Rescale resampling weights so that the sum becomes one.

For some reason, I had forgotten this, and it showed up because Qt
has buggy handling of pixels with alpha != 0xff. Add unit test
so it doesn't happen again.

I'm a bit concerned that rounding might cause problems so that we
should perhaps renormalize after the bilinear conversion, but we
can deal with that later if it should show up.

diff --git a/resample_effect.cpp b/resample_effect.cpp
index 6c490f141bc1a87b9f86db44b90b5d64f75d0dd7..10dc59c5a2dbab0a0533629b801357636ad31ef0 100644 (file)
--- a/resample_effect.cpp
+++ b/resample_effect.cpp
@@ -299,11 +299,18 @@ 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;
                }
        }
 

diff --git a/resample_effect_test.cpp b/resample_effect_test.cpp
index 952553681a87834beab57618120c85c42cb661b2..e4c23b10782c09e96fa91cd2a4cb030a7faf9c6c 100644 (file)
--- a/resample_effect_test.cpp
+++ b/resample_effect_test.cpp
@@ -166,3 +166,38 @@ TEST(ResampleEffectTest, UpscaleByThreeGetsCorrectPixelCenters) {
                }
        }
 }
+
+TEST(ResampleEffectTest, HeavyResampleGetsSumRight) {
+       const int swidth = 1280, sheight = 720;
+       const int dwidth = 36, dheight = 20;
+
+       float data[swidth * sheight], out_data[dwidth * dheight], expected_data[dwidth * dheight];
+       for (int y = 0; y < sheight; ++y) {
+               for (int x = 0; x < swidth; ++x) {
+                       data[y * swidth + x] = 0.5f;
+               }
+       }
+       for (int y = 0; y < dheight; ++y) {
+               for (int x = 0; x < dwidth; ++x) {
+                       expected_data[y * dwidth + x] = 0.5f;
+               }
+       }
+
+       EffectChainTester tester(NULL, dwidth, dheight, FORMAT_GRAYSCALE, COLORSPACE_sRGB, GAMMA_LINEAR);
+
+       ImageFormat format;
+       format.color_space = COLORSPACE_sRGB;
+       format.gamma_curve = GAMMA_LINEAR;
+
+       FlatInput *input = new FlatInput(format, FORMAT_GRAYSCALE, GL_FLOAT, swidth, sheight);
+       input->set_pixel_data(data);
+
+       tester.get_chain()->add_input(input);
+       Effect *resample_effect = tester.get_chain()->add_effect(new ResampleEffect());
+       ASSERT_TRUE(resample_effect->set_int("width", dwidth));
+       ASSERT_TRUE(resample_effect->set_int("height", dheight));
+       tester.run(out_data, GL_RED, COLORSPACE_sRGB, GAMMA_LINEAR);
+
+       expect_equal(expected_data, out_data, dwidth, dheight, 0.001);
+}
+