[Mesa-dev] [PATCH] vbo: fix another GL_LINE_LOOP bug

[Brian Paul]

On 11/04/2015 01:56 PM, Charmaine Lee wrote:
>
>> ________________________________________
>> From: Brian Paul <brianp at vmware.com>
>> Sent: Wednesday, November 4, 2015 12:03 PM
>> To: Charmaine Lee; mesa-dev at 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