opengl - having trouble implementing a perspective view - opengl

Question

I'm writing an OpenGL application. The program needs to have a simple shape at the center and the user can change the view from orthographic to perspective and vice versa. I don't have any problem in handling the orthographic view but I can't switch from ortho to perspective, even though gluPerspective seems pretty straightforward.
The code I wrote looks something like this:
void myDisplay(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glTranslatef(0, 0, -3);
glRotatef(xRot, 1.0, 0.0, 0.0);
glRotatef(yRot, 0.0, 1.0, 0.0);
glRotatef(zRot, 0.0, 0.0, 1.0);
glScalef(scale, scale, scale);
glPushAttrib(GL_ENABLE_BIT);
if (currentStyle) {
if (currentStyle == STYLE_DOTTED) glLineStipple(currentStyle, 0xAAAA);
else glLineStipple(currentStyle, 0x0F0F * currentStyle);
glEnable(GL_LINE_STIPPLE);
}
setColor();
drawObjectOfType();
glPopAttrib();
glFlush();
}
void myKeyboard(unsigned char key, int x, int y)
{
switch (key)
{
case 'p':
isOrtho = !isOrtho;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if(!isOrtho) {
gluPerspective(55,1,5,3);
gluLookAt(0,0,-7,0,0,0,0,1,0);
} else {
glOrtho(-2.0, 2.0, -2.0, 2.0, -10.0, 10.0);
}
glMatrixMode(GL_MODELVIEW);
break;
}
glutPostRedisplay();
}
The program also has lighting in it. Here's the init function that handles the lighting.
void init()
{
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat mat_shininess = { 100.0 };
GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light_position[] = { 0.0, 0.0, 10.0, 0.0 };
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialf(GL_FRONT, GL_SHININESS, mat_shininess);
glShadeModel(GL_SMOOTH);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
createMenus();
glEnable(GL_DEPTH_TEST);
glClearColor(0.0, 0.0, 0.0, 1.0);
}
What happens with this code is the view changes from ortho to perspective, but distance between the camera and the polygons seems to be reverted, meaning the closer the polygon to the camera, the smaller it looks. For example, I have a cube as the shape and the face that is closer to me looks smaller whereas the oppposite should have been the case.

Answer 1

The last two parameters to gluPerspective() are zNear and zFar. You have set the near clipping plane further away than the far clipping plane, which reversed the perspective correction.

Answer 2

gluLookAt defines a viewing transformation, and should therefore be part of your MODELVIEW transformation. Your code multiplies it into the PROJECTION transformation. Try a sequence like this for your perspective setup case:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(55,1,3,5);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0,0,-7,0,0,0,0,1,0);
Then use glPushMatrix and glPopMatrix instead of glLoadIdentity in your myDisplay function.
Your near and far plane are quite close to each other. We don't see in the code you posted what the range of the coordinates for your object are. You'll want to be careful that it ends up between your near and far plance, so that it won't get clipped away.