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libmemdraw: fix drawing to color mapped with alpha chan (m8a8)

--- a/sys/src/libmemdraw/draw.c

+++ b/sys/src/libmemdraw/draw.c

@@ -1541,8 +1541,8 @@

 static void

 writecmap(Param *p, uchar *w, Buffer src)

 {

-	uchar *cmap, *red, *grn, *blu;

-	int i, dx, delta;

+	uchar *cmap, *red, *grn, *blu, *alpha;

+	int i, dx, delta, a, m;

 	cmap = p->img->cmap->rgb2cmap;

@@ -1552,8 +1552,20 @@

 	blu = src.blu;

 	dx = p->dx;

-	for(i=0; i<dx; i++, red+=delta, grn+=delta, blu+=delta)

-		*w++ = cmap[(*red>>4)*256+(*grn>>4)*16+(*blu>>4)];

+	if(p->img->flags&Falpha){

+		alpha = src.alpha;

+		m = p->img->shift[CMap]/8;

+		a = p->img->shift[CAlpha]/8;

+		for(i=0; i<dx; i++, red+=delta, grn+=delta, blu+=delta, w+=2){

+			w[a] = *alpha;

+			if(alpha != &ones)

+				alpha+=delta;

+			w[m] = cmap[(*red>>4)*256+(*grn>>4)*16+(*blu>>4)];

+		}

+	} else {

+		for(i=0; i<dx; i++, red+=delta, grn+=delta, blu+=delta)

+			*w++ = cmap[(*red>>4)*256+(*grn>>4)*16+(*blu>>4)];

+	}

 }

 #define DBG if(0)

@@ -1752,7 +1764,7 @@

 {

 	if(img->depth < 8)

 		return writenbit;

-	if(img->chan == CMAP8)

+	if(img->nbits[CMap] == 8)

 		return writecmap;

 	return writebyte;

 }

-- 

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