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    <p>You have quite a few problems in this code, too many to list in comments in fact.</p> <ol> <li><p>Your projection matrices are changing the handedness of your post-projected coordinate space, which will affect polygon winding.</p> <ul> <li>Stick to a single handedness, unless you want to reverse the direction used for front- / back-facing polygon facets (<code>glFrontFace (...)</code>) - it is CCW by default, but you will have to change it to CW when you use <code>set3d (...)</code> to maintain consistent behavior.</li> <li>This problem arises because you have the Y-axis going different directions <br><br></li> </ul></li> <li><p>The behavior your are describing, where the object appears briefly and then disappears is indicative of an issue with your matrix stack.</p> <ul> <li>Are you calling <code>initGL (...)</code> 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. <br><br></li> </ul></li> <li><p>You cannot use <strong>0.0</strong> for zNear with a perspective projection matrix, this will result in wonky math during the perspective divide step that comes <em>after</em> transformation to clip-space; the depth buffer will not work correctly.</p> <ul> <li>In fact, <code>glFrustum (...)</code> will generate a <code>GL_INVALID_VALUE</code> error and do exactly <strong><em>nothing</em></strong> if you pass a value &#x2264; <strong>0.0</strong> for zNear. On a related note, <code>gluPerspective (...)</code> will not do this since it is not actually part of OpenGL, but it is equally invalid behavior to pass such a value to <code>gluPerspective (...)</code>.</li> </ul></li> </ol>
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    1. USAndon M. Coleman
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    1. COThank you Andon, and everyone else who answered. I had the Y-axis reversed in glFrustum() by mistake. Thank you for pointing that out. I am not calling initGL() more than once. I did double check all my code and found 2 places where I had glEnd() and glPopMatrix() reversed by mistake. In regards to point 3: I was able to get the cube to draw on top of the tiles by using glFrustum(0,SCREEN_WIDTH,SCREEN_HEIGHT,0, -1,1); in set3d() and glOrtho(0,SCREEN_WIDTH,SCREEN_HEIGHT,0,-1,1); in set2d(); so apparently you can have the zNear set to <0 in glFrustum. Probably not right, but it works for now.
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      1. USMike Deaton
    2. COand i think you can drop glPushMatrix() calls from initGl() altogether.. Keep in mind, thta minimum required size of matrix stacks in OpenGL is not that big, and for GL_PROJECTION matrices you really have only like 2 guaranteed spots in the stack...
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      1. USPavel Beliy
 

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