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    <p><code>glPushMatrix</code> duplicates the matrix on top of the stack (you're always working with the top one). Any other transformation you are doing modifies this top matrix, the duplicated one. When you do <code>glPopMatrix</code> we are back to the original matrix.</p> <p>For example, suppose you want to draw a car. You set up the matrix to draw the body of the car, let's call it <code>M1</code>. Now, you want to draw one wheel. You can either compute <code>M2</code> - the matrix needed for the wheel to be displayed correctly - or, since the wheel is relative to the body of the car (thus, there is a matrix <code>M3</code> such that <code>M2 = M1 * M3</code>) you modify <code>M1</code>. But the car has 4 wheels, you need to keep a copy of <code>M1</code>. You do this by doing a <code>glPushMatrix</code>, you get back the copy by doing a <code>glPopMatrix</code>.</p> <p><img src="https://i.stack.imgur.com/DXUHw.png" alt="glPushMatrix and glPopMatrix"></p> <p>When you draw anything on the screen, you are giving coordinates in object space. To really display something, those coordinates need to be transformed. For that we have some matrices.</p> <p>In the wheel example, you have only one wheel geometry but because you are using different matrices, there will be four wheels drawn. <code>glPushMatrix</code> and <code>glPopMatrix</code> work only with the matrix, the actual vertex data is kept into the GPU, each <code>glVertex</code> sends another one there and it cannot be removed. See the following image, the matrices are used only for transforming object coordinates to world coordinates (actually, all matrices can be pushed into a stack)</p> <p><img src="https://i.stack.imgur.com/yzCRP.png" alt="multiple matrices"></p>
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    1. POHow do glPushMatrix() and glPopMatrix() keep the scene the same?
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    1. USMihai Maruseac
    1. USMihai Maruseac
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    1. COthank you, i have a better understanding of how glPushMatrix() and glPopMatrix() work now, but i guess my question is - how is it that the wheels (in your example) would remain in the picture if we popped back the matrix to get M1? in my code, i push, translate, then pop ... but all previous translations have been recorded (as the box is moving across the screen). it seems to me that if i push, translate, then pop - then i am back to the original matrix (and thus, nothing should have happened).
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      1. USdrjrm3
    2. COUpdated now with more info.
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      1. USMihai Maruseac
    3. CO@Laurbert515 You constantly increase your box translation with the elapsed time, so why should it not move over the screen? You should also keep in mind, that OpenGL does not know anything of any objects. It just draws the vertices the moment you make the draw calls (`glVertex` in your case) and then forgets about them. So you can just change the matrix transformation and draw some other things.
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      1. USChristian Rau
 

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