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POOpenGL switching between ortho and perspective
 

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    <p>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.</p> <p>I'm initializing OpenGL with this:</p> <pre><code>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 ); } </code></pre> <p>I have a function for drawing a cube that works.</p> <pre><code>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(); } </code></pre> <p>I made 2 functions for setting the 2d and 3d projection modes.</p> <pre><code>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(); } </code></pre> <p>For the sake of completeness, some <strong><em>pseudocode</em></strong> of my textured quad drawing..</p> <pre><code>pushmatrix(); translate(); rotate(); -translate(); glbegin(); ...vertices glend(); popmatrix(); </code></pre> <p>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.</p> <p>Basically what I'm doing in the main loop is this:</p> <pre><code>move objects around, process collisions, etc. set2d(); draw textured quads, lines, text, etc. in 2d mode. set3d(); draw_cube(); </code></pre> <p>My question is: Why is the cube disappearing, and are there any errors in my set2d() and set3d() functions?</p>
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    1. USMike Deaton
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    1. COHave you tried calling `draw_cube()` first before you draw quads? Also, what depth are the 2D quads being drawn at? I think the cube is there, it's just behind your textured quads... maybe try just rendering the cube without the textured quads and see if it stays on screen.
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    2. COyour problem lies i think in glFrustum function, more specifically in zNear parameter. It NEVER should be equal to zero, and although the general rule is to make it small, you always have to be cautious with this; too short value of zNear WILL cause depth artifacts even in small models, a 0 will lead to singularities in mathematical calculations(basically, because of "divide by zero" kind of thing), which result, well, in complete disappearence of the image.
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      1. USPavel Beliy
    3. COWell spotted Pavel. I'd also be inclined to increase the `farVal`. It's worth noting both values should be positive.
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