draw: fix drawing of replicated source image on memlayer with a clip rectangle

when a replicated source image with a clipr with clipr.min > Pt(0, 0),
drawclip() would properly translate the src->clipr on the dstr
but then clamp the source rectangle back on src->r.

while traversing down multiple layers, this would cause the translation to
be applied multiple times to the dst rectangle giving the wrong image result.

this change adds a new drawclipnorepl() function that avoids the clamping
of source and mask rectangles to src->r and mask->r. this is then used in
libmemlayer.

the final memimagedraw() call will call drawclip() which will do the final
claming.

a testcase is provided:

#include <u.h>
#include <libc.h>
#include <draw.h>

Image *blue;
Image *red;

void
main(int, char *argv[])
{
	Image *i;

	if(initdraw(nil, nil, argv[0]) < 0)
		sysfatal("initdraw: %r");
	i = allocimage(display, screen->r, screen->chan, 1, DWhite);

	red = allocimage(display, Rect(0,0,1,1), screen->chan, 1, DRed);
	blue = allocimage(display, Rect(0,0,1,1), screen->chan, 1, DPaleblue);
	replclipr(red, 1, Rect(10, 10, 110, 110));
	replclipr(blue, 1, Rect(11, 11, 111, 111));

	/* draw on non-layer, works correctly */
	draw(i, i->r, red, nil, ZP);
	draw(i, i->r, blue, nil, ZP);
	draw(screen, screen->r, i, nil, i->r.min);
	flushimage(display, 1);

	/* draw on (screen) layer is too far to the right */
	draw(screen, screen->r, red, nil, ZP);
	draw(screen, screen->r, blue, nil, ZP);
	flushimage(display, 1);

	for(;;){
		sleep(1000);
	}
}

1 files changed, 15 insertions, 1 deletions

diff --git a/sys/man/2/memdraw b/sys/man/2/memdraw
index df68b699d..733ae0e78 100644
--- a/sys/man/2/memdraw
+++ b/sys/man/2/memdraw
@@ -25,6 +25,7 @@ memfillpoly,
 memimageline,
 memimagedraw,
 drawclip,
+drawclipnorepl,
 memlinebbox,
 memlineendsize,
 allocmemsubfont,
@@ -139,6 +140,9 @@ void	memimagedraw(Memimage *dst, Rectangle r, Memimage *src,
 int	drawclip(Memimage *dst, Rectangle *dr, Memimage *src,
 	   Point *sp, Memimage *mask, Point *mp,
 	   Rectangle *sr, Rectangle *mr)
+int	drawclipnorepl(Memimage *dst, Rectangle *dr, Memimage *src,
+	   Point *sp, Memimage *mask, Point *mp,
+	   Rectangle *sr, Rectangle *mr)
 Rectangle	memlinebbox(Point p0, Point p1, int end0, int end1,
 	   int radius)
 int	memlineendsize(int end)
@@ -396,8 +400,18 @@ but translated so the upper left corners are the returned
 .B sp
 and
 .BR mp .
+.I Drawclipnorepl
+does the same as
+.B drawclip
+but avoids clamping
+.B sp
+and
+.B mr
+within the image rectangle of source and mask when replicated.
 .I Drawclip
-returns zero when the clipped rectangle is empty.
+and
+.I drawclipnorepl
+return zero when the clipped rectangle is empty.
 .I Memlinebbox
 returns a conservative bounding box containing a line between
 two points