gluPerspective parameters- what do they mean?
I wonder about the gluPerspective parameters.
In all examples I see fovy is set to around 45-60degrees
I've tried to set it to different values and the object just disappears
what's the explanation for it?
The aspect value should always be the ratio? why would one change it?
zNear, zFar - once again the usual values are around 10 and 500+ what does it reflect?
Answer
The purpose of the 4 parameters is to define a view frustum, like this:
where nothing outside of the frustum should be is visible on screen ( To accomplish this, the parameters are used to calculate a 4x4 matrix, which is then used to transform each vertex into the so-called clip space. There, testing if a vertex is inside the frustum or not is trivial )
The field of view parameter is basically the angle in between a plane passing through the camera position as well as the top of your screen & another plane passing through the camera position and the bottom of your screen. The bigger this angle is, the more you can see of the world - but at the same time, the objects you can see will become smaller.
To observe its influence, I'd suggest coding a simple app where you can gradually increase / decrease the fov via keypress - then render some spheres or other basic objects & see what happens as you change it.
The aspect ratio is your viewport's aspect ratio. ( In the graphic above, the viewport is located at the the near clipping plane ) Being able to define it at will makes sense, since your viewport's aspect ratio may vary.
The zNear & zFar values define the distance between the camera position & the near and far clipping planes, respectively. Nothing that's closer to the camera than zNear or farther away than zFar will be visible. Both values must be > 0, and obviously, zFar > zNear. zFar should ideally be chosen so that everything you want to render is visible, but making it larger than necessary wastes depth buffer prescision & may lead to flickering effects, called z-fighting. Likewise, setting zNear too close to the camera may cause the same effect - in fact, having a reasonable zNear value is more important than zFar. If you want to know exactly why this happens, you should read some in-depth explanations, like this one or this one