Add absolute conversion matrices for ACEScg

CHANGELOG.md

@@ -17,7 +17,7 @@ This release has an [MSRV][] of 1.82.
 
 ### Added
 
-* Support converting between color spaces without chromatic adaptation, thereby representing the same absolute color in the destination color space as in the source color space. ([#139][], [#153][], [#156][] by [@tomcur][])
+* Support converting between color spaces without chromatic adaptation, thereby representing the same absolute color in the destination color space as in the source color space. ([#139][], [#153][], [#156][], [#164][], [#165][] by [@tomcur][])
 
   **Note to `ColorSpace` implementers:** the `WHITE_POINT` associated constant is added to `ColorSpace`, defaulting to D65.
   Implementations with a non-D65 white point should set this constant to get correct default absolute conversion behavior.
@@ -161,6 +161,8 @@ This is the initial release.
 [#157]: https://github.com/linebender/color/pull/157
 [#158]: https://github.com/linebender/color/pull/158
 [#159]: https://github.com/linebender/color/pull/159
+[#164]: https://github.com/linebender/color/pull/164
+[#165]: https://github.com/linebender/color/pull/165
 
 [Unreleased]: https://github.com/linebender/color/compare/v0.2.3...HEAD
 [0.2.3]: https://github.com/linebender/color/releases/tag/v0.2.3

color/src/colorspace.rs

@@ -956,6 +956,50 @@ impl ColorSpace for AcesCg {
         matvecmul(&LINEAR_SRGB_TO_ACESCG, src)
     }
 
+    fn to_linear_srgb_absolute(src: [f32; 3]) -> [f32; 3] {
+        // XYZ_to_lin_sRGB * ACEScg_to_XYZ
+        const ACESCG_TO_LINEAR_SRGB: [[f32; 3]; 3] = [
+            [
+                9_932_023_100_445. / 5_736_895_993_442.,
+                -1_732_666_183_650. / 2_868_447_996_721.,
+                -229_784_797_280. / 2_868_447_996_721.,
+            ],
+            [
+                -194_897_543_280. / 1_480_771_385_773.,
+                72_258_955_647_750. / 63_673_169_588_239.,
+                -552_646_980_800. / 63_673_169_588_239.,
+            ],
+            [
+                -68_657_089_110. / 2_794_545_067_783.,
+                -8082548957250. / 64_274_536_559_009.,
+                14_669_805_440. / 13_766_231_861.,
+            ],
+        ];
+        matvecmul(&ACESCG_TO_LINEAR_SRGB, src)
+    }
+
+    fn from_linear_srgb_absolute(src: [f32; 3]) -> [f32; 3] {
+        // XYZ_to_ACEScg * lin_sRGB_to_XYZ
+        const LINEAR_SRGB_TO_ACESCG: [[f32; 3]; 3] = [
+            [
+                2_095_356_009_722. / 3_474_270_183_447.,
+                17_006_614_853_437. / 52_114_052_751_705.,
+                71_464_174_897. / 1_488_972_935_763.,
+            ],
+            [
+                1_774_515_482_522. / 25_307_573_950_575.,
+                69_842_555_782_672. / 75_922_721_851_725.,
+                276_870_186_577. / 21_692_206_243_350.,
+            ],
+            [
+                101_198_449_621. / 4_562_827_993_584.,
+                31_778_718_978_443. / 273_769_679_615_040.,
+                1_600_138_878_851. / 1_700_432_792_640.,
+            ],
+        ];
+        matvecmul(&LINEAR_SRGB_TO_ACESCG, src)
+    }
+
     fn clip([r, g, b]: [f32; 3]) -> [f32; 3] {
         [
             r.clamp(-65504., 65504.),