Fix a bug where the first black pass of SOR would read junk data.

diff --git a/flow.cpp b/flow.cpp
index 76589429aed187faefeec955ac20d2943a5bcf83..2ed5bb855792aad0d97dbbc61fc2134c59ba27a5 100644 (file)
--- a/flow.cpp
+++ b/flow.cpp
@@ -764,7 +764,7 @@ private:
        GLuint uniform_diff_flow_tex;
        GLuint uniform_equation_red_tex, uniform_equation_black_tex;
        GLuint uniform_diffusivity_tex;
-       GLuint uniform_phase, uniform_zero_diff_flow;
+       GLuint uniform_phase, uniform_num_nonzero_phases;
 };
 
 SOR::SOR()
@@ -778,7 +778,7 @@ SOR::SOR()
        uniform_equation_black_tex = glGetUniformLocation(sor_program, "equation_black_tex");
        uniform_diffusivity_tex = glGetUniformLocation(sor_program, "diffusivity_tex");
        uniform_phase = glGetUniformLocation(sor_program, "phase");
-       uniform_zero_diff_flow = glGetUniformLocation(sor_program, "zero_diff_flow");
+       uniform_num_nonzero_phases = glGetUniformLocation(sor_program, "num_nonzero_phases");
 }
 
 void SOR::exec(GLuint diff_flow_tex, GLuint equation_red_tex, GLuint equation_black_tex, GLuint diffusivity_tex, int level_width, int level_height, int num_iterations, bool zero_diff_flow, ScopedTimer *sor_timer)
@@ -790,7 +790,9 @@ void SOR::exec(GLuint diff_flow_tex, GLuint equation_red_tex, GLuint equation_bl
        bind_sampler(sor_program, uniform_equation_red_tex, 2, equation_red_tex, nearest_sampler);
        bind_sampler(sor_program, uniform_equation_black_tex, 3, equation_black_tex, nearest_sampler);
 
-       glProgramUniform1i(sor_program, uniform_zero_diff_flow, zero_diff_flow);
+       if (!zero_diff_flow) {
+               glProgramUniform1i(sor_program, uniform_num_nonzero_phases, 2);
+       }
 
        // NOTE: We bind to the texture we are rendering from, but we never write any value
        // that we read in the same shader pass (we call discard for red values when we compute
@@ -803,6 +805,9 @@ void SOR::exec(GLuint diff_flow_tex, GLuint equation_red_tex, GLuint equation_bl
        for (int i = 0; i < num_iterations; ++i) {
                {
                        ScopedTimer timer("Red pass", sor_timer);
+                       if (zero_diff_flow && i == 0) {
+                               glProgramUniform1i(sor_program, uniform_num_nonzero_phases, 0);
+                       }
                        glProgramUniform1i(sor_program, uniform_phase, 0);
                        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
                        glTextureBarrier();
@@ -810,11 +815,13 @@ void SOR::exec(GLuint diff_flow_tex, GLuint equation_red_tex, GLuint equation_bl
                {
                        ScopedTimer timer("Black pass", sor_timer);
                        if (zero_diff_flow && i == 0) {
-                               // Not zero anymore.
-                               glProgramUniform1i(sor_program, uniform_zero_diff_flow, 0);
+                               glProgramUniform1i(sor_program, uniform_num_nonzero_phases, 1);
                        }
                        glProgramUniform1i(sor_program, uniform_phase, 1);
                        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+                       if (zero_diff_flow && i == 0) {
+                               glProgramUniform1i(sor_program, uniform_num_nonzero_phases, 2);
+                       }
                        if (i != num_iterations - 1) {
                                glTextureBarrier();
                        }

diff --git a/sor.frag b/sor.frag
index ef431d3346d96be4e0270ddd6ad4c9ebd1664a63..b072839a2b3b86bc510f260bbb30ae3a93c5bb60 100644 (file)
--- a/sor.frag
+++ b/sor.frag
@@ -9,7 +9,7 @@ uniform sampler2D diff_flow_tex, diffusivity_tex;
 uniform usampler2D equation_red_tex, equation_black_tex;
 uniform int phase;
 
-uniform bool zero_diff_flow;
+uniform int num_nonzero_phases;
 
 // See pack_floats_shared() in equations.frag.
 vec2 unpack_floats_shared(uint c)
@@ -70,7 +70,7 @@ void main()
 
        const float omega = 1.8;  // Marginally better than 1.6, it seems.
 
-       if (zero_diff_flow) {
+       if (num_nonzero_phases == 0) {
                // 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;
@@ -87,7 +87,12 @@ void main()
                b += smooth_r * textureOffset(diff_flow_tex, tc, ivec2( 1,  0)).xy;
                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;
+
+               if (num_nonzero_phases == 1) {
+                       diff_flow = vec2(0.0f);
+               } else {
+                       diff_flow = texture(diff_flow_tex, tc).xy;
+               }
 
                // From https://en.wikipedia.org/wiki/Successive_over-relaxation.
                float sigma_u = A12 * diff_flow.y;