Question

I am trying to write a blur shader for the iPad. I have it working but I am not very happy with the results. I get very choppy frame rates and the blur looks like crap when blur amount is high.

Any ideas on how to improve things?

Some sample output:

uniform sampler2D texture;
varying mediump vec2 fragTexCoord;
varying mediump vec3 eyespaceNormal;

varying highp float blurAmount;

void main(void)
{
    highp vec2 gaussFilter[7];
    gaussFilter[0] = vec2(-3.0, 0.015625);
    gaussFilter[1] = vec2(-2.0, 0.09375);
    gaussFilter[2] = vec2(-1.0, 0.234375);
    gaussFilter[3] = vec2(0.0, 0.3125);
    gaussFilter[4] = vec2(1.0, 0.234375);
    gaussFilter[5] = vec2(2.0, 0.09375);
    gaussFilter[6] = vec2(3.0, 0.015625);

    highp float blurSize = blurAmount * 1.0;

    /////////////////////////////////////////////////
    // 7x1 gaussian blur fragment shader
    /////////////////////////////////////////////////

    highp vec4 color = vec4(0,0,0,1);

    for( int i = 0; i < 7; i++ )
    {
        color += texture2D( texture, vec2( fragTexCoord.x+gaussFilter[i].x*blurSize, fragTexCoord.y+gaussFilter[i].x*blurSize ) )*gaussFilter[i].y;
    }

    gl_FragColor = color;
}

Edit:
A box blur may be the way to go.
Here is a box blur version of the shader:

highp vec4 color = vec4(0,0,0,1);

color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y - 4.0*blurAmount)) * 0.05;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y - 3.0*blurAmount)) * 0.09;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y - 2.0*blurAmount)) * 0.12;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y - blurAmount)) * 0.15;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y)) * 0.16;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y + blurAmount)) * 0.15;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y + 2.0*blurAmount)) * 0.12;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y + 3.0*blurAmount)) * 0.09;
color += texture2D(texture, vec2(fragTexCoord.x, fragTexCoord.y + 4.0*blurAmount)) * 0.05;

gl_FragColor = color;

Here is the box blur output(note it’s only a horizontal blur, but it might be enough for what i want) :

Answer 1

That shader needs to run twice for it to work, what you call blurSize should be a vec2 and the value of that should be vec2(0, 1.0/height) for vertical blur and vec2(1.0/width, 0) for horizontal blur.

See http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=240334

The idea behind doing a two pass blur is that it will dramatically reduce the number of texture lookups and hopefully increase the speed. A two pass blur with kernel size of 7×7 will require 14 texture lookups but if it done in a nested loop you’ll need to do 49 texture lookups.