CODEWITHSUNDEEP
openglmatrixrotation
Laurent Crivello
Laurent Crivello

Reputation: 3931

Relative rotation of OpenGL Camera

I am currently struggling in finding a formula to rotate my OpenGL "Camera" (I tried do do it via a scene rotation, but have the same issue). Basically my Camera is at a given position, looking a given point (all indicated to gluLookAt) and I would like to rotate the camera upwards for example, and still looking at the same point.

What should be the right process ? What input data should I take to decide the amount of movement ? 2D mouse coordinates evolution or 3D unprojected mouse coordinates evolution ?

Upvotes: 1

Views: 1763

Answers (2)

Raven
Raven

Reputation: 4903

If you're creating orbiting(oribitng around LookAt) camera for openGL I sugest you make it with these data:

LookAtPosition- 3D vector
CamUp - 3D unit vector
RelativeCamPosition - 3D unit vector
CamDistance - decimal number

LookAtPosition is a point on which you'll be looking. CamUp is vector that points up from camera, you can see it on this image. It's best to initialize camera at no rotation, so that CamUp = [0,1,0]. Note that it's unit vector so it's magnitude/size/length is always 1. RelativeCamPosition is again unit vector. You get it by taking LookAt to Camera vector and dividing by it's magnitude, which you'll save in CamDistance. In intialized state it might look as this:

LookAtPosition = [0,0,0]
CamUp = [0,1,0]
RelativeCamPosition = [1,0,0]
CamDistance = 10

You can now get camera position by

CamPosition = LookAtPosition + RelativeCamPosition * CamDistance

But you need to rotate that camera arround right? Well there's a reason for unit vectors - they are easy to use in calculations. I believe you use angles for rotating so you need to use only sine and cosine. Rotate function might look like this:

Rotate(angleX, angleY){
   RelativeCamPosition.x = sin(angleX)*cos(angleY);
   RelativeCamPosition.z = cos(angleX)*cos(angleY);
   RelativeCamPosition.y = sin(angleY);
}

where angleX and angleY are absolute (NOT RELATIVE) rotations in horizontal and vertical direction. You should always use absolute roations because there can be floating point errors while adding. Anyway I just made those calculations on scrap of paper so I hope they're allright.

Edit: I've just noticed that this will work just if your intiial state is like I wrote RelativeCamPosition = [1,0,0]. However it shouldn't be hard to edit them so it works for arbirtary initial state.

Upvotes: 2

Bernd Elkemann
Bernd Elkemann

Reputation: 23560

The trick is to see that a camera-rotation is the same as a scene rotation if you do it at the correct position. Move the camera into the point around which you want to rotate, then rotate the camera, then move back out by the same distance you moved in.

The amount by which you rotate depends on your application. Take G-Earth as an example: if you are close to the surface the rotation is (absolute) small, if you are far from the surface it is large.

Upvotes: 2

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