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Does anyone know how to apply a texture in Cycles that only effects the faces of a mesh that are facing upwards (ie in the global Z direction within an angular tolerance)?

This would be very useful for snow, frost, dirt or dust.

I've done some experiments using the Tangent node but the results are not ideal.

I believe Maya has a snow texture that produces this type of result.

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    $\begingroup$ You should accept this answer if it was helpful, and don't confuse your accounts. $\endgroup$ – stacker Nov 8 '13 at 22:35
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Use this node setup for your material:

Material nodes

The dot product of the face normal with vector (0, 0, 1) is 0 when the face normal is orthogonal (= horizontal) and maximal when equal. When pointing downwards the dot product is negative.

With the color ramp you can control the angular tolerance. If you don't want to have a smooth transition between texture and green you can move white handle to the black one.

This is used as the factor of a Mix Shader mixing the texture with a green diffuse material.

Result: Render result

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Great answer from Maccesch, helped me a lot, thanks. Alternatively, you can use the greater than operator if you want a hard separation. In the example below, only faces that are facing (i.e. their normal has same direction as) the x-axis:

example using greater than

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