But I'm unsure if this is a problem in Octave. The reason to believe this is
that Octave does not have format specific code, the code is all the same,
independent of the format chosen. However, if we write the image to a tif or
bmp file, everything is correct. By the way, using imread() to read the RGB
image where all 3 channels are the same, will read a grayscale image. This is
again, because GraphicsMagick will recognize it as such and report a grayscale
image to save memory.

Still, I'm marking this as a regression because apparently (according to the
OP on the mailing list)) Octave was saving the file correctly in 3.2.4 and
3.6.0.

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