//cam stuff

float near = 1.0;

float far = 100.0;

float fov = 60.0;

float asp = resolution.x/resolution.y;

 

float half_height = near*tan(fov*0.5*half_pi);

float half_width = half_height*asp;

vec2 projPlane = vec2(half_width*2.0, half_height*2.0);

...

void main(){

        vec3 camPos = vec3(0.0, 0.0, 4.0);

    vec3 pixel = vec3(-projPlane/2.0+(gl_FragCoord.xy/resolution.xy)*projPlane, camPos.z-near); //ray origin auf die nearplane setzen

       vec3 rayDir = normalize(pixel-camPos);

        ....raycasting

}