GRAPHICS: Fix dest alpha formula in blendPixelPtr

The original alpha computation formula had a problem:  If something was
drawn on top of a pixel that was already fully opaque, there would be
an overflow in the computed alpha, and the destination alpha would be
truncated to 0 (fully transparent).

In commit 264ba4a9 this formula was replaced with another one, which
did not have overflows but also was not correct.

This commits introduces a new formula, where the rounding errors have
been turned in another direction; drawing a fully opaque pixel on top
of a transparent one would result in a pixel which is almost, but not
fully, opaque.  However, this is no problem in practice, since drawing
fully opaque pixels can be achieved with much less code as a special
case, so add that (also improves rendering speed).

1 files changed, 10 insertions, 3 deletions

diff --git a/graphics/VectorRendererSpec.cpp b/graphics/VectorRendererSpec.cpp
index de249e8711..a3ec66a29d 100644
--- a/graphics/VectorRendererSpec.cpp
+++ b/graphics/VectorRendererSpec.cpp
@@ -620,7 +620,10 @@ applyScreenShading(GUI::ThemeEngine::ShadingStyle shadingStyle) {
 template<typename PixelType>
 inline void VectorRendererSpec<PixelType>::
 blendPixelPtr(PixelType *ptr, PixelType color, uint8 alpha) {
-	if (sizeof(PixelType) == 4) {
+	if (alpha == 0xff) {
+		// fully opaque pixel, don't blend
+		*ptr = color | _alphaMask;
+	} else if (sizeof(PixelType) == 4) {
 		const byte sR = (color & _redMask) >> _format.rShift;
 		const byte sG = (color & _greenMask) >> _format.gShift;
 		const byte sB = (color & _blueMask) >> _format.bShift;
@@ -628,15 +631,17 @@ blendPixelPtr(PixelType *ptr, PixelType color, uint8 alpha) {
 		byte dR = (*ptr & _redMask) >> _format.rShift;
 		byte dG = (*ptr & _greenMask) >> _format.gShift;
 		byte dB = (*ptr & _blueMask) >> _format.bShift;
+		byte dA = (*ptr & _alphaMask) >> _format.aShift;
 
 		dR += ((sR - dR) * alpha) >> 8;
 		dG += ((sG - dG) * alpha) >> 8;
 		dB += ((sB - dB) * alpha) >> 8;
+		dA += ((0xff - dA) * alpha) >> 8;
 
 		*ptr = ((dR << _format.rShift) & _redMask)
 		     | ((dG << _format.gShift) & _greenMask)
 		     | ((dB << _format.bShift) & _blueMask)
-		     | (*ptr & _alphaMask);
+		     | ((dA << _format.aShift) & _alphaMask);
 	} else if (sizeof(PixelType) == 2) {
 		int idst = *ptr;
 		int isrc = color;
@@ -651,7 +656,9 @@ blendPixelPtr(PixelType *ptr, PixelType color, uint8 alpha) {
 			(_blueMask & ((idst & _blueMask) +
 			((int)(((int)(isrc & _blueMask) -
 			(int)(idst & _blueMask)) * alpha) >> 8))) |
-			(idst & _alphaMask));
+			(_alphaMask & ((idst & _alphaMask) +
+			((int)(((int)(_alphaMask) -
+			(int)(idst & _alphaMask)) * alpha) >> 8))));
 	} else {
 		error("Unsupported BPP format: %u", (uint)sizeof(PixelType));
 	}