How to threshold a specific color in OpenGL/GLSL

Question

I made a particle trail effect in OpenGL.

The trick is to draw a semi transparent rectangle on the screen to gradually fade the previous frame:

    ofSetColor(0,0,0,255*(1.0-persistence));
    ofFill();
    ofRect(0, 0, ofGetScreenWidth(), ofGetScreenHeight());

and then draw the particles (roughly):

    ofEnableBlendMode(OF_BLENDMODE_ADD);    
    ofEnablePointSprites();
    vbo.draw(GL_POINTS, 0, (int)points.size());
    ofDisablePointSprites();

The problem is that the trails do not fade until being pure black, but remain grayish.

I could threshold the dark gray values, or fade them progressively, but what is the best way to do it? OpenGL blending functions/operations or shader? Speed is a major concern.

Answer 1

You need to allocate your fbo with a floating point format.

There is an example in openFrameworks examples directory that shows the difference between floating point and non floating point format and it seems to be your problem ( examples/gl/fboTrailsExample ).

Allocation with a floating point format:

ofFbo rgbaFboFloat;
rgbaFboFloat.allocate(400, 400, GL_RGBA32F_ARB);

and then in the update method you can draw to the fbo between rgbaFboFloat.begin(); and rgbaFboFloat.end();

To finish, you draw your fbo in the draw method: rgbaFboFLoat.draw(0,0);