light

When using the OpenGL fixed function pipeline for vertex setup, how a fragment program looks like that is compatible to the fixed function vertex setup? I guess that usually depends on the number of light sources and texture layers etc.. So for example how does a simple non-texture one-lightsource goraud shading fragment program look like that replaces GL's fixed function shader? While Gouraud shading calculates the light in the the vertex shader, Phong shading calculates the light in the fragment shader. The standard OpenGL light model is a Gouraud shading model, with a Blinn-Phong light

I am trying to read up this tutorial: https://aerotwist.com/tutorials/an-introduction-to-shaders-part-2/ but I am not able to follow up. Basically the code creates a directional light by using shaders that run directly on the GPU. This is the code: // same name and type as VS varying vec3 vNormal; void main() { // calc the dot product and clamp // 0 -> 1 rather than -1 -> 1 vec3 light = vec3(0.5,0.2,1.0); // ensure it's normalized light = normalize(light); // calculate the dot product of // the light to the vertex normal float dProd = max(0.0, dot(vNormal, light)); // feed into our frag colour