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cmm: Make log_100 and log_316 TFs symmetric #826

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cmm: Make log_100 and log_316 TFs symmetric #826

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12 changes: 10 additions & 2 deletions src/gfx_apis/vulkan/shaders/eotfs.glsl
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Original file line number Diff line number Diff line change
Expand Up @@ -67,7 +67,11 @@ vec3 inv_eotf_st240(vec3 c) {
}

vec3 eotf_log100(vec3 c) {
return pow(vec3(10), vec3(2.0) * (c - vec3(1.0)));
return mix(
vec3(0.0),
pow(vec3(10), vec3(2.0) * (c - vec3(1.0))),
greaterThan(c, vec3(0.0))
);
Comment on lines -70 to +74
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@mahkoh mahkoh Mar 22, 2026

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I feel like this should just clamp(c, 0.0, 1.0) like the other SDR transfer functions.

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@kennylevinsen kennylevinsen Mar 22, 2026

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The problem is that when c is zero, pow(vec3(10), vec3(2.0) * (c - vec3(1.0))) yields 0.01, so a clamp wouldn't help.

The approach is similar to compound_power_2_4. and matches what is done in their respective inverse EOTF's (a mix over greaterThanEqual(c, vec3(0.01)) and greaterThanEqual(c, vec3(sqrt(10)/1000)), respectively).

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@mahkoh mahkoh Mar 22, 2026

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The problem I see with your current code is that it creates a discontinuity at 0. I.e. E=0 maps to O=0 but E=epsilon maps to O=0.01. For a single black tile that would be fine, but I'm not sure if it would be fine for a gradient or any image that just so happens to have a black pixel.

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@kennylevinsen kennylevinsen Mar 22, 2026 •
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The step in log_316 is small enough that it is unlikely to matter, but log100 clearly shows the discontinuity in the gradient (images below).

Searching around a bit I don't see a whole lot of implementations to reference, but the fix matches gstreamer and the color-science python package. moxcms does neither and instead uses the midpoint to minimize the maximum error. Not a whole lot of support out there for these TFs though, so don't really have much in way of a benchmark or convention...

Plain gamma22:
image

Before this fix:
image

After this fix (note the left side of the gradient bar):
image

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@mahkoh mahkoh Mar 22, 2026

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If we want to use 0 as the fixed point, which is what all other EOTFs do right now, then maybe applying an affine function could work:

vec3 eotf_log100(vec3 c) {
    c = clamp(c, 0.0, 1.0);
    c = pow(vec3(10), vec3(2.0) * (c - vec3(1.0)));
    return (c - vec3(0.01)) / vec3(1.0 - 0.01);
}

vec3 inv_eotf_log100(vec3 c) {
    c = clamp(c, 0.0, 1.0);
    c = c * vec3(1.0 - 0.01) + vec3(0.01);
    return vec3(1.0) + log2(c) / vec3(log2(10)) / vec3(2.0);
}

However, I'd prefer to standardize these things in https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/main/staging/color-management/appendix.md.

}

vec3 inv_eotf_log100(vec3 c) {
Expand All @@ -80,7 +84,11 @@ vec3 inv_eotf_log100(vec3 c) {
}

vec3 eotf_log316(vec3 c) {
return pow(vec3(10), vec3(2.5) * (c - vec3(1.0)));
return mix(
vec3(0.0),
pow(vec3(10), vec3(2.5) * (c - vec3(1.0))),
greaterThan(c, vec3(0.0))
);
}

vec3 inv_eotf_log316(vec3 c) {
Expand Down
Binary file modified src/gfx_apis/vulkan/shaders_bin/out.frag.spv
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Binary file modified src/gfx_apis/vulkan/shaders_bin/tex.frag.spv
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2 changes: 1 addition & 1 deletion src/gfx_apis/vulkan/shaders_hash.txt
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
302a9f250bdc4f8e0e71a9f77c9a8a7aa55fd003bc91c2422a700c4abd83f54e src/gfx_apis/vulkan/shaders/alpha_modes.glsl
b6a0df1e231fab533499329636b7a580384784418baee06c147af5fcc384cf5c src/gfx_apis/vulkan/shaders/eotfs.glsl
87979e5a9cde1db1af97549294879362fbbcab72ad89aba64b5576a26b597d66 src/gfx_apis/vulkan/shaders/eotfs.glsl
8a38df18851cd13884499820f26939fb7319f45d913d867f254d8118d59fb117 src/gfx_apis/vulkan/shaders/fill.common.glsl
21c488d12aa5ad2f109ec44cb856dfe837e02ea9025b5ed64439d742c17cbf30 src/gfx_apis/vulkan/shaders/fill.frag
4fb481d8d73afdfb0d8f077eb8665d86f06c8a32a91e44ed369ef5dff554646d src/gfx_apis/vulkan/shaders/fill.vert
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12 changes: 10 additions & 2 deletions src/theme.rs
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Expand Up @@ -118,10 +118,18 @@ impl Color {
}
}
fn log100(c: f32) -> f32 {
10.0.powf(2.0 * (c - 1.0))
if c <= 0.0 {
0.0
} else {
10.0.powf(2.0 * (c - 1.0))
}
}
fn log316(c: f32) -> f32 {
10.0.powf(2.5 * (c - 1.0))
if c <= 0.0 {
0.0
} else {
10.0.powf(2.5 * (c - 1.0))
}
}
fn st428(c: f32) -> f32 {
c.powf(2.6) * 52.37 / 48.0
Expand Down
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