Add paint blend reference tests

tests/paint/blend_tests.cpp

@@ -0,0 +1,105 @@
+#include "paint/blend.h"
+#include "test_harness.h"
+
+#include <cmath>
+#include <string_view>
+
+using pp::paint::BlendMode;
+using pp::paint::Rgba;
+using pp::paint::blend_mode_name;
+using pp::paint::blend_pixels;
+
+namespace {
+
+bool near(float a, float b)
+{
+    return std::fabs(a - b) < 0.0001F;
+}
+
+void normal_blend_matches_source_over_alpha(pp::tests::Harness& h)
+{
+    const auto result = blend_pixels(
+        Rgba { .r = 0.2F, .g = 0.4F, .b = 0.6F, .a = 0.5F },
+        Rgba { .r = 0.8F, .g = 0.2F, .b = 0.1F, .a = 0.25F },
+        BlendMode::normal);
+
+    PP_EXPECT(h, near(result.a, 0.625F));
+    PP_EXPECT(h, near(result.r, 0.44F));
+    PP_EXPECT(h, near(result.g, 0.32F));
+    PP_EXPECT(h, near(result.b, 0.4F));
+}
+
+void zero_alpha_stroke_leaves_base_unchanged(pp::tests::Harness& h)
+{
+    const Rgba base { .r = 0.2F, .g = 0.3F, .b = 0.4F, .a = 0.5F };
+    const auto result = blend_pixels(
+        base,
+        Rgba { .r = 1.0F, .g = 1.0F, .b = 1.0F, .a = 0.0F },
+        BlendMode::screen);
+
+    PP_EXPECT(h, near(result.r, base.r));
+    PP_EXPECT(h, near(result.g, base.g));
+    PP_EXPECT(h, near(result.b, base.b));
+    PP_EXPECT(h, near(result.a, base.a));
+}
+
+void multiply_and_screen_are_bounded(pp::tests::Harness& h)
+{
+    const Rgba base { .r = 0.25F, .g = 0.5F, .b = 0.75F, .a = 1.0F };
+    const Rgba stroke { .r = 0.5F, .g = 0.5F, .b = 0.5F, .a = 1.0F };
+    const auto multiply = blend_pixels(base, stroke, BlendMode::multiply);
+    const auto screen = blend_pixels(base, stroke, BlendMode::screen);
+
+    PP_EXPECT(h, near(multiply.r, 0.125F));
+    PP_EXPECT(h, near(multiply.g, 0.25F));
+    PP_EXPECT(h, near(multiply.b, 0.375F));
+    PP_EXPECT(h, near(screen.r, 0.625F));
+    PP_EXPECT(h, near(screen.g, 0.75F));
+    PP_EXPECT(h, near(screen.b, 0.875F));
+}
+
+void color_dodge_and_overlay_handle_extremes(pp::tests::Harness& h)
+{
+    const auto dodge = blend_pixels(
+        Rgba { .r = 0.4F, .g = 0.5F, .b = 0.6F, .a = 1.0F },
+        Rgba { .r = 1.0F, .g = 0.5F, .b = 0.0F, .a = 1.0F },
+        BlendMode::color_dodge);
+    const auto overlay = blend_pixels(
+        Rgba { .r = 0.25F, .g = 0.5F, .b = 0.75F, .a = 1.0F },
+        Rgba { .r = 0.5F, .g = 0.5F, .b = 0.5F, .a = 1.0F },
+        BlendMode::overlay);
+
+    PP_EXPECT(h, near(dodge.r, 1.0F));
+    PP_EXPECT(h, near(dodge.g, 1.0F));
+    PP_EXPECT(h, near(dodge.b, 0.6F));
+    PP_EXPECT(h, near(overlay.r, 0.25F));
+    PP_EXPECT(h, near(overlay.g, 0.5F));
+    PP_EXPECT(h, near(overlay.b, 0.75F));
+}
+
+void clamps_inputs_and_names_modes(pp::tests::Harness& h)
+{
+    const auto result = blend_pixels(
+        Rgba { .r = -1.0F, .g = 2.0F, .b = 0.5F, .a = 2.0F },
+        Rgba { .r = 2.0F, .g = -1.0F, .b = 0.5F, .a = 2.0F },
+        BlendMode::normal);
+
+    PP_EXPECT(h, near(result.r, 1.0F));
+    PP_EXPECT(h, near(result.g, 0.0F));
+    PP_EXPECT(h, near(result.b, 0.5F));
+    PP_EXPECT(h, near(result.a, 1.0F));
+    PP_EXPECT(h, blend_mode_name(BlendMode::overlay) == std::string_view("overlay"));
+}
+
+}
+
+int main()
+{
+    pp::tests::Harness harness;
+    harness.run("normal_blend_matches_source_over_alpha", normal_blend_matches_source_over_alpha);
+    harness.run("zero_alpha_stroke_leaves_base_unchanged", zero_alpha_stroke_leaves_base_unchanged);
+    harness.run("multiply_and_screen_are_bounded", multiply_and_screen_are_bounded);
+    harness.run("color_dodge_and_overlay_handle_extremes", color_dodge_and_overlay_handle_extremes);
+    harness.run("clamps_inputs_and_names_modes", clamps_inputs_and_names_modes);
+    return harness.finish();
+}