FYI You can get gamma correct resizing (and other operations) in recent versions of photoshop by first switching the image to 32-bit colour depth mode.

I had a really long reply typed up with some specific workflow problems. But you can see people complain elsewhere in this thread about how Adobe handles stuff like this.

The basic problem is that the color space for blending is an application setting (can't be specified in the file) and higher bit depths will double or triple your file size.

You could do a rough approximation of gamma correction with squares and square roots, which wouldn't add to the processing time like a full sRGB conversion would.

Going from 8-bit sRGB channels to 32-bit IEEE float channels is a simple lookup in a table with 256 values.

Going the other way is only slightly harder. Scale to the range 0 ... 4095, properly round the float to an integer, verify the range or mask with 0xfff to protect against NaN and Inf, and then to a table lookup in a table with 4096 values.

Tux Paint contains GPL code for it. It's been there for more than a decade. All you GIMP and Photoshop users having trouble with this should have been using Tux Paint. :-)

Lookup tables are cache killers on modern CPUs. Arithmetic is often faster.

It's 5120 bytes for the both of them, the access is not at all random (due to color popularity), and the real cache killer is the image itself. You're dealing with a few megabytes for the image.

Oh, the lookup tables are cached just fine. It's just that they crowd out something else.

That "something else" is already crowded out by running through a multi-megabyte image.

At least the instruction cache is barely affected, which would not be the case for a math library function.

You're not running a multi MB image but a small block thereof. Not that 5-6KiB LUT matters, but the extra conditional multiply ops or power or logarithm function evaluations do. Latter are need a few MACs for a good approximation (at least 12 bit precise, preferably 16 bit - like 5th order polynomial), former have masks, worse for SIMD.

I had this idea a long time ago but never actually tested it. Now I have: http://marksblog.com/gamma-resize/. The square/sqrt method gets at least 80% of the quality but should be quick to compute on a modern processor.

sRGB gamma encode/decode is a natively provided hardware feature on GPUs, and is plenty fast. There’s really no excuse in 2019.

The irony is that all accelerated hardware resizers available on ARM SoCs that I know are also not gamma aware.

This is why downscaled videos feel to have more contrast, or solarised due to bruteforce compensation on most Android devices.

Those LUT or power or logarithm functions are not free... linear light scaling is about 2x slower in practice. Not that it matters until you get to 8K.

That's an interesting point. The GPU should do the resizing too if they're using it.

I first saw this post in 2009 or so, and it was eye-opening. I found it while googling for why colours shifted when I resized images, as back then there wasn't much awareness of this issue. But it's much less prevalent now. Related -- scaling algorithms: https://en.wikipedia.org/wiki/Image_scaling#Algorithms

You don't often find browsers disagreeing in 2019 on how to render things, but image colour -- even on the same MacOS (retina) platform -- appear to be one of those things they disagree on still: https://content.screencast.com/users/LouisStAmour/folders/Sn... Safari on the left, Firefox in the middle and Chrome on the right. So you can try it for yourself, the test page is at http://www.ericbrasseur.org/gamma_dalai_lama.html

If this isn't esoteric enough, I know of at least one major broadcast media company (Bell's crave.ca) that doesn't properly encode their HDTV 720p videos in MP4 to flag to the browser they're in Rec 709 colour space, and so the compression artifacts, designed to be invisible in Rec 709 are extremely visible on a wide gamut P3 display.

Of course, these flags were only recently respected on all platforms. Chrome on Windows as of Oct 2016: https://bugs.chromium.org/p/chromium/issues/detail?id=655417... and Firefox (err, actually still not fixed, it depends on the codec...) https://bugzilla.mozilla.org/show_bug.cgi?id=1300170#c87

But colour space is increasingly something you have to pay attention to, as displays improve beyond sRGB -- yet ironically, it's also getting easier to ignore as the latest stepping stone towards Rec 2020, the DCI-P3 colour space, is shared between PC and movie industries. https://en.wikipedia.org/wiki/DCI-P3#2015-2016 (Personally I refuse to buy any display or device that plays back video if it doesn't have a P3 display, ideally OLED)

FYI, you can test for colour gamut device support with CSS to serve wide-gamut colours -- but oddly enough, again, no Firefox support. :( https://developer.mozilla.org/en-US/docs/Web/CSS/@media/colo... FF bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1422237

> back then [in 2009] there wasn't much awareness of this issue

I haven’t noticed too much change in the general awareness of this problem between the 1990s and today.

Experts have always known the right thing to do and people have been talking about these problems as long as there has been image processing, (e.g. Poynton’s Gamma FAQ dates from sometime before 1998 http://poynton.ca/notes/colour_and_gamma/GammaFAQ.html) but it’s new to everyone at some point, and newcomers perennially screw up.

The annoying thing is that there are folks who continue to screw up (like Adobe Photoshop) who should really know better. Backwards compatibility shouldn’t be an excuse to continue with broken implementations for decades.

Oh, the broken implementations don't stop at backwards compatibility -- Lightroom CC, their new Cloud product, last I checked, prefers to send sRGB to Photoshop (!) unlike the traditional Lightroom which fully supports preserving embedded colour profiles when exporting. It's been a couple weeks since I looked into this but the instructions in this post https://lightroomkillertips.com/keeping-your-color-consisten... apply to Classic Lightroom if I recall correctly, and the modern "cloud" Lightroom CC only exports wide gamut correctly when set to export as uncompressed TIFF 16-bit. Everything else tries to export as sRGB automatically, which sucks for print or DCI-P3 workflows.

Also, I'm trying to find details on Apple's P3 profiles in how they differ from standard projection profiles for the movie industry to maintain compatibility with sRGB gamma, but there's not as much online as I expected about it.

In my search for info, I came across https://ninedegreesbelow.com/photography/lcms-make-icc-profi... which seems like a useful resource for Linux/FLOSS folks.

Ah. Found it, a source for the differences between "Display P3" and movie-theatre P3. Here's a link to the relevant section of the video, on colour spaces: https://developer.apple.com/videos/play/wwdc2016-501/?time=2...

And here's a good discussion on why D60 was chosen for film projection while D65 is used by modern computers for sRGB/Rec 709 compatibility: https://www.liftgammagain.com/forum/index.php?threads/dci-p3... and here's a bit on the technical details of ICC profiles and CIE XYZ numeric representations of colours: https://ninedegreesbelow.com/photography/xyz-rgb.html

https://apple.stackexchange.com/questions/252159/what-color-... describes the difference between P3 and what Apple uses.