Raytracing concepts and code, part 5, diffuse color

This is an article in a multipart series on the concepts of ray tracing. I am not sure where this will lead but I am open to suggestions. We will be creating code that will run inside Blender. Blender has ray tracing renderers of course but that is not the point: by reusing Python libraries and Blender's scene building capabilities we can concentrate on true ray tracing issues like shader models, lighting, etc.
I generally present stuff in a back-to-front manner: first an article with some (well commented) code and images of the results, then one or more articles discussing the concepts. The idea is that this encourages you to experiment and have a look at the code yourself before being introduced to theory. How well this works out we will see :-)

So far the series consists of the several articles labeled ray tracing concepts

Our rendered scene so far mainly consists of fifty shades of gray and that might be exciting enough for some but it would be better if we could spice it up with some color.

For this we will use the diffuse color of the first material slot of an object (if any). The code so far needs very little change to make this happen:
            # the default background is black for now
            color = np.zeros(3)
            if hit:
                # the get the diffuse color of the object we hit
                diffuse_color = Vector((0.8, 0.8, 0.8))
                mat_slots = ob.material_slots
                if len(mat_slots):
                    diffuse_color = mat_slots[0].material.diffuse_color
                color = np.zeros(3)
                light = np.ones(3) * intensity  # light color is white
                for lamp in lamps:
                    # for every lamp determine the direction and distance
                    light_vec = lamp.location - loc
                    light_dist = light_vec.length_squared
                    light_dir = light_vec.normalized()
                    # cast a ray in the direction of the light starting
                    # at the original hit location
                    lhit, lloc, lnormal, lindex, lob, lmat = scene.ray_cast(loc+light_dir*eps, light_dir)
                    # if we hit something we are in the shadow of the light
                    if not lhit:
                        # otherwise we add the distance attenuated intensity
                        # we calculate diffuse reflectance with a pure 
                        # lambertian model
                        # https://en.wikipedia.org/wiki/Lambertian_reflectance
                        color += diffuse_color * intensity * normal.dot(light_dir)/light_dist
            buf[y,x,0:3] = color
In lines 5-8 we check if there is at least one material slot on the object we hit and get its diffuse color. If there is no associated material we keep a default light grey color.
This diffuse color is what we use in line 28 if we are not in the shadow.

Code availability

The code is available on GitHub.

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