[Bf-blender-cvs] [5711025] master: Cycles: Use a bit better approach for erfinv()
Sergey Sharybin
noreply at git.blender.org
Fri Oct 10 13:40:16 CEST 2014
Commit: 571102576548c2188007c984a9359f33facf3104
Author: Sergey Sharybin
Date: Fri Oct 10 10:23:19 2014 +0200
Branches: master
https://developer.blender.org/rB571102576548c2188007c984a9359f33facf3104
Cycles: Use a bit better approach for erfinv()
Also reduce number of branching and multiplications a bit by inlining the branches.
This gives an unmeasurable speedup, which is in case of BMW is about 2% here.
===================================================================
M intern/cycles/kernel/closure/bsdf_microfacet.h
===================================================================
diff --git a/intern/cycles/kernel/closure/bsdf_microfacet.h b/intern/cycles/kernel/closure/bsdf_microfacet.h
index 2079d26..c48ebd0 100644
--- a/intern/cycles/kernel/closure/bsdf_microfacet.h
+++ b/intern/cycles/kernel/closure/bsdf_microfacet.h
@@ -43,30 +43,65 @@ CCL_NAMESPACE_BEGIN
* Some constants are baked into the code.
*/
-ccl_device_inline float approx_erff(float x)
+ccl_device_inline float approx_erff_do(float x)
{
- float s = 1.0f;
- if(x < 0.0f) {
- s = -1.0f;
- x = -x;
- }
/* Such a clamp doesn't give much distortion to the output value
* and gives quite a few of the speedup.
*/
if(x > 3.0f) {
- return s;
+ return 1.0f;
}
float t = 1.0f / (1.0f + 0.47047f*x);
- return s * (1.0f -
- t*(0.3480242f + t*(-0.0958798f + t*0.7478556f)) * expf(-x*x));
+ return (1.0f -
+ t*(0.3480242f + t*(-0.0958798f + t*0.7478556f)) * expf(-x*x));
+}
+
+ccl_device_inline float approx_erff(float x)
+{
+ if(x >= 0.0f) {
+ return approx_erff_do(x);
+ }
+ else {
+ return -approx_erff_do(-x);
+ }
+}
+
+ccl_device_inline float approx_erfinvf_do(float x)
+{
+ if(x <= 0.7f) {
+ const float x2 = x * x;
+ const float a1 = 0.886226899f;
+ const float a2 = -1.645349621f;
+ const float a3 = 0.914624893f;
+ const float a4 = -0.140543331f;
+ const float b1 = -2.118377725f;
+ const float b2 = 1.442710462f;
+ const float b3 = -0.329097515f;
+ const float b4 = 0.012229801f;
+ return x * (((a4 * x2 + a3) * x2 + a2) * x2 + a1) /
+ ((((b4 * x2 + b3) * x2 + b2) * x2 + b1) * x2 + 1.0f);
+ }
+ else {
+ const float c1 = -1.970840454f;
+ const float c2 = -1.624906493f;
+ const float c3 = 3.429567803f;
+ const float c4 = 1.641345311;
+ const float d1 = 3.543889200f;
+ const float d2 = 1.637067800f;
+ const float z = sqrtf(-logf((1.0f - x) * 0.5f));
+ return (((c4 * z + c3) * z + c2) * z + c1) /
+ ((d2 * z + d1) * z + 1.0f);
+ }
}
ccl_device_inline float approx_erfinvf(float x)
{
- float ln1_x2 = logf(1.0f - x*x);
- float term = 4.546884979448f + ln1_x2 * 0.5f;
- return copysignf(1.0f, x) *
- sqrtf(sqrtf(term*term - ln1_x2 * 7.142230224076f) - term);
+ if(x >= 0.0f) {
+ return approx_erfinvf_do(x);
+ }
+ else {
+ return -approx_erfinvf_do(-x);
+ }
}
/* Beckmann and GGX microfacet importance sampling from:
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