Finally get SOR working.

[nageru] / sor.frag

diff --git a/sor.frag b/sor.frag
index 6c5333fa5ead7c960a96361ae16fd501638d7a6a..fbcc2e4ca8b8532b829a7c653b88757310448fba 100644 (file)
--- a/sor.frag
+++ b/sor.frag
@@ -1,10 +1,13 @@
 #version 450 core
 
 in vec2 tc;
+in float element_sum_idx;
 out vec2 diff_flow;
 
 uniform sampler2D diff_flow_tex, smoothness_x_tex, smoothness_y_tex;
 uniform usampler2D equation_tex;
+uniform vec2 image_size;
+uniform int phase;
 
 // See pack_floats_shared() in equations.frag.
 vec2 unpack_floats_shared(uint c)
@@ -25,6 +28,16 @@ vec2 unpack_floats_shared(uint c)
 
 void main()
 {
+       // Red-black SOR: Every other pass, we update every other element in a
+       // checkerboard pattern. This is rather suboptimal for the GPU, as it
+       // just immediately throws away half of the warp, but it helps convergence
+       // a _lot_ (rough testing indicates that five iterations of SOR is as good
+       // as ~50 iterations of Jacobi). We could probably do better by reorganizing
+       // the data into two-values-per-pixel, so-called “twinning buffering”,
+       // but it makes for rather annoying code in the rest of the pipeline.
+       int color = int(round(element_sum_idx)) & 1;
+       if (color != phase) discard;
+
        uvec4 equation = texture(equation_tex, tc);
        float inv_A11 = uintBitsToFloat(equation.x);
        float A12 = uintBitsToFloat(equation.y);
@@ -35,7 +48,6 @@ void main()
        // (it couldn't be done earlier, because we didn't know
        // the values of the neighboring pixels; they change for
        // each SOR iteration).
-       // TODO: Multiply by some gamma.
        float smooth_l = textureOffset(smoothness_x_tex, tc, ivec2(-1,  0)).x;
        float smooth_r = texture(smoothness_x_tex, tc).x;
        float smooth_d = textureOffset(smoothness_y_tex, tc, ivec2( 0, -1)).x;
@@ -45,10 +57,7 @@ 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;
 
-       // FIXME: omega=1.6 seems to make our entire system diverge.
-       // Is this because we do Gauss-Seidel instead of Jacobi?
-       // Figure out what's going on.
-       const float omega = 1.0;
+       const float omega = 1.8;  // Marginally better than 1.6, it seems.
        diff_flow = texture(diff_flow_tex, tc).xy;
 
        // From https://en.wikipedia.org/wiki/Successive_over-relaxation.