Microoptimization in the SOR fragment shader.

diff --git a/sor.frag b/sor.frag
index 047526cb0899a32db07d9e8c143578b03a3e09e9..e1f86bbbfea8c3f6f019857369fe220eb69e5e4c 100644 (file)
--- a/sor.frag
+++ b/sor.frag
@@ -67,8 +67,12 @@ void main()
        float inv_A22 = uintBitsToFloat(equation.z);
        vec2 b = unpack_floats_shared(equation.w);
 
+       const float omega = 1.8;  // Marginally better than 1.6, it seems.
+
        if (zero_diff_flow) {
-               diff_flow = vec2(0.0f);
+               // Simplified version of the code below, assuming diff_flow == 0.0f everywhere.
+               diff_flow.x = omega * b.x * inv_A11;
+               diff_flow.y = omega * b.y * inv_A22;
        } else {
                // Subtract the missing terms from the right-hand side
                // (it couldn't be done earlier, because we didn't know
@@ -83,13 +87,11 @@ void main()
                b += smooth_d * textureOffset(diff_flow_tex, tc, ivec2( 0, -1)).xy;
                b += smooth_u * textureOffset(diff_flow_tex, tc, ivec2( 0,  1)).xy;
                diff_flow = texture(diff_flow_tex, tc).xy;
-       }
 
-       const float omega = 1.8;  // Marginally better than 1.6, it seems.
-
-       // From https://en.wikipedia.org/wiki/Successive_over-relaxation.
-       float sigma_u = A12 * diff_flow.y;
-       diff_flow.x += omega * ((b.x - sigma_u) * inv_A11 - diff_flow.x);
-       float sigma_v = A12 * diff_flow.x;
-       diff_flow.y += omega * ((b.y - sigma_v) * inv_A22 - diff_flow.y);
+               // From https://en.wikipedia.org/wiki/Successive_over-relaxation.
+               float sigma_u = A12 * diff_flow.y;
+               diff_flow.x += omega * ((b.x - sigma_u) * inv_A11 - diff_flow.x);
+               float sigma_v = A12 * diff_flow.x;
+               diff_flow.y += omega * ((b.y - sigma_v) * inv_A22 - diff_flow.y);
+       }
 }