How to create gamma (midtone) changes with help Pixel Bender?

Question

I'd like to create Level correction with help Pixel Bender. I made correction for each channel (r, g, b), it works. But I don't know how to make correction for midtone for each one.
Is there some formula or example?
Unfortunately google hasn't got solution
I use the next code for each channel, for instance, the red channel will be look like
dst.r = ((dst.r-red[0])/(red[1]-red[0])-rgb[1]-rgb[0]), where red[n], rgb[n] are the values which can be changed by user

Answer 1

thanks for your reply! so, if i wanna change gamma for each channel separately i have to change your code so:
dst.rgb = pow(dst.rgb, float3(red[1], green[1], blue[1])/gammaRGB)
where red[1], green[1], blue[1] are the parameters of gamma
i compared the result which i've got and it's different from photoshop's
so, now my code looks like this
    parameter float3 red
    <
        minValue: float3(0.0,0.0,1.0);
        maxValue: float3(1.0,1.0,1.0);
        defaultValue: float3(0.0,1.0,1.0);
    >;
    parameter float3 green
    <
        minValue: float3(0.0,0.0,0.0);
        maxValue: float3(1.0,1.0,1.0);
        defaultValue: float3(0.0,1.0,1.0);
    >;
    parameter float3 blue
    <
        minValue: float3(0.0,0.0,0.0);
        maxValue: float3(1.0,1.0,1.0);
        defaultValue: float3(0.0,1.0,1.0);
    >;
    parameter float3 RGB
    <
        minValue: float3(0.0,0.0,0.0);
        maxValue: float3(1.0,1.0,1.0);
        defaultValue: float3(0.0,1.0, 1.0);
    >;
    input image4 src;
    output pixel4 dst;
    void
    evaluatePixel()
        pixel4 p = sampleNearest(src,outCoord());
        float rgbdiff = RGB[2] - RGB[0];
        p.r = ( (p.r - red[0])/(red[2] - red[0]) - RGB[0])/rgbdiff;
        p.g = ( (p.g - green[0])/(green[2] - green[0]) - RGB[0])/rgbdiff;
        p.b = ( (p.b - blue[0])/(blue[2] - blue[0]) - RGB[0])/rgbdiff;
        p.rgb = pow(p.rgb, float3(red[1], green[1], blue[1]) / RGB[1]);
        dst = p;
upd: i tried to apply this filter again and it seems to me it works! Thank you very much for your correct answer. Maybe you know how to use photoshop's 'output panel', this parameter increases/decreases values of channel. Photoshop has this parameter in the bottom of level's panel.