problems with perspective

OpenGL Development JOGL Development
#1

Hi there,
First of all I should say that I’m pretty new to OpenGL and JOGL, so the problems I’m experiencing are probably familiar to anyone who’s not a novice.
I’ve been struggiling with getting accurate perspectives with diminuation of size. Basically, If I try to move an object in the negative z direction (just for the sake of an example) , rather than getting smaller, it will maintain it’s dimensions until it leaves the clipping volume, at which point it gradually gets clipped. I would appreciate some basic instruction on the matter. The following lines of code are a simplified version of what I had thought would fix the problem…
this is in display():

        gl.glClear(GL.GL_COLOR_BUFFER_BIT|GL.GL_DEPTH_BUFFER_BIT);
        gl.glMatrixMode(gl.GL_PROJECTION);
        gl.glLoadIdentity();      
        glu.gluPerspective(15.0f, 1.6f, -1.0, 10.0);
        gl.glMatrixMode(gl.GL_MODELVIEW); 
        gl.glLoadIdentity();
        gl.translate(some coordinates);
        glut.glutWireSphere(glu, 0.5d, 12, 8 );

Do I need to do anything else in order to activate an accurate rendition of perspective? Is there anything I might not be doing that I should be doing? Any help would be greatly appreciated.
Thanks
Sam

#2

[quote] gl.glClear(GL.GL_COLOR_BUFFER_BIT|GL.GL_DEPTH_BUFFER_BIT);
gl.glMatrixMode(gl.GL_PROJECTION);
gl.glLoadIdentity();
glu.gluPerspective(15.0f, 1.6f, -1.0, 10.0);
gl.glMatrixMode(gl.GL_MODELVIEW);
gl.glLoadIdentity();
gl.translate(some coordinates);
glut.glutWireSphere(glu, 0.5d, 12, 8 );
[/quote]
Theres a couple of things wrong with this. Firstly, and perhaps most importantly, glPerspective can’t take a negative value as the zNear value. Has to be >0. Generally you’d pick your zNear value depending on what your zFar value is. I generally go for something about between a 10th and 100th of the zFar value depending on the scene and where my ‘camera’ is. This will minimize artifacting in the depth buffer for similar z-values.

Secondly, your FOV is 15 degrees. As your FOV tends toward infinity , the perspective projection will tend toward an orthographic projection, so a nice round 45 or 60 degrees will give you more a pronounced perspective effect.

these are my default values in my camera class:
DEFAULT_FOV = 64f;
FAR_CLIP = 1000f;
NEAR_CLIP = 10f;

with the aspect set to whatever the aspect ratio of the viewport is.

D.

#3

Hey,
That fixed my problem. Thank you very much for the thoughtful response. Very helpful.
Take care