GLSL: Access to implicitly-sized array with function parameter

Question

I have the following code segment in a GLSL 4.0 core geometry shader:

void pushVertex(const int i)
{
    gl_Position = gl_in[i].gl_Position;
    // Some other stuff
    EmitVertex();
}

Later I basically call pushVertex(0); pushVertex(1); and so on.

This actually works on Windows with Nvidia drivers, however MaxOSX throws the error Indirect index into implicitly-sized array. Any ideas on a workaround?

Chris Dodd · Accepted Answer · 2016-05-20 22:58:41Z

1

Basically, you need to convince the GLSL compiler to inline this function so that the index becomes a constant, and indirect indexing isn't needed. Since the compiler apparently doesn't do it for you, you could try defining pushVertex as a macro:

#define pushVertex(I) if(1) {              \
    gl_Position = gl_in[I].gl_Position;    \
    /* other stuff */                      \
    EmitVertex();                          \
} else

Of course, if the compiler is so weak that it can't inline, it may have trouble with the macro.

answered May 20, 2016 at 22:58

Chris Dodd

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5 Comments

Marius Over a year ago

Yeah I solved it with a macro. I actually didn't know that GLSL supports macros to the extent of the C preprocessor. You might want to remove the trailing else though.

Marius Over a year ago

Oh okay, but wouldn't it work without the else? Wouldn't it also work without the if and just with curly braces?

Chris Dodd Over a year ago

Sure, as long as you don't try to use it like if (p) pushVertex(1); else pushVertex(2);

Marius Over a year ago

Oh I see. That is also the reason why people do it with do-while: #define doSomething(...) do {...} while(0). Honestly I think that is less confusing than the trailing else statement. Also: Macro programming has so many pitfalls that even I with now 10 years of C++ experience just learned something :D

Chris Dodd Over a year ago

The problem with do..while is that it's more likely to break from failing to optimize properly on a broken GLSL compiler that can't inline properly...

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