Fix feDisplacementMap applying scale twice

CHANGELOG.md

@@ -22,6 +22,8 @@ This changelog also contains important changes in dependencies.
 - Incorrect transform order on the outermost `svg` element with a `viewBox`. (#899)
 - Relative positioning of right-to-left text with `dx` and `dy`, including text on a path and text decorations. (#1093)
 - `text-anchor` now uses the actual extent of a text chunk, which `dx` and `textLength` can change. (#1145)
+- `feDisplacementMap` applied `scale` twice, displacing almost everything out of the filter region,
+  and read the displacement map with premultiplied colors. (#1019)
 
 ## [0.48.1] 2026-08-02
 

crates/resvg/src/filter/displacement_map.rs

@@ -9,13 +9,14 @@ use usvg::filter::{ColorChannel, DisplacementMap};
 /// - `map` pixels should have a **unpremultiplied alpha**.
 /// - `src` pixels can have any alpha method.
 ///
-/// `sx` and `sy` indicate canvas scale.
+/// `scale_x` and `scale_y` are the `scale` attribute already converted
+/// into the canvas coordinate system, i.e. the maximum displacement in pixels.
 ///
 /// `src`, `map` and `dest` dimensions must match.
 pub fn apply(
     fe: &DisplacementMap,
-    sx: f32,
-    sy: f32,
+    scale_x: f32,
+    scale_y: f32,
     src: ImageRef,
     map: ImageRef,
     dest: ImageRefMut,
@@ -42,8 +43,8 @@ pub fn apply(
 
         let dx = calc_offset(fe.x_channel_selector());
         let dy = calc_offset(fe.y_channel_selector());
-        let ox = (x as f32 + dx * sx * fe.scale()).round() as i32;
-        let oy = (y as f32 + dy * sy * fe.scale()).round() as i32;
+        let ox = (x as f32 + dx * scale_x).round() as i32;
+        let oy = (y as f32 + dy * scale_y).round() as i32;
 
         // TODO: we should use some kind of anti-aliasing when offset is on a pixel border
 

crates/resvg/src/filter/mod.rs

@@ -976,10 +976,14 @@ fn apply_displacement_map(
     input2: Image,
 ) -> Result<Image, Error> {
     let pixmap1 = input1.into_color_space(cs)?.take()?;
-    let pixmap2 = input2.into_color_space(cs)?.take()?;
+    let mut pixmap2 = input2.into_color_space(cs)?.take()?;
+
+    // The displacement map must be read using non-premultiplied color values.
+    demultiply_alpha(pixmap2.data_mut().as_rgba_mut());
 
     let mut pixmap = tiny_skia::Pixmap::try_create(region.width(), region.height())?;
 
+    // `scale` is in user units, convert it into pixels.
     let (sx, sy) = match scale_coordinates(fe.scale(), fe.scale(), ts) {
         Some(v) => v,
         None => return Ok(Image::from_image(pixmap1, cs)),

crates/resvg/tests/integration/render.rs

@@ -145,7 +145,12 @@ use crate::render;
 #[test] fn filters_feDiffuseLighting_surfaceScale_eq_1_33() { assert_eq!(render("tests/filters/feDiffuseLighting/surfaceScale=1.33"), 0); }
 #[test] fn filters_feDiffuseLighting_surfaceScale_eq_5() { assert_eq!(render("tests/filters/feDiffuseLighting/surfaceScale=5"), 0); }
 #[test] fn filters_feDiffuseLighting_tiny_element_with_spot_light() { assert_eq!(render("tests/filters/feDiffuseLighting/tiny-element-with-spot-light"), 0); }
+#[test] fn filters_feDisplacementMap_primitiveUnits_eq_objectBoundingBox() { assert_eq!(render("tests/filters/feDisplacementMap/primitiveUnits=objectBoundingBox"), 0); }
+#[test] fn filters_feDisplacementMap_scale_eq_0() { assert_eq!(render("tests/filters/feDisplacementMap/scale=0"), 0); }
+#[test] fn filters_feDisplacementMap_semi_transparent_map() { assert_eq!(render("tests/filters/feDisplacementMap/semi-transparent-map"), 0); }
 #[test] fn filters_feDisplacementMap_simple_case() { assert_eq!(render("tests/filters/feDisplacementMap/simple-case"), 0); }
+#[test] fn filters_feDisplacementMap_turbulence_map() { assert_eq!(render("tests/filters/feDisplacementMap/turbulence-map"), 0); }
+#[test] fn filters_feDisplacementMap_uniform_map() { assert_eq!(render("tests/filters/feDisplacementMap/uniform-map"), 0); }
 #[test] fn filters_feDistantLight_default_attributes() { assert_eq!(render("tests/filters/feDistantLight/default-attributes"), 0); }
 #[test] fn filters_feDistantLight_negative_azimuth_and_elevation() { assert_eq!(render("tests/filters/feDistantLight/negative-azimuth-and-elevation"), 0); }
 #[test] fn filters_feDistantLight_only_azimuth() { assert_eq!(render("tests/filters/feDistantLight/only-azimuth"), 0); }

crates/usvg/src/parser/filter.rs

@@ -721,8 +721,8 @@ fn convert_displacement_map(fe: SvgNode, scale: Size, primitives: &[Primitive])
         _ => ColorChannel::A,
     };
 
-    // TODO: should probably split scale to scale_x and scale_y,
-    //       but resvg doesn't support displacement map anyway...
+    // TODO: should probably split scale to scale_x and scale_y
+    //       to handle `primitiveUnits=objectBoundingBox` with a non-square bbox.
     let scale = (scale.width() + scale.height()) / 2.0;
 
     Kind::DisplacementMap(DisplacementMap {