Color blending with OpenGL

Programming Questions

zoltarpod

in Programming Questions • 2 years ago

So, bit of an intro here (this is my first post): I'm a freshman computer science student taking a CS elective course called "Interactive and Generative Art." We're doing a lot of stuff in Processing, and for a project, I'm making a color blending program.

It'll be quicker to explain with screenshots:
http://img28.imageshack.us/i/ex2i.jpg/
(That's not my actual program; I made that in MS Paint)

I'm using the ellipse function to make the colored circles. So my red circle is:
"fill(255, 0, 0);
ellipse(redX, redY, diameter, diameter);" etc. etc.

The blend function I'm currently using is
"gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE);"

That works perfectly for some combinations, for example:
It takes the red circle (255, 0, 0) and the blue circle (0, 0, 255) and comes up with a purple circle (like it should)(255, 0, 255).
But when I blend say, red (255, 0, 0) and yellow (255, 255, 0) I run into problems, because it produces (510, 255, 0). From my observations, it treats anything above 255 as 255, so the result is still yellow.

Does anyone have any suggestions? If the colors were (red, yellow, blue) instead of (red, green, blue) it seems like it'd work almost perfectly, but that's not the case.

Sorry if that's a bit unclear, I can clarify if needed.
Thanks in advance!

Replies(6)

jonobr1

Re: Color blending with OpenGL

2 years ago

Sounds like you need a run down of what the different blend functions do. Check this blog post out as a reference: http://www.machwerx.com/2009/02/11/glblendfunc/

Mix and match to get the ones that you want. It can help if you have Photoshop or Illustrator (Add and Subtract Blend Modes) to give you a space for more explorations in color theory.

Hope that helps.

—
http://jonobr1.com/

zoltarpod

Re: Re: Color blending with OpenGL

2 years ago

Well, I figured out what I need it to do, I need a blend func that takes the mean.
So red (255, 0, 0)
and yellow (255, 255, 0)
would produce (255, 128, 0) which would be orange!

I just can't figure out what function I need. Art isn't my thing, I'm a math/science guy :P

Does anybody know what I would use?

cedrickie..

Re: Color blending with OpenGL

2 years ago

you dont need a function to calculate that color. like jonobr1 mentioned you can use opengl blending.
i made an example sketch..

int numSpots =   3;
Spot[] spots = new Spot[numSpots];

import processing.opengl.*;
import javax.media.opengl.*;
PGraphicsOpenGL pgl;
GL gl;

void setup(){
size(400,400,OPENGL);
smooth();
noStroke();

color[] c=new color[3];
c[0]=color(255,0,0);
c[1]=color(0,255,0);
c[2]=color(0,0,255);

frameRate(125);
for(int i = 0; i<spots.length; i++){
    float x = random(0,width);
    float y= random(0,height);
    float rate = 1.5 + i*random(0.15,0.3);
    spots[i] = new Spot(x,y,220,rate,c[i]);
}
}

void draw(){

pgl = (PGraphicsOpenGL) g;
gl = pgl.gl;
pgl = (PGraphicsOpenGL) g;
gl = pgl.gl;

background(0);

pgl.beginGL();
// This fixes the overlap issue
gl.glDisable(GL.GL_DEPTH_TEST);
// Turn on the blend mode
gl.glEnable(GL.GL_BLEND);
// Define the blend mode
gl.glBlendFunc(GL.GL_SRC_ALPHA,GL.GL_ONE);
pgl.endGL();

for(int i = 0; i<spots.length; i++){
    fill(random(255),random(255),9);
    spots[i].move();
    spots[i].display();

}
}

class Spot{
float x, y;
float rad;
int dirY=1;
int dirX=1;
float speed;
color c;

Spot(float xpos, float ypos,float radius, float sp,color _c){
    x=xpos;
    y=ypos;
    rad=radius;
    speed=sp;
    c=_c;

}

void move(){
    y= y+speed * dirY*0.15 ;
    x= x+speed * dirX ;

    if(y>=height || y<=0){
      dirY =dirY * -1;
    }
    if(x>=width || x<=0){
      dirX =dirX * -1;
    }

}
void display(){
    fill(c);
    ellipse(x,y,rad,rad);

}

}

zoltarpod

Re: Re: Color blending with OpenGL

2 years ago

When I said "blend func," I was talking about one of OpenGL's functions.

The code you posted runs into exactly the same problem as my code does (the colors don't blend like they should, if I change the green circle to yellow, red and yellow make yellow, and not orange).

cedrickie..

Re: Color blending with OpenGL

2 years ago

alright i see.
you need to RYB RGB Conversion i guess
check this out :
http://threekings.tk/mirror/ryb_TR.pdf

zoltarpod

Re: Re: Color blending with OpenGL

2 years ago

Thanks for the link, it shed more light on my situation. Unfortunately, I think that's all a bit too advanced for me right now.

It'd work great, even in RGB mode, if one of OpenGL's blend functions blended the color by taking the mean of the colors. That'd give me the results that I'm looking for.

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