OpenGL switching between ortho and perspective - c++

Question

I have a tile engine using orthographic projection in immediate mode and I'm just trying to draw a 3d cube on top of my tile scene, in hopes that I can eventually incorporate 3d models into my engine instead of just sprites / textured quads. I would also like to make the tiles 3d for that slight bit of extra eye candy.. Hopefully I can eventually convert this to use modern OpenGL so I can take advantage of those extra features. Not on the top of my priority list at the moment. So on to the question.
I'm initializing OpenGL with this:
void initGL()
{
glDisable(GL_DEPTH_TEST);
glViewport( 0,0,SCREEN_WIDTH,SCREEN_HEIGHT);
//Initialize Projection Matrix
glMatrixMode( GL_PROJECTION );
glPushMatrix();
glLoadIdentity();
//Initialize Modelview Matrix
glMatrixMode( GL_MODELVIEW );
glPushMatrix();
glLoadIdentity();
glOrtho(0,SCREEN_WIDTH,SCREEN_HEIGHT,0,0,1);
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_FASTEST);
//...setting some various other attributes, omitted for brevity...
glEnable(GL_TEXTURE_2D);
glClearColor( 0, 0, 0, 0 );
}
I have a function for drawing a cube that works.
void draw_cube()
{
/* position object */
glRotatef(30.0F, 1.0F, 0.0F, 0.0F);
glRotatef(30.0F, 0.0F, 1.0F, 0.0F);
/* draw six faces of a cube */
glBegin(GL_QUADS);
...vertices... (removed for brevity)
glEnd();
}
I made 2 functions for setting the 2d and 3d projection modes.
void set3d()
{
glEnable(GL_DEPTH_TEST);
glClear(GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(0,SCREEN_WIDTH,0,SCREEN_HEIGHT, 0,1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void set2d()
{
glClear(GL_COLOR_BUFFER_BIT);
glDisable(GL_DEPTH_TEST);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0,SCREEN_WIDTH,SCREEN_HEIGHT,0,0,1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
For the sake of completeness, some pseudocode of my textured quad drawing..
pushmatrix();
translate();
rotate();
-translate();
glbegin();
...vertices
glend();
popmatrix();
So far I have managed to get the cube to draw on top of the tiles, but it disappears after approximately 1 second to never be seen again until I run the program again.
Basically what I'm doing in the main loop is this:
move objects around, process collisions, etc.
set2d();
draw textured quads, lines, text, etc. in 2d mode.
set3d();
draw_cube();
My question is: Why is the cube disappearing, and are there any errors in my set2d() and set3d() functions?

Answer 1

You have quite a few problems in this code, too many to list in comments in fact.
Your projection matrices are changing the handedness of your post-projected coordinate space, which will affect polygon winding.
Stick to a single handedness, unless you want to reverse the direction used for front- / back-facing polygon facets (glFrontFace (...)) - it is CCW by default, but you will have to change it to CW when you use set3d (...) to maintain consistent behavior.
This problem arises because you have the Y-axis going different directions
The behavior your are describing, where the object appears briefly and then disappears is indicative of an issue with your matrix stack.
Are you calling initGL (...) more than once in your software? You will run into a stack overflow if you do this enough times since you needlessly push the current matrix onto the stack and never pop it off.
You cannot use 0.0 for zNear with a perspective projection matrix, this will result in wonky math during the perspective divide step that comes after transformation to clip-space; the depth buffer will not work correctly.
In fact, glFrustum (...) will generate a GL_INVALID_VALUE error and do exactly nothing if you pass a value ≤ 0.0 for zNear. On a related note, gluPerspective (...) will not do this since it is not actually part of OpenGL, but it is equally invalid behavior to pass such a value to gluPerspective (...).