# Clicking Triangle Sides

**URL:** <https://discourse.threejs.org/t/clicking-triangle-sides/10971>\
**Category:** Questions\
**Tags:** math\
**Created:** [November 21, 2019, 11:00pm UTC](https://discourse.threejs.org/t/clicking-triangle-sides/10971 "2019-11-21T23:00:40Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![3DGraphics](https://avatars.discourse-cdn.com/v4/letter/3/97f17d/32.png) [@3DGraphics](https://discourse.threejs.org/u/3DGraphics)\
**Post date:** [November 21, 2019, 11:00pm UTC](https://discourse.threejs.org/t/clicking-triangle-sides/10971/1 "2019-11-21T23:00:40Z")

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Is there a way to determine which side of a triangle is clicked?

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**Author:** ![Usnul](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.threejs.org/usnul/32/2355_2.png) [@Usnul](https://discourse.threejs.org/u/Usnul)\
**Post date:** [November 22, 2019, 6:28am UTC](https://discourse.threejs.org/t/clicking-triangle-sides/10971/2 "2019-11-22T06:28:21Z")

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Compute dot product between face normal and and picking ray direction, if the product is \> 0: it’s the back side, if it’s \< 0: it’s the front side, if it’s 0 - the ray is along the the surface of the face.

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**Author:** ![3DGraphics](https://avatars.discourse-cdn.com/v4/letter/3/97f17d/32.png) [@3DGraphics](https://discourse.threejs.org/u/3DGraphics)\
**Post date:** [November 22, 2019, 2:18pm UTC](https://discourse.threejs.org/t/clicking-triangle-sides/10971/3 "2019-11-22T14:18:44Z")

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Thank you very much! I will try your suggestion.