Online Graphics Transforms 2: Rotations, Coordinate Frames
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Online Graphics Transforms 2: Rotations, Coordinate Frames

October 17, 2019


In this segment we are going to revisit rotations
in both 2D and 3D, and talk a little bit about coordinate frames. So far, all of our discussion has been in
terms of operating on points. But one can also change the coordinate system and in fact,
these are equivalent ways of thinking about transformations. For example, if I move towards
you, it can be interpreted in two different ways. Either you think of the coordinate system
as anchored to yourself and I am actually moving towards you, or you think of the coordinate
frame moving backwards and moving away from me. So let’s look at this with an example. Here
we have a point which is located at 2 units along the X axis, 1 unit along the Y axis. I can now translate that point to the left,
so I can move it to the location (1,1) in both axes. So this would be a left translation.
I can also see this as a change in coordinate frame, where the coordinate frame moves to
the right. And so the point item moves 1 unit to the left, or the coordinate frame moves
1 unit to the right. And both of these interpretations are equivalent, and you can use the interpretation
that’s more useful in a specific task. So the reason I brought this up is that in
many cases, you want to define a coordinate frame which is anchored to a specific person,
to an object, to the world, etc. And in many cases, you want a particular physical location
in the world to be transformed between coordinate frames. In general, coordinate frames can
have both translation and rotation. The center can be somewhere in the world and they can
be transformed by orientation; their axes can be different. So here you have the world, and you have an
origin in world space, x and y axes. In the camera coordinates you have some eye location
in u and v, which are the equivalent for x and y. So let’s look at a point p, and we want to
find the coordinates of a point p. So in world space, it’s let’s say 2 units along the x
axis, 0.9 units along the y axis. In the same point, in the camera coordinates,
it’s different. So it’s some location along u. Let’s say .5 along u, and -.6 along v.
And in many cases, one wants to do this. One has a geometric location in the world, even
in homework 1 on your teapot. And one wants to figure out where it lies in camera coordinates,
that has to take into account both the rotational coordinate system of the camera, as well as
where the camera is located, and what the eye coordinates are. Let’s go a little bit further and talk about
new interpretation of 2D rotations. If you remembered my derivation of 2D rotations,
I said a point P moves to P’, and the point P makes some angle alpha with the axis. The
angle of rotation is theta. And so you eventually make an angle of alpha plus theta. If you
write out the coordinates x and y and you simplify the trigonometric identities, eventually
you get a rotation matrix like this which is which is cos theta times x minus sin theta
times y sin theta times x plus cos theta times y. So this is your rotation axis. Of course following the key of rotating points
versus rotating co-ordinate systems, one can think about the point as being fixed, but
the coordinate system rotating in the inverse way. And, indeed, this is what the coordinate system
rotation looks like. This is the new u axis; this is the new v axis. And the coordinate
system is rotated clockwise by theta, whereas the point was rotating counterclockwise by
theta. So what are the coordinates of the new u and
v directions? And we’ll derive these in a moment. If we look at what the x coordinate
here is, this so since the axis are of unit norm, this distance is going to be cosine
of theta. And this length is going to be minus sine
of theta. So, the u coordinates are actually going to be given by cos theta minus sine
theta, that’s u. And v coordinates will be given by, this angle
will again be equal to theta and so you can see along the x direction, the v coordinate
will be given by sine theta, so I can write this as sine theta, and along the y direction
this will be cosine of theta. And in fact, that’s what this is. Sine theta,
cosine theta. So the rows of the rotation matrix can really be regarded as the coordinates
of the new axes. And in fact that’s what it is. So the uv coordinates
of the new axes are equal to cos theta x minus sin theta y, sin theta x cos theta y, of the
old axes. And this leads to the nice geometric interpretation
of 3D rotations which we also talked about earlier. Which is the rows of the matrix are
3 unit vectors of the new coordinate frame. And one can construct a rotation matrix from
any 3 orthonormal vectors. And in fact, the u coordinates are just x_u, y_u, z_u. v coordinates
similarly and w coordinates. Finally I want to briefly revisit the axis-angle
formula that we derived in the previous lecture. And I won’t go over all of the derivation
before, but the essential parts are there is a part which is the identity of cosine
theta, then there’s the a * a transpose part, then there’s the dual matrix and the cross
product part. I mainly put this slide up so that it can
be useful for you for revising when you do homework 1. The identity 1 minus cos theta
times this matrix. It has quadratic components and then sin theta times this cross product
matrix. That will be what the rotation matrix looks like. All of this gives you enough material
that we can actually derive the 4×4 matrix for gluLookAt which is a large component of
what you need to do in homework 1.

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