How would I create a function to return a Look-up table containing the transfer function for increasing/decreasing the brightness of an image in a given amount?

I am trying to create a function that returns a Look-up table containing the transfer function for increasing/decreasing brightness as follows;

   if inputvalue < -c
        outputvalue = 0
    else if inputvalue > 255 - c
        outputvalue = 255
    else
        outputvalue = inputvalue + c

Here is my attempt...

    function Lut = brightnessLUT(c)
    Lut = 1:256;
    if c < 0
        Lut = 0;
    else if c > 255
        Lut = 1:256;
    else
        Lut = 1:c;
    end
    Lut = uint8(Lut);
    end

Would this be correct? When I use another function I to enhance an inputted image with the amount of c it doesn't seem to affect the brightness at all and the inputted image just stays the same. Any help would be great.

    function Iout = enhanceBrightness(Iin,c)
    Lut = brightnessLUT(c);
    Iout = intlut(Iin,Lut);
    end