[Mesa-dev] GLSL 1.40 and fixed function

[Paul Berry]

On 16 April 2012 09:44, Ian Romanick <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.
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