2 files changed, 53 insertions, 10 deletions

diff --git a/graphics/VectorRendererSpec.cpp b/graphics/VectorRendererSpec.cpp
index 5ab6f6f2c9..5220beec5c 100644
--- a/graphics/VectorRendererSpec.cpp
+++ b/graphics/VectorRendererSpec.cpp
@@ -164,15 +164,15 @@ inline frac_t fp_sqroot(uint32 x) {
 
 // Color depending on y
 // Note: this is only for the outer pixels
-#define WU_DRAWCIRCLE_XCOLOR(ptr1,ptr2,ptr3,ptr4,x,y,px,py,a) do { \
-	this->blendPixelPtr(ptr1 + (y) - (px), color1, a); \
-	this->blendPixelPtr(ptr1 + (x) - (py), color2, a); \
-	this->blendPixelPtr(ptr2 - (x) - (py), color2, a); \
-	this->blendPixelPtr(ptr2 - (y) - (px), color1, a); \
-	this->blendPixelPtr(ptr3 - (y) + (px), color3, a); \
-	this->blendPixelPtr(ptr3 - (x) + (py), color4, a); \
-	this->blendPixelPtr(ptr4 + (x) + (py), color4, a); \
-	this->blendPixelPtr(ptr4 + (y) + (px), color3, a); \
+#define WU_DRAWCIRCLE_XCOLOR(ptr1,ptr2,ptr3,ptr4,x,y,px,py,a,func) do { \
+	this->func(ptr1 + (y) - (px), color1, a); \
+	this->func(ptr1 + (x) - (py), color2, a); \
+	this->func(ptr2 - (x) - (py), color2, a); \
+	this->func(ptr2 - (y) - (px), color1, a); \
+	this->func(ptr3 - (y) + (px), color3, a); \
+	this->func(ptr3 - (x) + (py), color4, a); \
+	this->func(ptr4 + (x) + (py), color4, a); \
+	this->func(ptr4 + (y) + (px), color3, a); \
 } while (0)
 
 // Color depending on corner (tl,tr,bl: color1, br: color2)
@@ -521,6 +521,23 @@ blendPixelPtr(PixelType *ptr, PixelType color, uint8 alpha) {
 
 template<typename PixelType>
 inline void VectorRendererSpec<PixelType>::
+blendPixelDestAlphaPtr(PixelType *ptr, PixelType color, uint8 alpha) {
+	int idst = *ptr;
+	// This function is only used for corner pixels in rounded rectangles, so
+	// the performance hit of this if shouldn't be too high.
+	// We're also ignoring the cases where dst has intermediate alpha.
+	if ((idst & _alphaMask) == 0) {
+		// set color and alpha channels
+		*ptr = (PixelType)(color & (_redMask | _greenMask | _blueMask)) |
+		                  ((alpha >> _format.aLoss) << _format.aShift);
+	} else {
+		// blend color with background
+		blendPixelPtr(ptr, color, alpha);
+	}
+}
+
+template<typename PixelType>
+inline void VectorRendererSpec<PixelType>::
 darkenFill(PixelType *ptr, PixelType *end) {
 	PixelType mask = (PixelType)((3 << _format.rShift) | (3 << _format.gShift) | (3 << _format.bShift));
 
@@ -1827,7 +1844,15 @@ drawRoundedSquareAlg(int x1, int y1, int r, int w, int h, PixelType color, Vecto
 				if (T < oldT || y == 1)
 					gradientFill(ptr_bl - y + px + 1, w - 2 * r + 2 * y - 1, x1 + r - y - x + 1, h - r + x);
 
-				WU_DRAWCIRCLE_XCOLOR(ptr_tr, ptr_tl, ptr_bl, ptr_br, x, y, px, py, a1);
+				// This shape is used for dialog backgrounds.
+				// If we're drawing on top of an empty overlay background,
+				// and the overlay supports alpha, we have to do AA by
+				// setting the dest alpha channel, instead of blending with
+				// dest color channels.
+				if (!g_system->hasFeature(OSystem::kFeatureOverlaySupportsAlpha))
+					WU_DRAWCIRCLE_XCOLOR(ptr_tr, ptr_tl, ptr_bl, ptr_br, x, y, px, py, a1, blendPixelPtr);
+				else
+					WU_DRAWCIRCLE_XCOLOR(ptr_tr, ptr_tl, ptr_bl, ptr_br, x, y, px, py, a1, blendPixelDestAlphaPtr);
 			}
 
 			ptr_fill += pitch * r;
diff --git a/graphics/VectorRendererSpec.h b/graphics/VectorRendererSpec.h
index 7812e77b22..d8fc82a6f1 100644
--- a/graphics/VectorRendererSpec.h
+++ b/graphics/VectorRendererSpec.h
@@ -121,6 +121,24 @@ protected:
 	inline void blendPixelPtr(PixelType *ptr, PixelType color, uint8 alpha);
 
 	/**
+	 * Blends a single pixel on the surface in the given pixel pointer, using supplied color
+	 * and Alpha intensity.
+	 * If the destination pixel has 0 alpha, set the color and alpha channels,
+	 * overwriting the destination pixel entirely.
+	 * If the destination pixel has non-zero alpha, blend dest with src.
+	 *
+	 * This is implemented to prevent blendPixel() to calculate the surface pointer on each call.
+	 * Optimized drawing algorithms should call this function when possible.
+	 *
+	 * @see blendPixel
+	 * @param ptr Pointer to the pixel to blend on top of
+	 * @param color Color of the pixel
+	 * @param alpha Alpha intensity of the pixel (0-255)
+	 */
+	inline void blendPixelDestAlphaPtr(PixelType *ptr, PixelType color, uint8 alpha);
+
+
+	/**
 	 * PRIMITIVE DRAWING ALGORITHMS
 	 *
 	 * Generic algorithms for drawing all kinds of aliased primitive shapes.