Blinn-Phong specular light bleeding on triangles facing-away from light: what is the canonical solution?

Question

In our implementation of the empirical Blinn-Phong shading model, we are facing a problem with light-bleeding of the specular component.

The model defines the half-vector $\vec{h} = \frac{\vec{v} + \vec{l}}{||\vec{v} + \vec{l}||}$, with:

• $\vec{v}$ the unit view vector
• $\vec{l}$ the unit light vector (defined from surface-point to light-source).

and the specular component is proportional to ${(\vec{n} \cdot \vec{h})^+}^p$ (with $p$ as Phong exponent)

Specular light bleeding situation

Light bleeding occurs in certain configurations when the light vector $\vec{l}$ is facing away from the surface (i.e. >90° angle from the normal $\vec{n}$).

Such a configuration is illustrated by the diagram below:

Here, we can see that although the light should not contribute to the surface lighting, the computed half-vector is close to the normal, and thus an important specular contribution will be added by the model.

We followed implementation advices from Fundamental of Computer Graphics and Real-Time Rendering, but we missed any mention of this potential problem:

• Are we misunderstanding how to implement this shading model?
• Or is this an inherent issue, and what would be the canonical approach to address it?

Edit bonus question:

• What tools to draw geometric diagrams would you recommend?

Answer 1

N dot L will be less then zero and must be computed for the diffuse component and is typically clamped to zero. This can be incorporated into the specular highlight calculation to resolve the issue. What you are describing doesn't come up always in descriptions of Blinn-Phong lighting, I assume, because the writers want to keep things simple. But it is a real issue, I wouldn't go so far as to call it a problem with the lightning model though.

A typical way to resolve it is to add this to the specular compuation (or something like it):

specular = specular * (float)(ndotl>0.0);