// maximum size supported by this shader
const int MaxKernelSize = 25;

// array of offsets for accessing the base image
uniform vec2 Offset[MaxKernelSize];

// size of kernel (width * height) for this execution
uniform int KernelSize;

// value for each location in the convolution kernel
uniform vec4 KernelValue[MaxKernelSize];

// image to be convolved
uniform sampler2D BaseImage;

void main()
{
    int i;
    vec4 sum = vec4(0.0);

    for (i = 0; i < KernelSize; i++)
    {
        vec4 tmp = texture2D(BaseImage, gl_TexCoord[0].st + Offset[i]);
        sum += tmp * KernelValue[i];
    }
    gl_FragColor = sum;
}

#version 110

const int KernelSize = 9;

uniform sampler2D samplerTexture;
uniform vec4 KernelValues[KernelSize];
uniform vec2 Offset[KernelSize];

vec4 convolution(void)
{
    //
    // Todo Convolution Filter
    //
   vec4 col = vec4(0.0, 0.0, 0.0, 0.0);

   for (int i = 0; i < KernelSize; i++)
    {
      col += texture2D(samplerTexture, gl_TexCoord[0].st + Offset[i]) * KernelValues[i];
   }
   return col;
}


void main(void)
{
    gl_FragColor = convolution();
    return;
}

//assume 2500px*1250px pictures, because we can't read size of picture in shader
void
loadShaderOffset()
{
   float offsets[18];

   for (int y=-1; y<=1; y++){
      for (int x=-1; x<=1; x++){
         int idx = (y+1)*3 + (x+1);

         offsets[2*idx]   = x/2500.0;
         offsets[2*idx+1] = y/1250.0;
      }
   }

   glUseProgramObjectARB(g_filterProgram);
   int offsetFBO = glGetUniformLocationARB(g_filterProgram, "Offset");
   glUniform4fvARB(offsetFBO, sizeof(offsets), &offsets[0]);
   glUseProgramObjectARB(0);
}

void
loadFilter(float filter[9])
{
   float offsets[36];

   for (int y=-1; y<=1; y++){
      for (int x=-1; x<=1; x++){
         int idx = (y+1)*3 + (x+1);
         
         offsets[4*idx]   = offsets[4*idx+1] = offsets[4*idx+2] = offsets[4*idx+3] = filter[idx];
      }
   }

   glUseProgramObjectARB(g_filterProgram);
   int kernelFBO = glGetUniformLocationARB(g_filterProgram, "KernelValues");
   glUniform4fvARB(kernelFBO, sizeof(offsets), &offsets[0]);
   glUseProgramObjectARB(0);
}

...
float kernel[9] = {0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0};
loadFilter(&kernel[0]);