diff --git a/crates/effects/src/time_fx.rs b/crates/effects/src/time_fx.rs
index cf7624d92..7fadc4f37 100644
--- a/crates/effects/src/time_fx.rs
+++ b/crates/effects/src/time_fx.rs
@@ -552,7 +552,8 @@ impl PixelMotion {
         let px = |id: &str| (g(id, 0.0).max(0.0) * scale).round() as u32;
         PixelMotion {
             tile,
-            window: ((g("tuning/blockSize", 8.0).max(4.0)) * scale).round().max(4.0) as u32,
+            // Block Size is declared 4..64; a project can store any number.
+            window: ((g("tuning/blockSize", 8.0).clamp(4.0, 64.0)) * scale).round().max(4.0) as u32,
             global_smoothness: g("tuning/smoothing/globalSmoothness", 0.5).clamp(0.0, 1.0) as f32,
             local_smoothness: g("tuning/smoothing/localSmoothness", 0.5).clamp(0.0, 1.0) as f32,
             iterations: g("tuning/smoothing/smoothingIterations", 20.0).clamp(0.0, 200.0) as u32,
diff --git a/crates/raster/src/flow.rs b/crates/raster/src/flow.rs
index feb033d65..85aada082 100644
--- a/crates/raster/src/flow.rs
+++ b/crates/raster/src/flow.rs
@@ -103,7 +103,8 @@ pub fn block_flow(a: &Image, b: &Image, block: u32, radius: i32) -> Flow {
 /// (Timewarp's Vector Detail and Block Size).
 pub fn block_flow_window(a: &Image, b: &Image, tile: u32, window: u32, radius: i32) -> Flow {
     let bs = tile.max(2) as usize;
-    let win = window.max(4) as i64;
+    // The matching cost per vector grows with the window's area; 512 is far above any block size the effects offer.
+    let win = i64::from(window.clamp(4, 512));
     // Window origin relative to the tile origin.
     let wo = bs as i64 / 2 - win / 2;
     let (w, h) = (a.width as usize, a.height as usize);
@@ -280,6 +281,13 @@ mod tests {
         s / n as f32
     }
 
+    #[test]
+    fn a_huge_matching_window_is_held_to_the_cap() {
+        let (a, b) = (texture(520, 520, 0.0), texture(520, 520, 2.0));
+        let capped = block_flow_window(&a, &b, 260, 512, 2);
+        assert_eq!(block_flow_window(&a, &b, 260, u32::MAX, 2).v, capped.v);
+    }
+
     #[test]
     fn flow_finds_translation() {
         let a = texture(128, 96, 0.0);