Recursion in GLSL prohibited?

Question

I ran into this error when trying to write the following recursive call. I have seen a lot of demos of implementations of recursive Ray tracing in GLSL so I assumed that GLSL supported recursion.

Is this not the case?

OpenGL is returning a compile time error message:

Error: Function trace(vec3, vec3, vec3, int) has static recursion

This is my function definition:

vec3 trace(vec3 origin, vec3 direction, vec3 illum, int order) 
{       
   float dist;  
   int s_index = getSphereIntersect(origin, direction, dist);   
   //if light hit
   float light_dist = 200;
   for(int k = 0; k < L_COUNT;k++)      
       if(s_intersects(l_center[k], l_radius[k], 
             origin, direction, 
             light_dist)) 
             if(light_dist < dist )             
                 return l_color[k]; //light is pure color  

   if (s_index != -1)
   {
       illum = s_color[s_index];
       for(int j = 0; j < L_COUNT; j++)
       {
           float ambient = 0.68;
           float diffuse = 0.5;
           vec3 poi = view + (direction * dist); 
           vec3 li_disp = normalize( poi - l_center[j]); 
           vec3 poi_norm = s_normal(s_center[s_index], s_radius[s_index], poi); 
            float shade=  dot(li_disp, normalize(poi_norm)); 
            if(shade < 0) shade = 0;
            illum = illum*l_color[j]*ambient + diffuse * shade; 
            //test shadow ray onto objects, if shadow then 0    
            if(order > 0)
                  illum = trace(poi+.0001*poi_norm, poi_norm, illum, order-1); 
        }   
    }   
    else
        illum = vec3(0,0,0);
    return illum; 
}

If you know maximum depth of your recursion you can replace it with a loop and an array served as a stack — Mr D, May 01 '17 at 06:09
@MrD Yep here small example of it [Reflection and refraction impossible without recursive ray tracing?](https://stackoverflow.com/a/45140313/2521214) — Spektre, Jul 21 '17 at 07:16

vallentin · Accepted Answer · 2018-06-27T21:46:12.260

I assumed that GLSL supported recursion

No. GLSL doesn't support or better said allow recursive function calls.

GLSL does not. The GLSL memory model does not allow for recursive function calls. This allows GLSL to execute on hardware that simply doesn't allow for recursion. It allows GLSL to function when there is no ability to write arbitrarily to memory, which is true of most shader hardware (though it is becoming less true with time).

So, no recursion in GLSL. Of any kind.

– OpenGL Wiki – Core Language (GLSL)

and

Recursion is not allowed, not even statically. Static recursion is present if the static function-call graph of a program contains cycles. This includes all potential function calls through variables declared as subroutine uniform (described below). It is a compile-time or link-time error if a single compilation unit (shader) contains either static recursion or the potential for recursion through subroutine variables.

– GLSL 4.5 Specification, Page 115

Some recursive functions can be converted into non-recursive functions using [tail call](https://en.wikipedia.org/wiki/Tail_call) elimination, but GLSL does not yet have this feature. — Anderson Green, Jul 24 '19 at 18:51

Recursion in GLSL prohibited?

1 Answers1