The sum of the angle formulas yields

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Hi guys, The parametric representation of the points on a circle with radius r . Mathematical Elements For Computer Graphics pg 216 reads: Noting that a circle is completely swept out of range of the parameter @ ( THETA ) from 0 to 2PI, and assuming that a fixed number of uniformly spaced points on the circumference are calculated, then DEL@ ( DELTA THETA ), the parameter increment between points, is a constant. The Cartesian coordinates of any point on an origin circle are then: x^i+1 = r cos( @^i + DEL@ ) y^i+1 = r sin( @^i + DEL@ ) where @^i is the value of the parameter that yields the point at x^i, y^i HERE IS WHERE I GET LOST... Using the sum of the angles formulas yields <-????? x^i+1 = r ( cos@^i cosDEL@ - sin@^i sinDEL@ ) y^i+1 = r ( cos@^i sinDEL@ - cosDEL@ sin@^i ) I don't get why and how this " x^i+1 = r cos( @^i + DEL@ ) " turns to this " x^i+1 = r ( cos@^i cosDEL@ - sin@^i sinDEL@ ) " Thank you in advance, DG

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It took me a while to decipher the equations (representing maths in plain text is such a pain, isn't it [smile]), but I think I get it now!

The missing step is a standard trignomatric expansion for the addition of angles in sines and cosines. The rule goes:

cos(x + y) = cos(x) cos(y) - sin(x) sin(y)

If you let x = theta and y = delta_theta, you get the trig. part of the equation you've got there.

I found a list of these functions (so I was sure I got it right!) at this link. Scroll down to the 'Addition Theorems' to find the relevant one.

Hope that's of some help!

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well, on this site:
http://www.geocities.com/SiliconValley/2151/math3d.html

it tells about matrix rotations, and the rotations look like this:
Rotatation about the x axis:

x' = x

y' = (cos é * y) - (sin é * z)

z' = (sin é * y) + (cos é * z)

Rotation about the y axis:

x' = (cos é * x) + (sin é * z)

y' = y

z' = -(sin é * x) + (cos é * z)

Rotation about the z axis:

x' = (cos é * x) - (sin é * y)

y' = (sin é * x) + (cos é * y)

z' = z

ignore me if im wrong plz!

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Quote:
 Original post by Trapper ZoidIt took me a while to decipher the equations (representing maths in plain text is such a pain, isn't it [smile]), but I think I get it now!The missing step is a standard trignomatric expansion for the addition of angles in sines and cosines. The rule goes:cos(x + y) = cos(x) cos(y) - sin(x) sin(y)If you let x = theta and y = delta_theta, you get the trig. part of the equation you've got there.I found a list of these functions (so I was sure I got it right!) at this link. Scroll down to the 'Addition Theorems' to find the relevant one.Hope that's of some help!

Thank you Trapper. That answers my question perfectly. I had an inkling it had to do with trig expansion but I wasn't sure.

DanielG

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