From 67f1ea1ae3cb4ace764c0c2c25ef766044ff2a2d Mon Sep 17 00:00:00 2001
From: Marco Arnulfo <marcoarnulfo1@gmail.com>
Date: Mon, 12 Oct 2026 18:28:53 +0200
Subject: [PATCH] render: Sharpen stays visible at reduced playback resolution
 (fixes #1351)

---
 crates/render/src/gpufx.rs                 |  4 +-
 crates/render/tests/sharpen_preview_res.rs | 65 ++++++++++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 crates/render/tests/sharpen_preview_res.rs

diff --git a/crates/render/src/gpufx.rs b/crates/render/src/gpufx.rs
index 6876a6d9..101575e7 100644
--- a/crates/render/src/gpufx.rs
+++ b/crates/render/src/gpufx.rs
@@ -355,7 +355,9 @@ impl FxOp {
                 }
             }
             "sharpen" => {
-                let r = cx.px_scale.max(0.35);
+                // below a sigma of ~0.58 the three boxes all round to radius 0 and the mask is a
+                // no-op, so Sharpen vanished at 1/2 playback resolution and below (#1351)
+                let r = cx.px_scale.max(0.6);
                 let (rx, ry) = gaussian_boxes(w, h, r, r);
                 FxOp::Unsharp { rx, ry, amount: f(e, "amount", cx) / 100.0, threshold: 0.0 }
             }
diff --git a/crates/render/tests/sharpen_preview_res.rs b/crates/render/tests/sharpen_preview_res.rs
new file mode 100644
index 00000000..d9d0c47a
--- /dev/null
+++ b/crates/render/tests/sharpen_preview_res.rs
@@ -0,0 +1,65 @@
+//! Sharpen must stay visible at reduced playback resolution (#1351).
+//!
+//! Playback at 1/2 renders with `px_scale` 0.5. Sharpen's blur radius scaled with it to a sigma the
+//! three-box Gaussian rounds to zero-width boxes, so the unsharp mask subtracted the image from
+//! itself and the effect vanished while playing, though it showed on the paused (Full) frame.
+
+use filmcraft_project::{ParamValue, find_effect};
+use filmcraft_render::effects::{FxCtx, apply};
+use filmcraft_render::image::Image;
+use filmcraft_time::Tick;
+
+fn cx(px_scale: f32) -> FxCtx<'static> {
+    FxCtx {
+        t: Tick::ZERO,
+        px_scale,
+        seconds: 0.0,
+        timecode: "00:00:01:00",
+        clip_name: "x",
+        project: None,
+        env: None,
+        working: filmcraft_color::WorkingSpace::Rec709,
+    }
+}
+
+/// Dark left half, light right half: the edge sits between x = 7 and x = 8.
+fn edge() -> Image {
+    let (w, h) = (16usize, 8usize);
+    let mut img = Image::filled(w, h, [0.25, 0.25, 0.25, 1.0]);
+    for y in 0..h {
+        for x in w / 2..w {
+            let i = (y * w + x) * 4;
+            img.px[i..i + 3].copy_from_slice(&[0.75, 0.75, 0.75]);
+        }
+    }
+    img
+}
+
+fn sharpened(px_scale: f32) -> Image {
+    let mut img = edge();
+    let mut e = find_effect("sharpen").expect("sharpen effect").instance();
+    e.params.get_mut("amount").expect("amount").value = ParamValue::Float(100.0);
+    apply(&mut img, &e, &cx(px_scale));
+    img
+}
+
+#[test]
+fn sharpen_shows_at_half_and_quarter_playback_resolution() {
+    let plain = edge();
+    for px_scale in [0.5, 0.25] {
+        let img = sharpened(px_scale);
+        let (dark, light) = (img.get(7, 4)[0], img.get(8, 4)[0]);
+        assert!(dark < plain.get(7, 4)[0] - 0.01, "px_scale {px_scale}: the dark side of the edge should darken (got {dark})");
+        assert!(light > plain.get(8, 4)[0] + 0.01, "px_scale {px_scale}: the light side of the edge should brighten (got {light})");
+    }
+}
+
+#[test]
+fn sharpen_at_full_resolution_is_unchanged() {
+    // values rendered before the fix: the Full (paused) frame must not change
+    let img = sharpened(1.0);
+    for (x, want) in [(6, 0.25), (7, 0.25 - 1.0 / 6.0), (8, 0.75 + 1.0 / 6.0), (9, 0.75)] {
+        let got = img.get(x, 4)[0];
+        assert!((got - want).abs() < 1e-5, "x {x}: got {got}, want {want}");
+    }
+}