sf.net # 998: Incorrect alpha blending in ALPHA put method

- avoid register overflow in gfx_put_alpha.c
- avoid register overflow in gfx_put_blend.c

changelog.txt

@@ -40,6 +40,7 @@ Version 1.20.0
 - sf.net #988: (regression) re-allow overloaded non-indexed array arguments - Can't pass UDT member array to an overloaded sub
 - gas64: 'SI' word found in cvtsi2ss wrongly releasing RSI register (SARG)
 - sf.net #997: gcc backend: handle EXTERN "rtlib": preserve the internally mangled name but emit an extra prototype to map the internal name to the external ASM name
+- gfxlib2: Incorrect alpha blending in ALPHA put method - fix C implementation to avoid overflowing int type
 
 
 Version 1.10.1

src/gfxlib2/gfx_put_alpha.c

@@ -22,13 +22,13 @@ static void fb_hPutAlpha4C(unsigned char *src, unsigned char *dest, int w, int h
 		for (x = w; x; x--) {
 			sc = *s++;
 			dc = *d;
-			a = (sc >> 24);
+			a = (sc >> 24) + 1;
 			srb = sc & MASK_RB_32;
 			sga = sc & MASK_GA_32;
 			drb = dc & MASK_RB_32;
 			dga = dc & MASK_GA_32;
 			srb = ((srb - drb) * a) >> 8;
-			sga = ((sga - dga) >> 8) * a;
+			sga = ((sga >> 8) - (dga >> 8)) * a;
 			*d++ = ((drb + srb) & MASK_RB_32) | ((dga + sga) & MASK_GA_32);
 		}
 		s += src_pitch;

src/gfxlib2/gfx_put_blend.c

@@ -83,7 +83,7 @@ static void fb_hPutBlend4C(unsigned char *src, unsigned char *dest, int w, int h
 				drb = dc & MASK_RB_32;
 				dga = dc & MASK_GA_32;
 				srb = ((srb - drb) * alpha) >> 8;
-				sga = ((sga - dga) >> 8) * alpha;
+				sga = ((sga >> 8) - (dga >> 8)) * alpha;
 				*d = ((drb + srb) & MASK_RB_32) | ((dga + sga) & MASK_GA_32);
 			}
 			d++;