libretro/scummvm
1
0
Fork 0
mirror of https://github.com/libretro/scummvm.git synced 2025-01-29 14:42:26 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

GRAPHICS: Fix dest alpha formula in blendPixelPtr

Browse Source 
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).
This commit is contained in:
Marcus Comstedt 2014-01-10 20:02:24 +01:00
parent 51d7405a7b
commit 4b81b1299d
1 changed files with 10 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
graphics/VectorRendererSpec.cpp
View File

@ -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));
}