[frameworks/kdeclarative] src/qmlcontrols/graphicaleffects: graphicaleffects: Avoid complicated matrix multiply

Git commit e8c0120d4c8b581eca6cdfb8bf73bec04cc803ae by Kai Uwe Broulik.
Committed on 28/07/2026 at 05:54.
Pushed by broulik into branch 'master'.

graphicaleffects: Avoid complicated matrix multiply

It doesn't seem work in all profiles/configurations.
Implement it manually instead.

M  +15   -5    src/qmlcontrols/graphicaleffects/lanczos2sharp.frag

https://invent.kde.org/frameworks/kdeclarative/-/commit/e8c0120d4c8b581eca6cdfb8bf73bec04cc803ae

diff --git a/src/qmlcontrols/graphicaleffects/lanczos2sharp.frag b/src/qmlcontrols/graphicaleffects/lanczos2sharp.frag
index b6477e24..0c5771e3 100644
--- a/src/qmlcontrols/graphicaleffects/lanczos2sharp.frag
+++ b/src/qmlcontrols/graphicaleffects/lanczos2sharp.frag
@@ -56,7 +56,16 @@ const vec3 dtt = vec3(65536.0, 255.0, 1.0);
 
 vec4 reduce4(vec3 A, vec3 B, vec3 C, vec3 D)
 {
-    return dtt * mat4x3(A, B, C, D);
+    return vec4(dot(dtt, A), dot(dtt, B), dot(dtt, C), dot(dtt, D));
+}
+
+vec3 weightedSum4(vec4 w, vec3 a, vec3 b, vec3 c, vec3 d)
+{
+    return vec3(
+        dot(w, vec4(a.x, b.x, c.x, d.x)),
+        dot(w, vec4(a.y, b.y, c.y, d.y)),
+        dot(w, vec4(a.z, b.z, c.z, d.z))
+    );
 }
 
 vec3 min4(vec3 a, vec3 b, vec3 c, vec3 d)
@@ -149,10 +158,11 @@ void main()
     vec3 min_sample = min4(c11, c21, c12, c22);
     vec3 max_sample = max4(c11, c21, c12, c22);
 
-    color = weights[0] * transpose(mat4x3(c00, c10, c20, c30));
-    color += weights[1] * transpose(mat4x3(c01, c11, c21, c31));
-    color += weights[2] * transpose(mat4x3(c02, c12, c22, c32));
-    color += weights[3] * transpose(mat4x3(c03, c13, c23, c33));
+    color  = weightedSum4(weights[0], c00, c10, c20, c30);
+    color += weightedSum4(weights[1], c01, c11, c21, c31);
+    color += weightedSum4(weights[2], c02, c12, c22, c32);
+    color += weightedSum4(weights[3], c03, c13, c23, c33);
+
     color = color / dot(vec4(1.0) * weights, vec4(1.0));
 
     // Anti-ringing