From 5e9a1b6ec7a95cd8e04d64849420d4c3d73a86d3 Mon Sep 17 00:00:00 2001
From: wstock <1394687+wstock@users.noreply.github.com>
Date: Tue, 13 Oct 2026 07:24:41 +0000
Subject: [PATCH] Visualize: the all-masks overlay shows the union of the
 masks, not the last one
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`Overlay::AllMasks` painted each mask's plane in turn, and every mask view
rewrites the whole image from its alpha (White on Black writes `255·a`,
Image on Black multiplies by `a`…), so each plane wiped the ones before it
and only the last visible mask showed. The planes are now merged (per-pixel
max) and painted once; the GPU path goes through the same `apply`.

The test added with #919 could not pass: besides the above, its hidden-mask
check expected the plain pixel under White on Black, where everything
outside the masks is black by definition (as the single-mask test asserts).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NH3itd85jPuM6wNjgcGfEH
---
 crates/pipeline/src/visualize.rs | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/crates/pipeline/src/visualize.rs b/crates/pipeline/src/visualize.rs
index 15c164bb1..8f3cf1884 100644
--- a/crates/pipeline/src/visualize.rs
+++ b/crates/pipeline/src/visualize.rs
@@ -199,13 +199,18 @@ pub fn apply(img: &mut Rgba8, o: Overlay, plan: &Plan<'_>, mask: Option<&Plane>,
             }
         }
         Overlay::AllMasks { view, color, opacity } => {
-            if let Some(planes) = masks {
-                for a in planes {
-                    if (a.width, a.height) == (img.width, img.height) {
-                        mask_view(img, a, view, color, opacity);
-                    }
+            // one view of the union of the masks: painting the planes one after another
+            // overwrites (White on Black, Image on Black…) or compounds (Color) the earlier ones
+            let mut union: Option<Plane> = None;
+            for a in masks.unwrap_or_default().iter().filter(|a| (a.width, a.height) == (img.width, img.height)) {
+                match &mut union {
+                    None => union = Some(a.clone()),
+                    Some(u) => u.data.iter_mut().zip(&a.data).for_each(|(u, a)| *u = u.max(*a)),
                 }
             }
+            if let Some(u) = &union {
+                mask_view(img, u, view, color, opacity);
+            }
         }
     }
 }
@@ -425,7 +430,6 @@ mod tests {
             components: vec![MaskComponent { name: None, op: MaskOp::Add, invert: false, shape: right }],
             ..Default::default()
         });
-        let plain = render(&src, &info, &s, &RenderRequest::fit(80, 40)).image;
         let o = Overlay::AllMasks { view: MaskView::WhiteOnBlack, color: [255, 0, 0], opacity: 100 };
         let img = render(&src, &info, &s, &RenderRequest { overlay: o, ..RenderRequest::fit(80, 40) }).image;
         let (in_l, in_r, mid) = (20 * 80 + 12, 20 * 80 + 60, 20 * 80 + 40);
@@ -435,7 +439,7 @@ mod tests {
         // a hidden mask is skipped
         s.masks[1].visible = false;
         let img2 = render(&src, &info, &s, &RenderRequest { overlay: o, ..RenderRequest::fit(80, 40) }).image;
-        assert_eq!(img2.data[in_r], plain.data[in_r], "hidden right mask not painted");
+        assert_eq!(img2.data[in_r], [0, 0, 0, 255], "hidden right mask not painted: black like everything outside");
         assert_eq!(img2.data[in_l], [255; 4], "visible left mask still painted");
         // parts round trip and a stable, distinct cache key
         let (k, v) = o.to_parts();