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libmemdraw: avoid buffering in alphadraw() unless strictly necessary

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

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

@@ -595,19 +595,21 @@

 	isgrey = dst->flags&Fgrey;

 	/*

-	 * Buffering when src and dst are the same bitmap is sufficient but not 

-	 * necessary.  There are stronger conditions we could use.  We could

-	 * check to see if the rectangles intersect, and if simply moving in the

-	 * correct y direction can avoid the need to buffer.

+	 * Buffering is only necessary when src and dst are the same

+	 * bitmap and both r and sr intersect each other and begin at

+	 * the same scanline.  We could even avoid this if it was

+	 * possible to walk the scanline pixels in the -x direction.

+	 *

+	 * The vector û = sr.min - r.min determines the safest

+	 * direction to follow on each axis.

 	 */

-	needbuf = (src->data == dst->data);

+	needbuf = (src->data == dst->data && r.min.y == sr.min.y && rectXrect(r, sr));

+	dir = (src->data == dst->data && r.min.y > sr.min.y && rectXrect(r, sr)) ? -1 : 1;

 	ndrawbuf = 0;

 	getparam(&z->spar, src, sr, isgrey, needbuf, &ndrawbuf);

 	getparam(&z->dpar, dst, r, isgrey, needbuf, &ndrawbuf);

 	getparam(&z->mpar, mask, mr, 0, needbuf, &ndrawbuf);

-

-	dir = (needbuf && byteaddr(dst, r.min) > byteaddr(src, sr.min)) ? -1 : 1;

 	z->spar.dir = z->mpar.dir = z->dpar.dir = dir;

 	/*

@@ -2027,13 +2029,13 @@

 		case CAlpha:

 			v |= (a>>(8-nb))<<d;

 			break;

+		case CGrey:

+			m = RGB2K(r,g,b);

+			v |= (m>>(8-nb))<<d;

+			break;

 		case CMap:

 			p = img->cmap->rgb2cmap;

 			m = p[(r>>4)*256+(g>>4)*16+(b>>4)];

-			v |= (m>>(8-nb))<<d;

-			break;

-		case CGrey:

-			m = RGB2K(r,g,b);

 			v |= (m>>(8-nb))<<d;

 			break;

 		}

-- 

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