Another GLSL question

think maybe I should try asking something more pointed.

I'm really struggling with glsl and I know I'm making a huge problem for myself since I'm also using metal in runtime for rendering but it doesn't matter if I set the renderer to opengl either I get the same result.

So to simplify life I found a shader on a site called "shadertoy" It creates a cartoony water effect and I'd like to use it. But I get errors when I do. This is the shader:

#define TAU 6.28318530718
#define MAX_ITER 5


void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
  float time = iGlobalTime * .5+23.0;
    // uv should be the 0-1 uv of texture...
  vec2 uv = fragCoord.xy / iResolution.xy;

#ifdef SHOW_TILING
  vec2 p = mod(uv*TAU*2.0, TAU)-250.0;
#else
    vec2 p = mod(uv*TAU, TAU)-250.0;
#endif
  vec2 i = vec2(p);
  float c = 1.0;
  float inten = .005;


  for (int n = 0; n < MAX_ITER; n++)
  {
  float t = time * (1.0 - (3.5 / float(n+1)));
  i = p + vec2(cos(t - i.x) + sin(t + i.y), sin(t - i.y) + cos(t + i.x));
  c += 1.0/length(vec2(p.x / (sin(i.x+t)/inten),p.y / (cos(i.y+t)/inten)));
  }
  c /= float(MAX_ITER);
  c = 1.17-pow(c, 1.4);
  vec3 colour = vec3(pow(abs(c), 8.0));
    colour = clamp(colour + vec3(0.0, 0.35, 0.5), 0.0, 1.0);



  #ifdef SHOW_TILING
  // Flash tile borders...
  vec2 pixel = 2.0 / iResolution.xy;
  uv *= 2.0;


  float f = floor(mod(iGlobalTime*.5, 2.0)); // Flash value.
  vec2 first = step(pixel, uv) * f;   // Rule out first screen pixels and flash.
  uv  = step(fract(uv), pixel); // Add one line of pixels per tile.
  colour = mix(colour, vec3(1.0, 1.0, 0.0), (uv.x + uv.y) * first.x * first.y); // Yellow line

  #endif
  fragColor = vec4(colour, 1.0);
}

I save it in a file called water.shader (but I have tried water.glsl too.)

In my container object for scnnodes I have a method I call :

-(void)initModifiers
{
    SCNNode * node = self.nodeObject;
    _surfModifier = [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"sm_surf" ofType:@"shader"] encoding:NSUTF8StringEncoding error:nil];
    _lightModifier= [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"sm_light" ofType:@"shader"] encoding:NSUTF8StringEncoding error:nil];

    _geomModifier = [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"sm_geom" ofType:@"shader"] encoding:NSUTF8StringEncoding error:nil];
    _fragModifier = [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"sm_frag" ofType:@"shader"] encoding:NSUTF8StringEncoding error:nil];
    _waterModifier = [NSString stringWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"water" ofType:@"shader"] encoding:NSUTF8StringEncoding error:nil];
    [node.geometry setValue:@0.0 forKey:@"Amplitude"];
    [node.geometry setValue:@0.0 forKey:@"lightIntensity"];
    [node.geometry setValue:@0.0 forKey:@"surfIntensity"];
    [node.geometry setValue:@0.0 forKey:@"fragIntensity"];
}

This is nearly a direct c&p from an apple example

Then for other modifiers, specifically one that uses surfmodifier and lightmodifier I can call it with:

-(void)addWaterToNode:(SCNNode*)node
{
    [SCNTransaction begin];
    [SCNTransaction setAnimationDuration:3];
    [node.geometry setValue:@0.25 forKey:@"Amplitude"];
    [SCNTransaction setCompletionBlock:^{
        [SCNTransaction begin];
        [SCNTransaction setAnimationDuration:3];
      
        node.geometry.shaderModifiers = @{SCNShaderModifierEntryPointSurface : _surfModifier,
                                          SCNShaderModifierEntryPointLightingModel : _lightModifier};
        [node.geometry setValue:@0.25 forKey:@"surfIntensity"];
        [SCNTransaction commit];
    }];
  
    [SCNTransaction commit];
}

The combination _surfModifier and _lightModifier has a water-ish look but it's not very good.

But the problem is the water modifier that I listed above, I don't know how to call it or add it to the geometry so that I can use it instead. Or honestly any shader at all even if I write it myself I guess I don't understand how to use them.