Normalize the resample weight after bilinear combining.

We introduce a small bit of error in the combining (due to having to
compensate for lack of subpixel sampling precision), so normalize
after it rather than before it. Also, do a second normalization pass,
which seemingly helps sometimes (probably due to inaccuracies in the
float sum).

This seems to kill about half the precision loss on Intel, at least.

diff --git a/resample_effect.cpp b/resample_effect.cpp
index 10dc59c5a2dbab0a0533629b801357636ad31ef0..367e8b4d52604043d6529fcdb5b392ed77136ec2 100644 (file)
--- a/resample_effect.cpp
+++ b/resample_effect.cpp
@@ -229,7 +229,6 @@ void SingleResamplePassEffect::update_texture(GLuint glsl_program_num, const str
                assert(false);
        }
 
-
        // For many resamplings (e.g. 640 -> 1280), we will end up with the same
        // set of samples over and over again in a loop. Thus, we can compute only
        // the first such loop, and then ask the card to repeat the texture for us.
@@ -299,19 +298,13 @@ void SingleResamplePassEffect::update_texture(GLuint glsl_program_num, const str
                int base_src_y = lrintf(center_src_y);
 
                // Now sample <int_radius> pixels on each side around that point.
-               double sum = 0.0;
                for (int i = 0; i < src_samples; ++i) {
                        int src_y = base_src_y + i - int_radius;
                        float weight = lanczos_weight(radius_scaling_factor * (src_y - center_src_y), LANCZOS_RADIUS);
                        weights[(y * src_samples + i) * 2 + 0] = weight * radius_scaling_factor;
                        weights[(y * src_samples + i) * 2 + 1] = (src_y + 0.5) / float(src_size);
-                       sum += weights[(y * src_samples + i) * 2 + 0];
                }
 
-               // Normalize so that the sum becomes one.
-               for (int i = 0; i < src_samples; ++i) {
-                       weights[(y * src_samples + i) * 2 + 0] /= sum;
-               }
        }
 
        // Now make use of the bilinear filtering in the GPU to reduce the number of samples
@@ -337,6 +330,18 @@ void SingleResamplePassEffect::update_texture(GLuint glsl_program_num, const str
                        src_samples,
                        src_samples - src_bilinear_samples);
                assert(int(src_samples) - int(num_samples_saved) == src_bilinear_samples);
+
+               // Normalize so that the sum becomes one. Note that we do it twice;
+               // this sometimes helps a tiny little bit when we have many samples.
+               for (int normalize_pass = 0; normalize_pass < 2; ++normalize_pass) {
+                       float sum = 0.0;
+                       for (int i = 0; i < src_bilinear_samples; ++i) {
+                               sum += bilinear_weights[(y * src_bilinear_samples + i) * 2 + 0];
+                       }
+                       for (int i = 0; i < src_bilinear_samples; ++i) {
+                               bilinear_weights[(y * src_bilinear_samples + i) * 2 + 0] /= sum;
+                       }
+               }
        }       
 
        // Encode as a two-component texture. Note the GL_REPEAT.