libmemdraw bug?

[qwx <qwx@sciops.net>]

Hello,

I'm looking at rsc's implementation of pico, which reads in images with
readmemimage().  It only handles 248x248 images, but if extended to
arbitrary dimensions, the images aren't displayed correctly unless their
dimensions don't have the lowest 3 bits set.  In other words, if dx & 7
and/or dy & 7, the image seems to be read/loaded incorrectly.

Pico uses libmemdraw to load and manipulate images, then displays them with
libdraw.  I'm inlining below a simple program that demonstrates the issue.

What is going on?

Thanks,

qwx

/*
; hget http://9front.org/img/ninefront.bit > /tmp/ninefront.bit
; 6c -FTVw test.c && 6l test.6 && 6.out /tmp/ninefront.bit
*/
#include <u.h>
#include <libc.h>
#include <draw.h>
#include <memdraw.h>

void
main(int argc, char **argv)
{
	int fd;
	Memimage *m;
	Image *i;

	if(argc != 2)
		sysfatal("usage");
	if(memimageinit() < 0)
		sysfatal("memimageinit: %r");
	if((fd = open(argv[1], OREAD)) < 0)
		sysfatal("open: %r");
	m = readmemimage(fd);
	close(fd);
	if(m == nil)
		sysfatal("readmemimage: %r");
	if(initdraw(nil, nil, nil) < 0)
		sysfatal("initdraw: %r");
	if((i = allocimage(display, m->r, m->chan, 0, DNofill)) == nil)
		sysfatal("allocimage: %r");
	if(loadimage(i, i->r, byteaddr(m, m->r.min), Dx(m->r)*Dy(m->r)*m->depth/8) < 0)
		sysfatal("loadimage: %r");
	draw(screen, screen->r, i, nil, i->r.min);
	flushimage(display, 1);
	for(;;)
		sleep(1);
}