til: fixup til_fb_fragment_t.texture fragmenting

Until now when fragmenting with a texture present the texture
pointer was simply copied through to the new logical fragment.
The problem with that is when sampling pixels from the texture in
a nested frame scenario, the locations didn't align with the
placement of the logical fragment.

With this change when the incoming fragment has a texture, the
output fragment gets some uninitialized memory attached in the
outgoing fragment's texture pointer.  Then the fragmenter is
expected to do the same populating of res_fragment->texture it
already did for res_fragment, just relative to
fragment->texture->{buf,stride,pitch} etc.

It's a bit hairy/janky because til_fb_fragment_t.texture is just
a pointer to another til_fb_fragment_t.  So the ephemeral/logical
fragments fragmenting/tiling produces which tend to just be
sitting on the stack need to get another til_fb_fragment_t
instance somewhere and made available at the ephemeral
til_fb_fragment_t's .texture member.  We don't want to be
allocating and freeing these things constantly, so for now I'm
just ad-hoc stowing the pointer of an adjacent on-stack texture
fragment in the .texture member when the incoming fragment has a
texture.  But this is gross because the rest of the fragment
contents don't get initialized _at_all_, and currently if the
incoming fragment has no texture the res_fragment->texture member
isn't even initialized.  The fragmenters aren't really supposed
to be expecting anything sensible in *res_fragment, but now we're
making use of res_fragment->texture *if* fragment->texture is
set.  This is just gross.

So there's a bunch of asserts sprinkled around to help police
this fragility for now, but if someone writes new fragmenters
there's a good chance this will trip them up.

1 files changed, 56 insertions, 2 deletions

diff --git a/src/til_fb.c b/src/til_fb.c
index 99bf4d3..536a6f2 100644
--- a/src/til_fb.c
+++ b/src/til_fb.c
@@ -575,11 +575,38 @@ int til_fb_fragment_slice_single(const til_fb_fragment_t *fragment, unsigned n_f
 	unsigned	slice = fragment->height / n_fragments;
 	unsigned	yoff = slice * number;
 
+	assert(fragment);
+	assert(res_fragment);
+
 	if (yoff >= fragment->height)
 		return 0;
 
+	if (fragment->texture) {
+		assert(res_fragment->texture);
+		assert(fragment->frame_width == fragment->texture->frame_width);
+		assert(fragment->frame_height == fragment->texture->frame_height);
+		assert(fragment->width == fragment->texture->width);
+		assert(fragment->height == fragment->texture->height);
+		assert(fragment->x == fragment->texture->x);
+		assert(fragment->y == fragment->texture->y);
+
+		*(res_fragment->texture) = (til_fb_fragment_t){
+				.buf = fragment->texture->buf + yoff * fragment->texture->pitch,
+				.x = fragment->x,
+				.y = yoff,
+				.width = fragment->width,
+				.height = MIN(fragment->height - yoff, slice),
+				.frame_width = fragment->frame_width,
+				.frame_height = fragment->frame_height,
+				.stride = fragment->texture->stride,
+				.pitch = fragment->texture->pitch,
+				.cleared = fragment->texture->cleared,
+		};
+
+	}
+
 	*res_fragment = (til_fb_fragment_t){
-				.texture = fragment->texture,
+				.texture = fragment->texture ? res_fragment->texture : NULL,
 				.buf = fragment->buf + yoff * fragment->pitch,
 				.x = fragment->x,
 				.y = yoff,
@@ -602,6 +629,9 @@ int til_fb_fragment_tile_single(const til_fb_fragment_t *fragment, unsigned tile
 	unsigned	w = fragment->width / tile_size, h = fragment->height / tile_size;
 	unsigned	x, y, xoff, yoff;
 
+	assert(fragment);
+	assert(res_fragment);
+
 	if (w * tile_size < fragment->width)
 		w++;
 
@@ -617,8 +647,32 @@ int til_fb_fragment_tile_single(const til_fb_fragment_t *fragment, unsigned tile
 	xoff = x * tile_size;
 	yoff = y * tile_size;
 
+	if (fragment->texture) {
+		assert(res_fragment->texture);
+		assert(fragment->frame_width == fragment->texture->frame_width);
+		assert(fragment->frame_height == fragment->texture->frame_height);
+		assert(fragment->width == fragment->texture->width);
+		assert(fragment->height == fragment->texture->height);
+		assert(fragment->x == fragment->texture->x);
+		assert(fragment->y == fragment->texture->y);
+
+		*(res_fragment->texture) = (til_fb_fragment_t){
+					.buf = fragment->texture->buf + (yoff * fragment->texture->pitch) + (xoff),
+					.x = fragment->x + xoff,
+					.y = fragment->y + yoff,
+					.width = MIN(fragment->width - xoff, tile_size),
+					.height = MIN(fragment->height - yoff, tile_size),
+					.frame_width = fragment->frame_width,
+					.frame_height = fragment->frame_height,
+					.stride = fragment->texture->stride + (fragment->width - MIN(fragment->width - xoff, tile_size)),
+					.pitch = fragment->texture->pitch,
+					.cleared = fragment->texture->cleared,
+				};
+
+	}
+
 	*res_fragment = (til_fb_fragment_t){
-				.texture = fragment->texture,
+				.texture = fragment->texture ? res_fragment->texture : NULL,
 				.buf = fragment->buf + (yoff * fragment->pitch) + (xoff),
 				.x = fragment->x + xoff,
 				.y = fragment->y + yoff,