how to use a fragment shader without main function - c++

Question

i found a shader to do a drop shadow from http://madebyevan.com/shaders/fast-rounded-rectangle-shadows/
// License: CC0 (http://creativecommons.org/publicdomain/zero/1.0/)
// This approximates the error function, needed for the gaussian integral
vec4 erf(vec4 x) {
vec4 s = sign(x), a = abs(x);
x = 1.0 + (0.278393 + (0.230389 + 0.078108 * (a * a)) * a) * a;
x *= x;
return s - s / (x * x);
}
// Return the mask for the shadow of a box from lower to upper
float boxShadow(vec2 lower, vec2 upper, vec2 point, float sigma) {
vec4 query = vec4(point - lower, upper - point);
vec4 integral = 0.5 + 0.5 * erf(query * (sqrt(0.5) / sigma));
return (integral.z - integral.x) * (integral.w - integral.y);
}
i thought that a shader need a main function and should return color.
My question is how to use the function boxShadow in c++ code with opengl given a box
thanks

Answer 1

A function can be defined in a shader just as you would do with a C function. I mean, the code for the function is in the same "unit" as the rest of the shader.
#version XX-YY
//MyFunc(.....)
whateverreturn MyFunc(.....)
{
do something and return a whateverreturn
}
void main(void)
{
//use MyFunc
whateverreturn var = MyFunc(....)
}
A bit different case is when you have a function that can be part of several shaders, but it isn't a "full" shader, it has no main() function. This function lives in a file or in string-array or something similar.
Say you have the function in a specific file:
#version XX-YY
//MyFunc(.....)
whateverreturn MyFunc(.....)
{
do something and return a whateverreturn
}
And the file with the shader where you want to use it:
#version XX-YY
//declare the function
whateverreturn MyFunc(.....);
void main(void)
{
//use MyFunc
whateverreturn var = MyFunc(....)
}
Like you do with any common GLSL code, use glShaderSource and glCompileShader.
Now the key step is how to integrate the code within a full (with 'main') shader: just use glAttachShader (again, like you do with VS or FS) before glLinkProgram and that'as all.