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May 15th, 2017, 02:43 AM   #1
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Compounding angles in 3D

Hi folks,

I'm trying to get my head round this little problem.

Hopefully very simple but I can't seem to picture it.

Here's the short back story:

I'm a VFX artist, like video compositing. I have a system called Flame and in this we have a 3D environment with surfaces, objects and cameras and stuff.

So, imagine a camera looking a shape. The shape has coords 0,0,0 and the Camera is say 0,0,1000 and looking at the shape. Both start with zero rotation on all axis and the object is facing the camera directly.

Basically what I'd like to know is:

If the object rotates by theta degrees on its x axis and then alpha degrees on its y axis, what angle would its z axis form with a line drawn from camera to the object?

The reason I ask, if you're interested, is:

I want to write an expression (like a function that goes instead of a variable) that will turn the shape transparent when turned away from the camera.

I wrongly assumed that I could treat x and y separately.

So I looked at the object's y rotation first. It's facing the camera with 0 degrees rotation so if it was simply:

if -90 < y rotation <90 then transparency = 0

otherwise transparency =100 (completely transparent on my system.)

I used a cos function to get this to work for all angles. Pretty straight forward.

Then I added looked at moving the object sideways in x. If it was off to the left it wouldn't need to rotate as far as 90 degrees before it should become transparent so I incorporated the angle from where it was straight on to where it moved to.

This seemed to work. I did the same from moving in y and x rotation.

I thought these to would simply compound like +ve and -ve numbers.


cos (y rotation)*cos(x rotation)

-ve means you can't see the object, +ve means you can.

This also worked well until you rotate both. Once you rotate on x say the object's y axis also rotates.

So I think what I need is the angle from the line from camera to object and the line perpendicular to the face of the object.

So I think I need to compound the angles.

It's tricky to get my head round though as I can't seem to draw it or picture it.

If this makes any kind of sense and you have any insight your input would be greatly appreciated.


Last edited by skipjack; August 7th, 2017 at 10:34 PM.
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August 7th, 2017, 10:35 PM   #2
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