3b1b-manim/manimlib/shaders/inserts/get_gl_Position.glsl

// Assumes the following uniforms exist in the surrounding context:
// uniform vec2 frame_shape;
// uniform float focal_distance;
// uniform float is_fixed_in_frame;

const vec2 DEFAULT_FRAME_SHAPE = vec2(8.0 * 16.0 / 9.0, 8.0);

float perspective_scale_factor(float z, float focal_distance){
    // TODO, what happens when focal_distance < z?
    return focal_distance / (focal_distance - z);
}


vec4 get_gl_Position(vec3 point){
    vec4 result = vec4(point, 1.0);
    if(!bool(is_fixed_in_frame)){
        result.x *= 2.0 / frame_shape.x;
        result.y *= 2.0 / frame_shape.y;
        result.xy *= perspective_scale_factor(result.z, focal_distance);
        // TODO, what's the better way to do this?
        // This is to keep vertices too far out of frame from getting cut.
        result.z *= 0.01;
    } else{
        result.x *= 2.0 / DEFAULT_FRAME_SHAPE.x;
        result.y *= 2.0 / DEFAULT_FRAME_SHAPE.y;
    }
    result.z *= -1;
    return result;
}