From: Steinar H. Gunderson <sgunderson@bigfoot.com>
Date: Sat, 25 Jan 2014 11:42:56 +0000 (+0100)
Subject: Rescale resampling weights so that the sum becomes one.
X-Git-Tag: 1.0~55
X-Git-Url: https://git.sesse.net/?p=movit;a=commitdiff_plain;h=aa3e630eb0c455921c721f9ccd84e872bc9757bc

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 6c490f1..10dc59c 100644
--- 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 9525536..e4c23b1 100644
--- 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);
+}
+