modules/submit: fix bilerp mode out-of-bounds access

Found via -fsanitize=address, this is a quick and dirty way to
prevent the OOB access without adding more conditionals, just
prevent scaling the fragment dimensions to the full grid
dimensions.

This could be done better by reworking things a bit and putting
zero at the center of the grid with a -1..+1 mapping, so rounding
towards zero would land in the middle as opposed to off the
start, with the existing .5f offset.

But for now just fix the bug, nobody will notice the slight LCD
overscan-style difference of bilerp=on vs. off due to this way.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/modules/submit/submit.c b/src/modules/submit/submit.c
index 1f3a2ae..af2d044 100644
--- a/src/modules/submit/submit.c
+++ b/src/modules/submit/submit.c
@@ -208,15 +208,15 @@ static void draw_grid(submit_context_t *ctxt, til_fb_fragment_t *fragment)
 
 static void draw_grid_bilerp(submit_context_t *ctxt, til_fb_fragment_t *fragment)
 {
-	float	xscale = ((float)GRID_SIZE - 1.f) / (float)fragment->frame_width;
-	float	yscale = ((float)GRID_SIZE - 1.f) / (float)fragment->frame_height;
+	float	xscale = ((float)GRID_SIZE - 2.f) / (float)fragment->frame_width;
+	float	yscale = ((float)GRID_SIZE - 2.f) / (float)fragment->frame_height;
 
 	for (int y = 0; y < fragment->height; y++) {
 		for (int x = 0; x < fragment->width; x++) {
 			uint32_t	color;
 
 			/* TODO: this could be optimized a bit! i.e. don't recompute the y for every x etc. */
-			color = sample_grid_bilerp(ctxt, .5f + ((float)(fragment->x + x)) * xscale, .5f + ((float)(fragment->y + y)) * yscale);
+			color = sample_grid_bilerp(ctxt, 1.f + ((float)(fragment->x + x)) * xscale, 1.f + ((float)(fragment->y + y)) * yscale);
 			til_fb_fragment_put_pixel_unchecked(fragment, 0, fragment->x + x, fragment->y + y, color);
 		}
 	}