Add paint blend reference tests

src/paint/blend.cpp

@@ -0,0 +1,109 @@
+#include "paint/blend.h"
+
+#include <algorithm>
+#include <cmath>
+
+namespace pp::paint {
+
+namespace {
+
+[[nodiscard]] float saturate(float value) noexcept
+{
+    if (!std::isfinite(value)) {
+        return value < 0.0F ? 0.0F : 1.0F;
+    }
+
+    return std::clamp(value, 0.0F, 1.0F);
+}
+
+[[nodiscard]] float mix(float a, float b, float t) noexcept
+{
+    return a * (1.0F - t) + b * t;
+}
+
+[[nodiscard]] float blend_channel(float base, float stroke, BlendMode mode) noexcept
+{
+    switch (mode) {
+    case BlendMode::normal:
+        return stroke;
+    case BlendMode::multiply:
+        return base * stroke;
+    case BlendMode::screen:
+        return 1.0F - (1.0F - base) * (1.0F - stroke);
+    case BlendMode::color_dodge:
+        if (stroke >= 1.0F) {
+            return 1.0F;
+        }
+        return saturate(base / (1.0F - stroke));
+    case BlendMode::overlay:
+        return base < 0.5F
+            ? 2.0F * base * stroke
+            : 1.0F - 2.0F * (1.0F - base) * (1.0F - stroke);
+    }
+
+    return stroke;
+}
+
+[[nodiscard]] float blend_rgb(float base, float stroke, float base_alpha, float stroke_alpha, float alpha_total, BlendMode mode) noexcept
+{
+    if (alpha_total <= 0.0F) {
+        return 0.0F;
+    }
+
+    const auto stroke_weight = stroke_alpha / alpha_total;
+    const auto base_weight = base_alpha / alpha_total;
+    if (mode == BlendMode::normal) {
+        return saturate(mix(base, stroke, stroke_weight));
+    }
+
+    const auto mode_value = blend_channel(base, stroke, mode);
+    return saturate(mix(stroke, mix(base, mode_value, stroke_weight), base_weight));
+}
+
+}
+
+Rgba blend_pixels(Rgba base, Rgba stroke, BlendMode mode) noexcept
+{
+    base.r = saturate(base.r);
+    base.g = saturate(base.g);
+    base.b = saturate(base.b);
+    base.a = saturate(base.a);
+    stroke.r = saturate(stroke.r);
+    stroke.g = saturate(stroke.g);
+    stroke.b = saturate(stroke.b);
+    stroke.a = saturate(stroke.a);
+
+    if (stroke.a == 0.0F) {
+        return base;
+    }
+
+    const auto contribution = (1.0F - base.a) * stroke.a;
+    const auto alpha_total = saturate(base.a + contribution);
+
+    return {
+        blend_rgb(base.r, stroke.r, base.a, stroke.a, alpha_total, mode),
+        blend_rgb(base.g, stroke.g, base.a, stroke.a, alpha_total, mode),
+        blend_rgb(base.b, stroke.b, base.a, stroke.a, alpha_total, mode),
+        alpha_total,
+    };
+}
+
+const char* blend_mode_name(BlendMode mode) noexcept
+{
+    switch (mode) {
+    case BlendMode::normal:
+        return "normal";
+    case BlendMode::multiply:
+        return "multiply";
+    case BlendMode::screen:
+        return "screen";
+    case BlendMode::color_dodge:
+        return "color_dodge";
+    case BlendMode::overlay:
+        return "overlay";
+    }
+
+    return "unknown";
+}
+
+}