How does the Modified Phong lighting model also known as the Blinn-Phong differ from the Phong Lighting Model?
How can I distinguish the two?
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1 Answer
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The Phong lighting model computes the specular response as the dot product between the mirror reflection direction and the viewing direction, raised to a power.
For example, if $\vec{V}$ is the viewing direction, $\vec{L}$ the incoming light direction and $\vec{R}$ the perfect specular reflection direction for $\vec{L}$, then the specular response is $\text{max}(\vec{V}.\vec{R},0)^p$, where $p$ is the exponent that controls the fall-off of the specular response.
The Blinn-Phong model uses a half vector $\vec{H}$, which is computed as $\vec{H} = \frac{\vec{L}+\vec{V}}{|\vec{L}+\vec{V}|}$, which is then used to compute the specular response as $\text{max}(\vec{H}.\vec{N},0)^p$, where $\vec{N}$ is the surface normal. The idea is that when the eye vector $\vec{V}$ is aligned perfectly with the perfect mirror direction $\vec{R}$, the half vector $\vec{H}$ would be exactly aligned with the surface normal $\vec{N}$. Hence the Blinn-Phong can get away with computing an expensive mirror direction $\vec{R}$ everytime by using the half vector which can model the specular response.
