Copys TIFF Lab with transparency and alpha output is wrong

[angelmixu]

Hi!

I'm using some TIFF files in LAB format with a transparency and additional alphas, and the output when copying the images are wrong.
Some channels are missed, or empty.

Here it is a .zip with the files:
tiff_files.zip

I'm using this version from homebrew in macOS:

% vips -v
vips-8.9.2-Tue Apr 21 09:26:11 UTC 2020

And using these command-line for copying the files:

vips copy "TIF LAB 8b TRANSP.tif" lab8_out.tif
vips copy "TIF LAB 16b TRANSP.tif" lab16_out.tif

[boxerab]

These are interesting files. Here are screenshots from opening these files in Krita: The first pair is 8 bit, the second pair is 16 bit. So, it looks like there is a problem with 16 bit.

btw, both lab8 and lab16 are 16 bit images.

Are these files being displayed as expected ?

[boxerab]

The other odd thing about these images is that the data is signed, including the alpha channels.

[boxerab]

It looks like vips is converting the alpha channel from 16 to 8 bit precision, and is not handling the signed nature of the data.

[angelmixu]

Hi @boxerab , this is how I see the images in photoshop (the original images "TIF LAB*.tif" were generated using Photoshop):

In this case, the selected transparency channel is wrong and a channel is lost:

Here the color information is lost, maybe because of choosing the wrong transparency channel? But still all channels are listed:

Original 8bit:

Original 16bit:

The issue about the output for an 8bit LAB being a 16bit is this one: #1473

[angelmixu]

Hi! This commit in our custom branch looks like fixes this issue :)
angelmixu@b031680

At this moment I can't put it in libvips master as we have to fix some issues because our branch is way behind master and there will be some conflicts.