Blur frame buffer content

Question

Can I blur not a texture but the frame buffer? The following shader blurs a texture:

#ifdef GL_ES
precision mediump float;
#endif

varying vec4 v_fragmentColor;
varying vec2 v_texCoord;

uniform vec2 resolution;
uniform float blurRadius;
uniform float sampleNum;

vec3 blur(vec2);

void main(void)
{
    vec3 col = blur(v_texCoord);
    gl_FragColor = vec4(col, 1.0) * v_fragmentColor;
}

vec3 blur(vec2 p)
{
    if (blurRadius > 0.0 && sampleNum > 1.0)
    {
        vec3 col = vec3(0);
        vec2 unit = 1.0 / resolution.xy;

        float r = blurRadius;
        float sampleStep = r / sampleNum;

        float count = 0.0;

        for(float x = -r; x < r; x += sampleStep)
        {
            for(float y = -r; y < r; y += sampleStep)
            {
                float weight = (r - abs(x)) * (r - abs(y));
                col += texture2D(CC_Texture0, p + vec2(x * unit.x, y * unit.y)).rgb * weight;
                count += weight;
            }
        }

        return col / count;
    }

    return texture2D(CC_Texture0, p).rgb;
}

How can I blur not a texture pixels, but Frame Buffer pixels that is already drawn?

Answer 1

You can apply that blur shader to texture that you've attached to the framebuffer as drawing to the framebuffer writes the pixel data to the selected colour attachment.

This page provides info on one way that it can be done by drawing the framebuffers colour attachment to the screen with a shader. It's in opengl 3.3 though but all the functions used should exist in opengl es 2.0.