Add stroke alpha blend reference tests

src/paint/blend.cpp

@@ -44,6 +44,67 @@ namespace {
     return stroke;
 }
 
+[[nodiscard]] float blend_stroke_screen(float base, float stroke) noexcept
+{
+    return base + stroke - (base * stroke);
+}
+
+[[nodiscard]] float blend_stroke_hard_light(float base, float stroke) noexcept
+{
+    return stroke < 0.5F
+        ? base * (stroke * 2.0F)
+        : blend_stroke_screen(base, 2.0F * stroke - 1.0F);
+}
+
+[[nodiscard]] float blend_stroke_hard_mix(float base, float stroke) noexcept
+{
+    if (base == 0.0F) {
+        return 0.0F;
+    }
+
+    return base + stroke < 0.5F ? 0.0F : saturate(base + stroke);
+}
+
+[[nodiscard]] float blend_stroke_color_dodge(float base, float stroke) noexcept
+{
+    if (base == 0.0F) {
+        return 0.0F;
+    }
+
+    if (stroke == 1.0F) {
+        return 1.0F;
+    }
+
+    return base / (1.0F - stroke);
+}
+
+[[nodiscard]] float blend_stroke_color_burn(float base, float stroke) noexcept
+{
+    if (base == 1.0F) {
+        return 1.0F;
+    }
+
+    if (stroke == 0.0F) {
+        return 0.0F;
+    }
+
+    return 1.0F - std::min(1.0F, (1.0F - base) / stroke);
+}
+
+[[nodiscard]] float blend_stroke_linear_height(float base, float stroke, float depth) noexcept
+{
+    const auto partial = (1.0F - stroke) * std::pow(depth, 0.25F) + (base * depth * 10.0F);
+    return base * partial;
+}
+
+[[nodiscard]] float blend_stroke_height(float base, float stroke, float depth) noexcept
+{
+    const auto a = std::pow(1.0F - stroke, std::max(1.0F, (1.0F - depth) * 10.0F))
+        * std::pow(depth, 0.25F);
+    const auto b = base * depth * 5.0F;
+    return base * (a + b);
+}
+
 [[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) {
@@ -88,6 +149,44 @@ Rgba blend_pixels(Rgba base, Rgba stroke, BlendMode mode) noexcept
     };
 }
 
+float blend_stroke_alpha(
+    float base,
+    float stroke,
+    float depth,
+    StrokeBlendMode mode) noexcept
+{
+    base = saturate(base);
+    stroke = saturate(stroke);
+    depth = saturate(depth);
+
+    switch (mode) {
+    case StrokeBlendMode::normal:
+        return saturate(mix(base, stroke, depth));
+    case StrokeBlendMode::multiply:
+        return saturate(mix(base, base * stroke, depth));
+    case StrokeBlendMode::subtract:
+        return saturate(mix(base, std::max(0.0F, base - stroke), depth));
+    case StrokeBlendMode::darken:
+        return saturate(mix(base, std::min(base, stroke), depth));
+    case StrokeBlendMode::overlay:
+        return saturate(mix(base, blend_stroke_hard_light(stroke, base), depth));
+    case StrokeBlendMode::color_dodge:
+        return saturate(mix(base, blend_stroke_color_dodge(base, stroke), depth));
+    case StrokeBlendMode::color_burn:
+        return saturate(mix(base, blend_stroke_color_burn(base, stroke), depth));
+    case StrokeBlendMode::linear_burn:
+        return saturate(mix(base, saturate(base + stroke - 1.0F), depth));
+    case StrokeBlendMode::hard_mix:
+        return saturate(mix(base, blend_stroke_hard_mix(base, stroke), depth));
+    case StrokeBlendMode::linear_height:
+        return saturate(blend_stroke_linear_height(base, stroke, depth));
+    case StrokeBlendMode::height:
+        return saturate(blend_stroke_height(base, stroke, depth));
+    }
+
+    return 1.0F;
+}
+
 const char* blend_mode_name(BlendMode mode) noexcept
 {
     switch (mode) {
@@ -106,4 +205,34 @@ const char* blend_mode_name(BlendMode mode) noexcept
     return "unknown";
 }
 
+const char* stroke_blend_mode_name(StrokeBlendMode mode) noexcept
+{
+    switch (mode) {
+    case StrokeBlendMode::normal:
+        return "normal";
+    case StrokeBlendMode::multiply:
+        return "multiply";
+    case StrokeBlendMode::subtract:
+        return "subtract";
+    case StrokeBlendMode::darken:
+        return "darken";
+    case StrokeBlendMode::overlay:
+        return "overlay";
+    case StrokeBlendMode::color_dodge:
+        return "color_dodge";
+    case StrokeBlendMode::color_burn:
+        return "color_burn";
+    case StrokeBlendMode::linear_burn:
+        return "linear_burn";
+    case StrokeBlendMode::hard_mix:
+        return "hard_mix";
+    case StrokeBlendMode::linear_height:
+        return "linear_height";
+    case StrokeBlendMode::height:
+        return "height";
+    }
+
+    return "unknown";
+}
+
 }

src/paint/blend.h

@@ -12,6 +12,20 @@ enum class BlendMode : std::uint8_t {
     overlay,
 };
 
+enum class StrokeBlendMode : std::uint8_t {
+    normal,
+    multiply,
+    subtract,
+    darken,
+    overlay,
+    color_dodge,
+    color_burn,
+    linear_burn,
+    hard_mix,
+    linear_height,
+    height,
+};
+
 struct Rgba {
     float r = 0.0F;
     float g = 0.0F;
@@ -20,6 +34,12 @@ struct Rgba {
 };
 
 [[nodiscard]] Rgba blend_pixels(Rgba base, Rgba stroke, BlendMode mode) noexcept;
+[[nodiscard]] float blend_stroke_alpha(
+    float base,
+    float stroke,
+    float depth,
+    StrokeBlendMode mode) noexcept;
 [[nodiscard]] const char* blend_mode_name(BlendMode mode) noexcept;
+[[nodiscard]] const char* stroke_blend_mode_name(StrokeBlendMode mode) noexcept;
 
 }