OpenGL two pass shader effect with FBO render to texture gives noise in result on Windows only

Question

What is the correct way of doing the following:

Render a scene into a texture using a FBO (fbo-a)
Then apply an effect using the texture (tex-a) and render this into another texture (tex-b) using the same fbo (fbo-a)
Then render this second texture, with the applied effect (tex-b) as a full screen quad.

My approach is this, but this gives me a texture filled with "noise" on window + the applied effect (all pixels are randomly colored red, green, blue white, black).

I'm using one FBO, with two textures set to GL_COLOR_ATTACHENT0 (tex-a) and GL_COLOR_ATTACHMENT1 (tex-b)
I bind my fbo, make sure it's rendered into the tex-a using glDrawBuffer(GL_COLOR_ATTACHMENT0)
Then I apply the effect in a shader with tex-a bound and set as 'sampler2D'. Using texture unit 1, and switch to the second color attachment (glDrawBuffer(GL_COLOR_ATTACHMENT1)). and render a full screen quad. Everything is now rendered into tex-b
Then I switch back to the default FBO (0) and use tex-b with a full screen quad to render the result.

Example of the result when applying my shader enter noise

This is the shader I'm using. I'm not aware this could be what is causing this, but maybe the noise is caused by a overflow?

Vertex shader

attribute vec4 a_pos; 
attribute vec2 a_tex; 
varying vec2 v_tex; 

void main() { 
    mat4 ident = mat4(1.0); 
    v_tex = a_tex; 
    gl_Position = ident * a_pos; 
}

Fragment shader

uniform int u_mode; 
uniform sampler2D u_texture; 
uniform float u_exposure; 
uniform float u_decay; 
uniform float u_density; 
uniform float u_weight; 
uniform float u_light_x; 
uniform float u_light_y; 
const int NUM_SAMPLES = 100; 
varying vec2 v_tex; 
void main() { 
    if (u_mode == 0) { 
        vec2 pos_on_screen = vec2(u_light_x, u_light_y); 
        vec2 delta_texc = vec2(v_tex.st - pos_on_screen.xy); 
        vec2 texc = v_tex; 
        delta_texc *= 1.0 / float(NUM_SAMPLES) * u_density; 
        float illum_decay = 1.0; 
        for(int i = 0; i < NUM_SAMPLES; i++) { 
            texc -= delta_texc; 
            vec4 sample = texture2D(u_texture, texc); 
            sample *= illum_decay * u_weight; 
            gl_FragColor += sample; 
            illum_decay *= u_decay; 
        } 
        gl_FragColor *= u_exposure; 
    } 
    else if(u_mode == 1) { 
        gl_FragColor = texture2D(u_texture, v_tex); 
        gl_FragColor.a = 1.0; 
    } 

}

I've read this FBO article on opengl.org, where they describe a feedback loop at the bottom of the article. The description is not completely clear to me and I'm wondering if I'm exactly doing what they describe there.

Update 1:

Link to source code

Update 2:

When I first set gl_FragColor.rgb = vec3(0.0, 0.0, 0.0); before I start the sampling loop (with NUM_SAMPLES), it works find. No idea why though.

This looks like it could be an incorrectly bound texture, could you post your FBO/texture setup code? — StuGrey

Chris Dodd Chris Dodd · Accepted Answer · 2012-07-16T18:38:30

The problem is that you're not initializing gl_FragColor, and you're modifying it with the lines

gl_FragColor += sample;

and

gl_FragColor *= u_exposure;

both of which depend on the previous value of gl_FragColor. So you're getting some random junk (whatever happened to be in the register that the shader compiler decided to use for the gl_FragColor computation) added in. This has a strong possibility of working fine on some driver/hardware combinations (because the compiler decided to use a register that was always 0 for some reason) and not on others.

OpenGL two pass shader effect with FBO render to texture gives noise in result on Windows only

1 Answers