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Normal Question?

Started by Chris1066 Mar 27, 2000 at 3:19 PM 1 reply 600+ views

Original Post

Mar 27, 2000 at 3:19 PM

I have a triangle, whos coordinates are [2,0,0] [0,2,0] [0,0,2] and when I make a face normal for this triangle I get [0.577,0.577,0.577] as expected. The face is at an angle of 45 degrees to the local axis, but if I do a dotproduct of the face normal with the vector [0,1,0] I will get an angle of 54.7 degrees instead of 45, where am I going wrong. Chris

Spellbound

Mar 28, 2000 at 11:11 AM

The error that you''re making is that you''re assuming that the triangle is at an angle of 45 degrees to the local axis. It is in fact 54.7 degrees as you have calculated with the dotproduct.

If you rotate the triangle 45 degrees around the y-axis you''ll get these coordinates [1.414,0,1.414][0,2,0][-1.414,0,1.414]. The normal to the triangle is now [0,0.577,0.816].

I hope you see your error now.

evaclear

Mar 28, 2000 at 12:15 PM

Just wanted to point out you shouldn''t use degrees in your programs. Everything should be converted into radians, makes life alot easier.

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