I Can't Escape: Darkness Indirect Lighting Shader

varying vec2 vcoord; //passed from the vertex shader and is pos.xy*0.5+0.5, where pos is the vertex coordinate of the screen quad used to render the pass.
uniform sampler2D color_buffer; //the back buffer (the previous pass's lighting).
uniform sampler2D mask; //the conduction mask (with the conduction in the alpha channel because it is a GL_ALPHA texture).
uniform vec2 radius; //vec2(1.0/mask.width, 1.0/mask.height) - the size of a single pixel
void main(void){
    vec4 ret = vec4(0.0); float sum = 0.0; vec2 coord = vcoord; float me = texture2D(mask,coord).a;
    //left pixel (weight 2)
    coord.x -= radius.x; float m = me*texture2D(mask,coord).a*2.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //top left pixel (weight 4)
    coord.y -= radius.y; m = me*texture2D(mask,coord).a*4.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //bottom left pixel (weight 4)
    coord.y += radius.y*2.0; m = me*texture2D(mask,coord).a*4.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //center pixel (weight 1)
    coord.y -= radius.y; coord.x += radius.x; sum += 1.0; ret += texture2D(color_buffer,coord);
    //top pixel (weight 2)
    coord.y -= radius.y; m = me*texture2D(mask,coord).a*2.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //bottom pixel (weight 2)
    coord.y += radius.y*2.0; m = me*texture2D(mask,coord).a*2.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //right pixel (weight 2)
    coord.y -= radius.y; coord.x += radius.x; m = me*texture2D(mask,coord).a*2.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //top right pixel (weight 4)
    coord.y -= radius.y; m = me*texture2D(mask,coord).a*4.0; sum += m; ret += m*texture2D(color_buffer,coord);
    //bottom right pixel (weight 4)
    coord.y += radius.y*2.0; m = me*texture2D(mask,coord).a*4.0; sum += m; ret += m*texture2D(color_buffer,coord);
    sum = 1.0/sum; gl_FragColor = ret*sum;
}