Re: [Algorithms] Common Src/DestAlphaBlend mode combinations

[Alen L. <ale...@cr...>]

Saturday, December 6, 2008, 1:12:17 AM, Wessam wrote:
>> The last time I checked (CS2?), an option for linear space operation was available only for some operations
>> (image resizing?), but not for general blending, and was in some obscure place in the prefs.
> It’s been in Photoshop a while ago.
Edit >> Color Settings > (Check) Advanced Mode > (Check) Blend RGB Colors Using Gamma.
> It affects all transparency blends (including layer transparencies).

Sorry, my bad. Yes that's the option. I've misremembered the way it
works. It applies to blending, but it curiously doesn't apply to image
resizing.

I remember it seemed strange, because resizing is the textbook example
of how operating in gamma space can produce wrong results:
#1 Create an image filled with pixel-wide black/white stripe pattern.
#2 Downscale it 2x2 using bilinear filtering.
#3 Upscale it back.
#4 Compare it visually with the original when viewed from a distance.
If resampling was done in gamma space, the result will _much_ darker
than the original.

When I was trying to demonstrate this kind of artefact to our artists,
I was expecting that I would be able to demonstrate how Photoshop is
able to do the resizing correctly if the color settings are right, but
alas.... To do a correct image resize in PS, one has to do strange
tricks with converting the image to linear color using the levels
dialog (a strange place for a gamma conversion tool, isn't it?), then
resize, then go back to gamma space. And this has to be done with
16-bit per component precision of course. In the end it is really
complicated, so I gave up and let our artists continue to do image
resizing the wrong way. Sadly, Gimp seems to follow the stray path of
PS, and even the gamma conversion is placed in the same dialog, etc.
:(

If you know of a way to do resizing correctly in PS, please do let me
know!

Cheers,
Alen