Radius calculation in the blur was about 10% off. Probably impossible to notice,...

[movit] / blur_effect.cpp

diff --git a/blur_effect.cpp b/blur_effect.cpp
index b48dc956e59dade84e49ac507de998dc4241f45d..579521fde00b2e952824681bf5dcaa1aea178e7e 100644 (file)
--- a/blur_effect.cpp
+++ b/blur_effect.cpp
@@ -114,12 +114,16 @@ void SingleBlurPassEffect::set_gl_state(GLuint glsl_program_num, const std::stri
        } else {
                float sum = 0.0f;
                for (unsigned i = 0; i < NUM_TAPS + 1; ++i) {
-                       float z = i / radius;
-
                        // Gaussian blur is a common, but maybe not the prettiest choice;
                        // it can feel a bit too blurry in the fine detail and too little
                        // long-tail. This is a simple logistic distribution, which has
                        // a narrower peak but longer tails.
+                       //
+                       // We interpret the radius as sigma, similar to Gaussian blur.
+                       // Wikipedia says that sigma² = pi² s² / 3, which yields:
+                       const float s = (sqrt(3.0) / M_PI) * radius;
+                       float z = i / (2.0 * s);
+
                        weight[i] = 1.0f / (cosh(z) * cosh(z));
 
                        if (i == 0) {