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+--- Thread: Reproduce the old behavior of "color to alpha" (GIMP 2.6) (/Thread-Reproduce-the-old-behavior-of-color-to-alpha-GIMP-2-6)
Reproduce the old behavior of "color to alpha" (GIMP 2.6) - leiopar - 11-29-2022
I use GIMP since the first version 2.
I noticed recently a change in the behavior of the "color to alpha" command from the 2.6 vertion. When I want to extract the white from a layer, the result is too pale when we put it over a white layer (which should however visually cancel the transparency effect)
See attached file : http://leiopar.free.fr/DISKETTE/gimp_couleur-vers_alpha.zip
I asked the question here, with no results.
Please, how to reproduce the behavior of GIMP 2.6 with GIMP 2.10 ? (it is now not found in portable vertion)
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - rich2005 - 11-29-2022
The only way I get a 'lighter' color-to-alpha' is when the transparency threshold is increased
[attachment=9101]
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - leiopar - 11-29-2022
(11-29-2022, 02:34 PM)rich2005 Wrote: The only way I get a 'lighter' color-to-alpha' is when the transparency threshold is increased
It's even worse !
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - rich2005 - 11-29-2022
I did mean that the threshold should be zero.
Did these images originate in Gimp 2.6 ?
I can not find the reason behind the difference from your xcf files but the fix is the layer mode.
Set the mode to legacy - the right side icon
[attachment=9103]
Can you post the original 2.6 image before opening in Gimp 2.10 ?
If you are using Windows then there are Gimp 2.6 installers https://download.gimp.org/gimp/v2.6/windows/ and these do co-exist with Gimp 2.10
screenshot Win10 / Gimp 2.6.8 (64 bit) https://i.imgur.com/3VvTAco.jpg
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - Ofnuts - 11-29-2022
(11-29-2022, 02:04 PM)leiopar Wrote: I use GIMP since the first version 2.
I noticed recently a change in the behavior of the "color to alpha" command from the 2.6 vertion. When I want to extract the white from a layer, the result is too pale when we put it over a white layer (which should however visually cancel the transparency effect)
See attached file : http://leiopar.free.fr/DISKETTE/gimp_couleur-vers_alpha.zip
I asked the question [/url][url=https://forums.commentcamarche.net/forum/affich-35991037-couleur-vers-alpha-trop-pale]here, with no results.
Please, how to reproduce the behavior of GIMP 2.6 with GIMP 2.10 ? (it is now not found in portable vertion)
Bucket fill with the bucket tool in Color Erase mode (and with the color to erase of course). This is the best way in 2.10 to get the exact erasure (in other words if you put the result over a background of the erased color, you get back exactly the initial image).
In case you wonder, one thing that changes in 2.10 is that now all computations are done in "linear light". Up to Gimp 2.8, channel values where gamma-encoded and computations were done directly on these values (which is conceptually wrong). A well known result was the red-to-green gradient that goes through brown instead of going through orange but this had other implications such as computing the correct composition of semi-transparent pixels.
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - rich2005 - 11-30-2022
Since I installed a Gimp 2.6, I made a 2.6 xcf file to play with (attached for info)
Open in Gimp 2.10 and it is in legacy mode.
Leave it in legacy mode and color-to-alpha (c2a) which is now a GEGL procedure works as in Gimp 2.6 https://i.imgur.com/cQYOBxx.jpg
Change the mode to default, there are good reasons for the enhanced Gimp 2.10 color options, apply c2a and get that faded effect. https://i.imgur.com/BIpYGR9.jpg
It is possible to get close to the legacy version by reducing the opacity threshold, say 0.5, not quite an exact match, needs a little more transparency. https://i.imgur.com/Ldln5jA.jpg
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - leiopar - 12-01-2022
(11-29-2022, 05:59 PM)rich2005 Wrote: I can not find the reason behind the difference from your xcf files but the fix is the layer mode.
It works
I would never have looked there ...It’s crazy that the "normal mode" is so different in the management of Alpha channel !!!
(11-29-2022, 09:36 PM)Ofnuts Wrote: Bucket fill with the bucket tool in Color Erase mode
It work fine to (with the advantage of staying in the new "normal mode by default")
(11-29-2022, 09:36 PM)Ofnuts Wrote: In case you wonder, one thing that changes in 2.10 is that now all computations are done in "linear light"...
Interesting ... I didn’t understand everything but it’s good to know.
Thank you all
RE: Reproduce the old behavior of "color to alpha" (GIMP 2.6) - rich2005 - 12-03-2022
Still looking for a correct solution. Ofnuts did mention that Gimp 2.10 now uses rgb linear
With the layer mode in default, right-click in layer, up at the top is Composite Space, select RGB(Percetual)
[attachment=9113]