Help with fragment shaders

Programming Questions

digitalte..

in Programming Questions • 11 months ago

I'm rather new to shaders and am having some difficulty with the whole thing. I made a basic julia fractal explorer and I got it to work in OpenFrameworks, but it doesn't work in processing (it is giving me the error "Object is not a valid shader"). Any idea why?

Processing code:

PShader shades;
void setup(){
size(640,480,P2D);
shades=loadShader("shader.glsl");
}
void draw(){
background(255);
shades.set("iterations",40.0);
shades.set("c",map(mouseX,0,width,-2,2),map(mouseY,0,width,-2,2));
shades.set("xRange",-2,2,width);
shades.set("yRange",-2,2,height);
filter(shades);
}

Shader code:

uniform float iterations;
uniform vec2 c;
uniform vec3 xRange,yRange; // form of {x1,x2,width} where the point=map(x-coord,0,width,x1,x2)
void main() {
float x=gl_FragCoord.x;
float y=gl_FragCoord.y;
x=mix(xRange.x,xRange.y,x/xRange.z);
y=mix(yRange.x,yRange.y,y/yRange.z);
float col=0.0,i=0.0;
float nx=x, x2=x,ny=y;
gl_FragColor=vec4(0.0,0.0,0.0,1.0);
for(i=0.0;i<iterations;i++){
x2=x;
x=x*x-y*y+c.x;
y=2.0*x2*y+c.y;
if(x*x+y*y>4.0){
col=1.0-i/iterations;
gl_FragColor=vec4(col,col,col,1.0);
break;
}
}
}

Thank you!

Replies(2)

velvetkev..

Re: Help with fragment shaders

11 months ago

Trying to get my head round this myself, but I believe (based on this post and this one) that there are certain variables which have to be exposed in the shader so Processing knows how to deal with them. Otherwise Processing reverts to the default shaders. Hopefully we'll get some better documentation for the PShader stuff soon.

andres

Re: Help with fragment shaders

11 months ago

The filter(PShader) function is meant to apply post-processing effects on the current frame, and the shaders to use with it should have a texture sampler to read the frame image from.

What you are doing is closer to the procedural shaders that generate geometry entirely in the fragment shader. These shaders need you to draw a rectangle covering the entire screen so each pixel is processed once. The following sketch code should work:

PShader shades;
void setup(){
size(640, 480, P2D);
shades = loadShader("shader.glsl");
}
void draw(){
background(255);
shades.set("iterations",40.0);
shades.set("c",map(mouseX,0,width,-2,2),map(mouseY,0,width,-2,2));
shades.set("xRange",-2.0,2.0,float(width));
shades.set("yRange",-2.0,2.0,float(height));
shader(shades);
rect(0, 0, width, height);
}

Also note that I'm making sure that the arguments of the set("xRange") and set("yRange") are floats, otherwise Processing will think that the uniform type for those two are int[3] instead of vec3.

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