<p>The term D is a <a href="https://en.wikipedia.org/wiki/Probability_density_function" rel="nofollow noreferrer">probability density function</a> (PDF) of microfacets' directions. Vaguely speaking, it says what "portion" of all microfacets points in the given direction.</p>

<p>Since it is a PDF, it doesn't have to be less or equal to 1. It can, and often is, higher than 1 for some directions.</p>

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2019-12-04T17:30:23.837Z
2019-12-07T03:29:34.110Z
2019-12-07T03:29:34.110Z
ivokabel
<p>Is the normal distribution term D in the Cook-Torrance BRDF supposed to be between 0 and 1? What does this number actually represent?</p>

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Value range of normal distribution function
dblouis
# Value range of normal distribution function

(Asked by: username_0 on 2019-11-25)

Is the normal distribution term D in the Cook-Torrance BRDF supposed to be between 0 and 1? What does this number actually represent?

**Tags:** brdf


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## 1 answers

### Answer by username_1 on 2019-12-04. Score: 2 | Accepted answer

The term D is a [probability density function](https://en.wikipedia.org/wiki/Probability_density_function) (PDF) of microfacets' directions. Vaguely speaking, it says what "portion" of all microfacets points in the given direction.

Since it is a PDF, it doesn't have to be less or equal to 1. It can, and often is, higher than 1 for some directions.


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