HDR Highlight Compression on the GPU: a NaN Amount keeps the layer, as on the CPU

Doc comments on the touched helpers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/effects/src/utility.rs

@@ -60,6 +60,7 @@ const HDR_LUMA: [f32; 3] = [0.30, 0.59, 0.11];
 /// Exponent of the domain the Amount blend happens in (out^0.4 is mixed linearly).
 const HDR_BLEND_GAMMA: f32 = 0.4;
 
+/// `v^e` with the sign of `v` kept (negative input stays negative, as on the GPU).
 #[inline]
 fn hdr_spow(v: f32, e: f32) -> f32 {
     v.signum() * v.abs().powf(e)
@@ -82,6 +83,8 @@ fn hdr_compress_pixel(c: [f32; 3], t: f32) -> [f32; 3] {
     })
 }
 
+/// HDR Highlight Compression: every straight-colour pixel through [`hdr_compress_pixel`] at
+/// Amount / 100; Amount 0 or NaN returns the layer unchanged.
 fn hdr_compression(ctx: &EffectCtx, mut b: Buf) -> Buf {
     let t = (ctx.params.f("amount") / 100.0).clamp(0.0, 1.0) as f32;
     if t <= 0.0 || !t.is_finite() {

crates/gpu/src/fx_pixel2.rs

@@ -215,6 +215,8 @@ fn composite_mode(m: u32) -> BlendMode {
     }
 }
 
+/// The per-pixel effects of `fp2_point` (Cineon, HDR Compander and Highlight Compression, CC
+/// Overbrights, CC Composite, Color Link): their parameters packed for the kernel, or `None`.
 fn point(e: &mut Enc, id: &str, ctx: &EffectCtx, b: GBuf) -> Option<GBuf> {
     let pr = ctx.params;
     let mut p = Params::default();
@@ -238,7 +240,8 @@ fn point(e: &mut Enc, id: &str, ctx: &EffectCtx, b: GBuf) -> Option<GBuf> {
         }
         "ec.utility.hdrcompression" => {
             let t = (pr.f("amount") / 100.0).clamp(0.0, 1.0) as f32;
-            if t <= 0.0 {
+            // A NaN Amount keeps the layer, as on the CPU.
+            if t <= 0.0 || !t.is_finite() {
                 return Some(b);
             }
             p.u[0][0] = 2;

crates/gpu/src/tests/fx_pixel2.rs

@@ -185,6 +185,8 @@ fn composites_and_colour() {
     effect_case("ec.utility.hdrcompression", &[]);
     effect_case("ec.utility.hdrcompression", &[("amount", n(40.0))]);
     effect_case("ec.utility.hdrcompression", &[("amount", n(90.0))]);
+    // A NaN Amount keeps the layer (the CPU returns it unchanged).
+    effect_case("ec.utility.hdrcompression", &[("amount", n(f64::NAN))]);
     effect_case("ec.utility.growbounds", &[("pixels", n(9.0))]);
     effect_case("ec.utility.ccoverbrights", &[]);
     for ch in 1..=5 {