[Mesa-dev] [PATCH] mesa/vbo: Treat attribute 0 and vertex as the same

[Brian Paul]

On 10/13/2011 05:34 PM, Ian Romanick wrote:
> From: Ian Romanick<ian.d.romanick at intel.com>
>
> This is supported by the pseudo-code on pages 27 and 28 (pages 41 and
> 42 of the PDF) of the OpenGL 2.1 spec.  The last part of the
> implementation of ArrayElement is:
>
>      if (generic attribute array 0 enabled) {
>        if (generic vertex attribute 0 array normalization flag is set, and
> 	  type is not FLOAT or DOUBLE)
> 	VertexAttrib[size]N[type]v(0, generic vertex attribute 0 array element i);
>        else
> 	VertexAttrib[size][type]v(0, generic vertex attribute 0 array element i);
>      } else if (vertex array enabled) {
>        Vertex[size][type]v(vertex array element i);
>      }
>
> Page 23 (page 37 of the PDF) of the same spec says:
>
>      "Setting generic vertex attribute zero specifies a vertex; the
>      four vertex coordinates are taken from the values of attribute
>      zero. A Vertex2, Vertex3, or Vertex4 command is completely
>      equivalent to the corresponding VertexAttrib* command with an
>      index of zero."
>
> Fixes piglit test attribute0.
>
> NOTE: This is a candidate for stable branches.
>
> Signed-off-by: Ian Romanick<ian.d.romanick at intel.com>
> ---
>   src/mesa/vbo/vbo_exec_array.c |    3 ++-
>   1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/src/mesa/vbo/vbo_exec_array.c b/src/mesa/vbo/vbo_exec_array.c
> index 18719d5..ceb6a64 100644
> --- a/src/mesa/vbo/vbo_exec_array.c
> +++ b/src/mesa/vbo/vbo_exec_array.c
> @@ -538,7 +538,7 @@ recalculate_input_bindings(struct gl_context *ctx)
>            }
>         }
>
> -      for (i = 0; i<  MAX_VERTEX_GENERIC_ATTRIBS; i++) {
> +      for (i = 1; i<  MAX_VERTEX_GENERIC_ATTRIBS; i++) {
>   	 if (exec->array.generic_array[i]->Enabled)
>   	    inputs[VERT_ATTRIB_GENERIC0 + i] = exec->array.generic_array[i];
>   	 else {
> @@ -547,6 +547,7 @@ recalculate_input_bindings(struct gl_context *ctx)
>            }
>         }
>
> +      inputs[VERT_ATTRIB_GENERIC0] = inputs[0];
>         ctx->NewState |= _NEW_ARRAY;
>         break;
>      }

Tested-by: Brian Paul <brianp at vmware.com>


This looks good to me.  I hacked the attribute0 test to do some 
additional experiments and it worked as expected.

There's two interesting points (IMHO) to note regarding generic 
attribute 0 that I don't think are spelled out in the specs.  Please 
correct me if I'm wrong.

1. If a vertex shader uses gl_Vertex then the first generic attribute 
that's allocated will be 1.  But if the gl_Vertex is not used, the 
first generic attribute that's allocated is 0.  If quering 
GL_MAX_VERTEX_ATTRIBS returns 16, you really only have 15 generic 
attribs available if gl_Vertex is used.

2. If gl_Vertex is not used in the vertex shader, you can only draw 
with vertex arrays - you can't use glBegin/End rendering.  Section 2.7 
of the OpenGL 2.1 spec it says that glVertexAttrib(0, ...) specifies a 
vertex so if you have a vertex shader like this:

   attribute vec4 vertex;
   attribute vec4 color;
   void main() {
      gl_Position = vertex;
      gl_FrontColor = color;
   }

and draw like this:

   vertPos = glGetAttribLocation(prog, "vertex");
   colPos = glGetAttribLocation(prog, "color");
   glBegin(GL_TRIANGLE_STRIP);
   for (i = 0; i < n; i++) {
      glVertexAttrib3fv(colAttr, colors[i]);
      glVertexAttrib2fv(vertAttr, verts[i]);
   }
   glEnd();

It may or may not work depending on whether "vertex" is in location 0 
or 1.

-Brian