Images loaded from palletized TGA files lack alpha channels #110437
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Reference: blender/blender#110437
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System Information
Operating system: Windows 11 Home
Graphics card: Intel ARC A750 8GB
Blender Version
Broken: 3.6.1 and 4.0.0
2ed108cb84Worked: None tested
Short description of error
When a palletized TGA file is loaded as an image, any transparent pixels come through as opaque black.
This does not occur for unpalletized TGA files; they come through transparent as expected.
An example of the same file in both palletized and unpalletized formats is attached. The files look identical in Aseprite.
Exact steps for others to reproduce the error
This can additionally be confirmed by looking at the histogram, with the unpalletized file having a small spike at 0% for alpha while the palletized file is flat until 100%.
Can confirm that palletized tga doesn't load with alpha. (But not sure if pallet+alpha is to spec)
imagemagickandgeeqieis loading both correctlyGIMPfailed to load palletized tga completely, doesn't show correct image.Here's what GIMP shows on palletized tga when alpha channel is manually removed after loading:
This is a problematic texture, with an unfortunate history.
The texture itself declares in its header (byte offset 17) that it contains 0 bits of alpha [1]. Our imaging library is now respecting this data and so no alpha channel is created for it.
Previous blender versions: Older Blender versions were "broken" in 2 ways actually, but by accident, allowed things to work in some cases. First this texture will not work in Cycles if the alpha channel is expected. Eevee and the Image Editor work by accident. You can see that it's actually being loaded as RGB with no Alpha [4] but by luck happens to work due to a quirk of the old image loading code.
What about GIMP: The nightly 2.99.x versions will load it because they have a workaround to bypass the broken textures :( [2]
What about Krita: Krita actually refuses to load the palettized image entirely and issues an error [3]
Others? : Other software, like current released versions of GIMP, are affected too (see Yiming's comment above)
[1] Truevision TGA, File Format Specification, Version 2.0 as PDF, Page 8: https://www.loc.gov/preservation/digital/formats/fdd/fdd000180.shtml
[2] Gimp workaround: https://gitlab.gnome.org/GNOME/gimp/-/blob/master/plug-ins/common/file-tga.c#L550
[3] Krita error logs
[4] Blender incorrect format from 3.3.8 LTS
So it might be an Aseprite bug that it is creating this file at all? Tonight I’ll experiment with changing the header byte called out see what happens. If palletized TGA cannot have an alpha channel I’ll get a bug raised on the Aseprite side. Cheers.
Which version of GIMP is that? Testing with 2.10.32, both files loaded with the alpha channel preserved.
Ah, so, reading the spec, it seems there is supposed to be an "extension area" following the image data which should contain the following field:
I'll raise this as a bug on the Aseprite side, thanks.
I tried using GIMP 2.10.18 which failed to load it properly but 2.99.16 succeeds as they have their own workaround in place.
The part of the spec is probably this one -- in the main TGA file header:
Bits 3-0 in the provided file are 0.
That said, I think even after this is fixed our image library will have issues doing the proper thing. I'll have to work with the upstream project to fix the issue (which can take some time) but having a "valid" set of image files is a good first step for Aseprite to address first.