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# Why reverse the light direction?

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### #1arkerone  Members   -  Reputation: 113

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Posted 02 August 2012 - 06:24 PM

Hello,

I have a question why do reverse the direction of a directional light?

[source lang="cpp"] // Invert the light direction for calculations. lightDir = -lightDirection; // Calculate the amount of light on this pixel. lightIntensity = saturate(dot(input.normal, lightDir));[/source]

Thx!

Edited by arkerone, 02 August 2012 - 06:25 PM.

### #2zacaj  Members   -  Reputation: 570

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Posted 02 August 2012 - 06:31 PM

They probably subtract in the wrong order somewhere earlier (possibly for another calculation that needs it the other way)
for instance lightPos-vertPos instead of vertPos-lightPos

### #3L. Spiro  Crossbones+   -  Reputation: 5144

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Posted 02 August 2012 - 07:00 PM

When calculating the half vector, the vectors are both from the object to the eye or light.
That is, the half vector is created by adding the vector from the object to the light and the vector from the object to the eye.

Here, pay attention to only the V and the L vectors. V = View = Eye. The H vector is the half vector, half-way between the V and L vectors.
But notice that they all point away from the surface. Towards the eye and the sun/light.

Some people think better when visualizing the sun vector (or light vector of any kind) as going out of the sun in some direction.
Those people need to reverse the sun’s direction for lighting equations, because that line of thinking makes the L vector point towards the surface.

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### #4Postie  Members   -  Reputation: 568

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Posted 02 August 2012 - 07:26 PM

In comes down to the difference between the direction of a light vs its position relative to a surface being rendered. It's typical to define a directional light as a vector that points in the direction the light is shining. However, for lighting calculations we need to know the position of the light relative to the surface. For global lights this turns out to simply be the reverse of the light's direction vector.
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### #5SIC Games  Members   -  Reputation: 328

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Posted 02 August 2012 - 07:38 PM

I can guess the calculation could come bouncing off a surface. If a surface contains more reflective material then the shader tells the light to bounce off and go into another direction. Just a guess.
-Paul

### #6arkerone  Members   -  Reputation: 113

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Posted 02 August 2012 - 08:06 PM

thank you everyone I have just realized

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