<p>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.</p>

<p>For example, if <span class="math-container">$\vec{V}$</span> is the viewing direction, <span class="math-container">$\vec{L}$</span> the incoming light direction and <span class="math-container">$\vec{R}$</span> the perfect specular reflection direction for <span class="math-container">$\vec{L}$</span>, then the specular response is <span class="math-container">$\text{max}(\vec{V}.\vec{R},0)^p$</span>, where <span class="math-container">$p$</span> is the exponent that controls the fall-off of the specular response. </p>

<p>The Blinn-Phong model uses a half vector <span class="math-container">$\vec{H}$</span>, which is computed as <span class="math-container">$\vec{H} = \frac{\vec{L}+\vec{V}}{|\vec{L}+\vec{V}|}$</span>, which is then used to compute the specular response as <span class="math-container">$\text{max}(\vec{H}.\vec{N},0)^p$</span>, where <span class="math-container">$\vec{N}$</span> is the surface normal. The idea is that when the eye vector <span class="math-container">$\vec{V}$</span> is aligned perfectly with the perfect mirror direction <span class="math-container">$\vec{R}$</span>, the half vector <span class="math-container">$\vec{H}$</span> would be exactly aligned with the surface normal <span class="math-container">$\vec{N}$</span>. Hence the Blinn-Phong can get away with computing an expensive mirror direction <span class="math-container">$\vec{R}$</span> everytime by using the half vector which can model the specular response.</p>

CC BY-SA 4.0
2018-12-03T18:36:23.487Z
2018-12-03T18:36:23.487Z
sriravic
<p>How does the Modified Phong lighting model also known as the Blinn-Phong differ from the Phong Lighting Model? </p>

<p>How can I distinguish the two?</p>

<lighting><model>
How does the Modified Phong Lighting Model from the Phong Lighting Model?
user9778
# How does the Modified Phong Lighting Model from the Phong Lighting Model?

(Asked by: username_0 on 2018-12-03)

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?

**Tags:** lighting, model


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## 1 answers

### Answer by username_1 on 2018-12-03. Score: 1 | Accepted answer

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.


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