Converting shaders from other programs?

Question

I'm just getting started with Pixel bender, but I've played around with fairly simplified GLSL Shaders in Max/MSP and Quartz Composer before. I'm trying to convert this one shader for use in after effects, but I'm a bit stuck on converting some of the variable types and things like that. I'm also not sure if the effect is even possible to recreate in after effects because it requires a sort of feedback loop when I use it in Max/MSP. The effect basically creates video trails off of bright elements in the scene. Here is the code as it appears for the max/msp fragment shader. If anyone has some hints of whether it looks possible, let me know
uniform float slide_up;
uniform float slide_down;
varying vec2 texcoord0;
varying vec2 texcoord1;
uniform sampler2DRect tex0;
uniform sampler2DRect tex1;
void main(void)
    vec4 su, sd, up, down, amount;
    // sample inputs at texcoords
    vec4 input0 = texture2DRect(tex0, texcoord0);
    vec4 input1 = texture2DRect(tex1, texcoord1);
    // get contribution
    amount.x = input0.x > input1.x ? 1.0 : 0.0;
    amount.y = input0.y > input1.y ? 1.0 : 0.0;
    amount.z = input0.z > input1.z ? 1.0 : 0.0;
    amount.w = input0.w > input1.w ? 1.0 : 0.0;
    // calculate slide down
    float d = max(1.0, abs(slide_down));
    sd = vec4(1.0 / d);
    down = input1 + ((input0 - input1) * sd);
    // calculate slide up
    float u = max(1.0, abs(slide_up));
    su = vec4(1.0 / u);
    up = input1 + ((input0 - input1) * su);
    // mix between down and up
    gl_FragColor = mix(down, up, amount);

Answer 1

Actually I managed to convert the right areas, but now I gues my question is more about setting up a feedback loop in after effects so that the effect can accumulate. Is this a possibility or should I be doing it another way? This is the code that I ended up with:
    input image4 src;
    input image4 feedback;
    output pixel4 dst;
    parameter float slide_up;
    parameter float slide_down;
    void
    evaluatePixel()
        float4 su, sd, amount, up, down;
    // sample inputs at texcoords
    float4 input0 = sampleNearest(src, outCoord());
    float4 input1 = sampleNearest(feedback, outCoord());
    // get contribution
    amount.x = input0.x > input1.x ? 1.0 : 0.0;
    amount.y = input0.y > input1.y ? 1.0 : 0.0;
    amount.z = input0.z > input1.z ? 1.0 : 0.0;
    amount.w = input0.w > input1.w ? 1.0 : 0.0;
    // calculate slide down
    float d = max(1.0, abs(slide_down));
    sd = float4(1.0 / d);
    down = input1 + ((input0 - input1) * sd);
    // calculate slide up
    float u = max(1.0, abs(slide_up));
    su = float4(1.0 / u);
    up = input1 + ((input0 - input1) * su);
    // mix between down and up
        dst = mix(down, up, amount);