Cycles: Add Blackman-Harris filter, fix Gaussian filter

This commit adds the Blackman-Harris windows function as a pixel filter to Cycles. On some cases, such as wireframes or high-frequency textures,
Blackman-Harris can give subtle but noticable improvements over the Gaussian window.
Also, the gaussian window was truncated too early, which degraded quality a bit, therefore the evaluation region is now three times as wide.
To avoid artifacts caused by the wider curve, the filter table size is increased to 1024.

Reviewers: #cycles

Differential Revision: https://developer.blender.org/D1453

1 files changed, 12 insertions, 1 deletions

diff --git a/intern/cycles/render/film.cpp b/intern/cycles/render/film.cpp
index ce3f5b180ff..abdd1c2cc8a 100644
--- a/intern/cycles/render/film.cpp
+++ b/intern/cycles/render/film.cpp
@@ -209,10 +209,16 @@ static float filter_func_box(float /*v*/, float /*width*/)
 
 static float filter_func_gaussian(float v, float width)
 {
-	v *= 2.0f/width;
+	v *= 6.0f/width;
 	return expf(-2.0f*v*v);
 }
 
+static float filter_func_blackman_harris(float v, float width)
+{
+	v = M_2PI_F * (v / width + 0.5f);
+	return 0.35875f - 0.48829f*cosf(v) + 0.14128f*cosf(2.0f*v) - 0.01168f*cosf(3.0f*v);
+}
+
 static vector<float> filter_table(FilterType type, float width)
 {
 	vector<float> filter_table(FILTER_TABLE_SIZE);
@@ -224,6 +230,11 @@ static vector<float> filter_table(FilterType type, float width)
 			break;
 		case FILTER_GAUSSIAN:
 			filter_func = filter_func_gaussian;
+			width *= 3.0f;
+			break;
+		case FILTER_BLACKMAN_HARRIS:
+			filter_func = filter_func_blackman_harris;
+			width *= 2.0f;
 			break;
 		default:
 			assert(0);