[Mesa-dev] GLSL 1.40 and fixed function

[Ian Romanick]

On 04/16/2012 10:01 AM, Paul Berry wrote:
> On 16 April 2012 09:44, Ian Romanick <idr at freedesktop.org
> <mailto:idr at freedesktop.org>> wrote:
>
>     On 04/15/2012 03:25 AM, nobled wrote:
>
>         I'm not sure if I'm missing something, but the non-compatibility
>         OpenGL 3.1 Specification seems pretty straightforward about this:
>
>         page 42/55 section 2.11 "Vertex Shaders":
>         "If the program object
>         has no vertex shader, or no program object is currently in use, the
>         results of vertex
>         shader execution are undefined."
>
>         page 160/173 section 3.9 "Fragment Shaders":
>         "If the program
>         object has no fragment shader, or no program object is currently in
>         use, the results
>         of fragment shader execution are undefined."
>
>         page 45/58 section 2.11.2 "Program Objects":
>         "If UseProgram is called with program set to 0, then the current
>         rendering state refers to an invalid program object, and the results
>         of vertex and
>         fragment shader execution are undefined. However, this is not an
>         error."
>
>
>     Right.  It looks like this language persists all the way through
>     OpenGL 4.2.
>
>     I did a bit more spec spelunking, and it seems the only link error
>     required by desktop OpenGL is for geometry shaders and tessellation
>     shaders.
>
>     "Linking will also fail if the program object contains objects
>         to form a geometry shader (see section 2.12), and
>
>             * the program contains no objects to form a vertex shader;
>
>             ..."
>
>
>     "Linking can fail for a variety of reasons as specified in the
>         OpenGL Shading Language Specification, as well as any of the
>         following reasons:"
>
>             * ...
>
>             * The program object contains objects to form a tessellation
>               control shader (see section 2.12.1), and
>
>                 - the program is not separable and contains no objects
>                   to form a vertex shader;
>
>             ..."
>
>
>         I'm not sure if "undefined" is used in the "undefined rendering
>         results" sense or the "summoning nasal demons" sense, though.
>
>
>     That's not defined. :p  Sorry, I couldn't resist.
>
>     Here's my new proposal:
>
>         * Don't generate a linker error or warning for incomplete programs.
>
>         * For draw calls that use incomplete programs, drop the
>     rendering on the floor and emit a message to the debug log.
>
>
> A point of clarification: if the program is incomplete because it lacks
> a fragment shader, is transform feedback expected to work?  My read of
> the spec is that the phrase "the results of fragment shader execution
> are undefined" means that transform feedback is still expected to work
> in this situation.  So we need to make sure that we only drop fragments
> on the floor, not vertices.

Ah, good catch.  That sounds like a nice test case. :)