diff --git a/crates/effects/src/lib.rs b/crates/effects/src/lib.rs
index 4c1ea60b..5a083392 100644
--- a/crates/effects/src/lib.rs
+++ b/crates/effects/src/lib.rs
@@ -110,7 +110,7 @@ pub use time_fx::{PixelMotion, TimewarpPlan, TwStep, pmb_plan, posterized_time,
 // effectcraft-gpu fx_tone.
 pub use ocio::color_stabilizer_maps;
 // GPU effects, part B (3D Channel, Immersive Video, colour management, simulation render passes).
-pub use channel3d::{CRYPTO_LAYERS, crypto_aux_image, crypto_aux_image_with, id_color, selection_hashes};
+pub use channel3d::{CRYPTO_LAYERS, CryptoAux, crypto_aux_image, crypto_aux_image_with, id_color, selection_hashes};
 pub use ocio::{ColorOp, Straight, Tf, color_program};
 pub use sim::{Acc, Blob, Bubble, Post, Shape, Sprite, SpritePlan, Tint, bubble_list, drizzle_drops, mercury_blobs, sprite_plan};
 pub use sim2::{
diff --git a/crates/gpu/src/fx_depth.rs b/crates/gpu/src/fx_depth.rs
index a74ff717..1bb0e49e 100644
--- a/crates/gpu/src/fx_depth.rs
+++ b/crates/gpu/src/fx_depth.rs
@@ -280,8 +280,9 @@ fn cryptomatte(e: &mut Enc, ctx: &EffectCtx, b: GBuf, a: &AuxChannels) -> Option
     // cores with the rank planes resolved once.
     // The ID colours are only read by the Colors display; Matte, Preview and Matte Only need the
     // matte alone.
-    let colors = pr.e("display") == 0 && !pr.b("matteOnly");
-    let img = effectcraft_effects::crypto_aux_image_with(a, &layer, &sel, colors);
+    // `tests_fx_depth::cryptomatte_every_display_matches_the_cpu` guards this against the shader.
+    let what = effectcraft_effects::CryptoAux::for_display(pr.e("display"), pr.b("matteOnly"));
+    let img = effectcraft_effects::crypto_aux_image_with(a, &layer, &sel, what);
     let tex = e.g.upload_image(&img)?;
     let plane = lookup(e, &b, a, &tex, [0.5, 0.5], [0.0; 4]);
     let mut p = op(7);
diff --git a/crates/gpu/src/tests_fx_depth.rs b/crates/gpu/src/tests_fx_depth.rs
index b84b4849..8a666565 100644
--- a/crates/gpu/src/tests_fx_depth.rs
+++ b/crates/gpu/src/tests_fx_depth.rs
@@ -251,6 +251,18 @@ fn ids_and_cryptomatte() {
     case("ec.3d.cryptomatte", &[("selection", Value::Str("alpha".into())), ("matteOnly", on())]);
 }
 
+/// The GPU skips the ID colours for displays that do not read them
+/// ([`effectcraft_effects::CryptoAux::for_display`]): every display × Matte Only must still match
+/// the CPU effect, so a shader that starts reading RGB in another mode fails here.
+#[test]
+fn cryptomatte_every_display_matches_the_cpu() {
+    for display in 0..3 {
+        for matte_only in [false, true] {
+            case("ec.3d.cryptomatte", &[("selection", Value::Str("beta, delta".into())), ("display", e(display)), ("matteOnly", Value::Bool(matte_only))]);
+        }
+    }
+}
+
 #[test]
 fn extractor() {
     case("ec.3d.extractor", &[("red", Value::Str("Z".into())), ("green", Value::Str("UV.U".into())), ("whitePoint", n(1000.0))]);