What's the shading algorithm behind Smooth Shading?

Question

When I flip between flat and smooth shading, I see that the vertex normals flip between n vertices for n meeting faces and single (averaged) normal. Once the vertex normals are deduced, how're objects with Smooth Shading rendered? I'm interested in the shading algorithm/interpolation model (Phong, Gouraud, etc.) than the illumination model (like Blinn-Phong, etc.) used. When smooth shading, are lighting calculations done per-vertex or per-fragment?

I'm a graphics programmer (OpenGL) who recently started learning Blender. I'd like to understand how Blender works under the hood for better understanding.

I searched in the manual but couldn't find anything regarding this, understandably. The Blender 3D Wikibook says Phong while an answer here says Gouraud.

When flat shading, the usual technique is to just shade the entire tri or quad with one lighting calculation performed with the face normal. I hope this holds true with Blender too.

Answer 1

As phong uses interpolated vertex normals this is the method used checking the docs of the source code.

source: https://github.com/blender/blender/blob/01a2bd03695f217616acc2581fe8746f5cd226d7/scripts/modules/bpy_types.py#L672

def shade_smooth(self):
        """
        Render and display faces smooth, using interpolated vertex normals,
        removing the "sharp_face" attribute
        """
        _name_convention_attribute_remove(self.attributes, "sharp_face")

Answer 2

"Phong" and "Gouraud" are not always used consistently between sources. If we consider Phong shading to be a shading algorithm where the interpolation of vertex normal is used to light fragments, and Gouraud shading to be a shading algorithm where the interpolation of vertex lighting is used to light fragments, then Workbench uses Phong shading.

We can see this by looking at the specular highlight of a smooth shaded object in Workbench:

The vertices of this smooth shaded cube are all darker than the specular highlight in the middle of the face. That highlight cannot be created by interpolation of vertex lighting. As a comparison with Gouraud shading, you can see the demonstration of specular artifacts at https://en.wikipedia.org/wiki/Gouraud_shading .

As for how we get that fragment normal, it is the renormalized weighted average of vertex normals of its (triangulated) face's vertices. The weights are the barycentric coordinates of the fragment. If we'd like, we can see these coordinates by using a geometry/parametric node in a shader.