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

this causes problems in libmemlayer when it drawclips() at each iteration
when traversing down the layers. the clamping of sr back to src->r (which is 1x1
in the example below) it will shift the destination rectangle to the right
as it is reapplied at each loop iteration.

#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);
        }
}

in 9front, i added drawclipnorepl() function that is like drawclip() but does not clamp
the source and mask rectangles to be in src->r and mask->r and use it in libmemlayer.

http://code.google.com/p/plan9front/source/detail?r=c18ae5b7ea0ee138080824c4f164628f899b7770

--
cinap