moving object in the world towards a stationary camera

Question

I want to move the camera forward, which is equivalent to moving the world back towards camera. I'm using Glut and glTranslate would do the job, but my question is how should I use it?

Suppose initially I start with glLoadIdentity(), then I set up the look at point using gluLookAt, and then I did some translation/rotation to the model. In this case how should I use glTranslate to translate the object in the world so that they can move with respect to the camera instead of their own origin/coordinate?

I thought I could save the current matrix using glGet, load Identity matrix, then do the translation I wanted, and then multiply the previous matrix back using glMultmatrix. But this didn't work for me.

And also if I want to enable yaw/pitch using glrotate, how should I do? (Also in the sense to rotate the world to make it seems rotating camera)

Sorry for my poor wording or conceptual mistake if there is any. I'm quite new to opengl and graphic programming in general and I'm still trying to fully understand the opengl pipeline, especially the matrix part. Any detailed explanation to that will also be greatly appreciated!

Answer 1

From reading your question, it sounds to me like what you're trying to do is simulate camera movement by translating every other object in the world about a fixed point (the camera)

While you're correct in saying that moving the camera actually moves everything else in the world about it, you seem to be going about it the wrong way. After all, look how much difficulty you're having just moving one box. Now imagine you have hundreds! Not much fun :)

Fortunately, there is a function that can help you, and you're already using it! gluLookAt (http://www.opengl.org/sdk/docs/man2/xhtml/gluLookAt.xml) is your guy. What it does under the hood is it creates a matrix (Not sure what a matrix is? Give this a read: http://solarianprogrammer.com/2013/05/22/opengl-101-matrices-projection-view-model/) that every other point in the world is multiplied by. This multiplication translates each point until its in its correct position relative to the camera. So you are correct in saying that moving the camera actually moves the whole world relative to the camera, this way we can do it all in one pass instead of having to calculate the new positions of each point manually.

So, you want to move the camera forward on the z axis? Just call gluLookAt, but pass in a value of eyez that is less than when you previously called gluLookAt. Here's an example:

   gluLookAt(0,3,0,0,0,0,0,1,0);//This is out starting position, (0,3,0)
   gluLookAt(0,2,0,0,0,0,0,1,0);//And this is out ending position. Notice that the eyez value has decreased by one

As for how to rotate, take a look at the second group of three parameters, the "center" parameters. Those determine what point is in the center of the camera, that is, what the camera is looking at. In the previous example, the center point was (0,0,0). You can rotate the camera by moving these points around. How you do it is a pretty complicated topic with a good bit of math thrown in, but the following links should help a bit:

http://ogldev.atspace.co.uk/www/tutorial15/tutorial15.html

http://www.fastgraph.com/makegames/3drotation/

http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation

Don't get discouraged if it seems too hard, keep at it! Feel free to ask me if you need clarification on this answer.