On 11/04/2015 01:56 PM, Charmaine Lee wrote:
________________________________________
From: Brian Paul <bri...@vmware.com>
Sent: Wednesday, November 4, 2015 12:03 PM
To: Charmaine Lee; mesa-dev@lists.freedesktop.org
Subject: Re: [Mesa-dev] [PATCH] vbo: fix another GL_LINE_LOOP bug
On 11/04/2015 12:40 PM, Charmaine Lee wrote:
I concur with Sinclair and Roland that the vbo code is quite tricky.
I do have a question, so when the line loop spans multiple vertex buffers,
where is 0th vertex stored exactly?
It depends. If the line loop started in a new/empty VB, the 0th vertex
is at the start of the buffer. But if the line loop started later in
the buffer (because another primitive before it), the 0th vertex will be
at the prim->start position.
I didn't find the bug that this patch fixes earlier, because my test
programs all hit the first case. We have to draw at least two
primitives to hit some of the corner cases.
In vbo_exec_End, you are changing src = exec->vtx.buffer_map + last_prim->start *
exec->vtx.vertex_size;
If the 0th vertex is always copied to the beginning of the buffer, why do you
need this change?
To account for the second case, above.
If it was not copied to the beginning of the buffer, couldn't it be overwritten
by the subsequent vertices?
This patch actually fixes a case where subsequent vertices _were_
overwriting previous vertices when another primitive followed the line
loop. That's the change at line 846.
So if last_prim->start is 100, and if the line loop spans 3 vertex buffers,
so at the second vbo_exec_wrap_buffers(), can't the original vertex 0 which
is at buffer_map + 100 * vertex_size be already overwritten by the
vertices that are now in the second vertex buffer?
When the VB is filled, we convert the partial line loop to a line strip
and draw it. Then, we reset the VB and copy two vertices into it, and
resume the glBegin/End primitive. By time we start filling the second
VB, the only thing we care about is the two vertices we just copied into it.
-Brian
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