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POHow do glPushMatrix() and glPopMatrix() keep the scene the same?
 

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  1. POHow do glPushMatrix() and glPopMatrix() keep the scene the same?
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    <p>I found some code online which will move a box across the screen, then reset it after the box hits the end of the screen.<br> Here is the code:</p> <pre class="lang-cpp prettyprint-override"><code>void display(void) { int sign = 1; if (lastFrameTime == 0) { /* * sets lastFrameTime to be the number of milliseconds since * Init() was called; */ lastFrameTime = glutGet(GLUT_ELAPSED_TIME); } int now = glutGet(GLUT_ELAPSED_TIME); int elapsedMilliseconds = now - lastFrameTime; float elapsedTime = float(elapsedMilliseconds) / 1000.0f; lastFrameTime = now; int windowWidth = glutGet(GLUT_WINDOW_WIDTH); if (boxX &gt; windowWidth) { boxX -= windowWidth; } boxX += (sign)*256.0f * elapsedTime; glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glPushMatrix(); //creates a new matrix at the top that we can do things to? glTranslatef(boxX, 0.0f, 0.0f); /* * draw a "quad" (rectangle) */ glBegin(GL_QUADS); glVertex2f(0.0f, 0.0f); glVertex2f(128.0f, 0.0f); glVertex2f(128.0f, 128.0f); glVertex2f(0.0f, 128.0f); glEnd(); glPopMatrix(); //pops that matrix off the stack so we can have a "clean" version to do something next time.? glutSwapBuffers(); } </code></pre> <p>Now, the way I understand <a href="http://www.opengl.org/sdk/docs/man2/xhtml/glPushMatrix.xml" rel="noreferrer"><code>glPushMatrix()</code></a> and <a href="http://www.opengl.org/sdk/docs/man2/xhtml/glPopMatrix.xml" rel="noreferrer"><code>glPopMatrix()</code></a> is that <code>glPushMatrix()</code> puts (or pushes) a new matrix on the stack for you to do things to, so that after you pop it back off you have a "clean" slate again. This is why, if I neglect the <code>glPopMatrix()</code> after <code>glEnd()</code>, my square seems to accelerate rather than move at a constant velocity.</p> <p>How is it, however, that the changes I make inside of <code>glPushMatrix()</code> and <code>glPopMatrix()</code> are kept? When I use <code>glPushMatrix()</code> and make a change to the top matrix, it visualizes the changes, but when i use <code>glPopMatrix()</code>, aren't all those changes gone? When I am restored to a "clean" slate again, how is it that my box moves across the screen? </p> <p>How is the state of that translation recorded if i just pop the matrix off again after making the change?</p>
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    1. COBy the way. You know, that Your are learning OpenGL by reading depreciated code? Because PushMatrix and PopMatrix is deprecated in OpenGL 3.0. http://www.opengl.org/registry/doc/glspec30.20080923.pdf (406 page) Your way of drawing things (begin; vertex; end) is deprecated too. It is actually worst way to do it. So, I am just pointing problems, but I do not have good solution for them. Try to google for openGL 3.1 tutorials. You could even try to find good OpenGL ES tutorials. Try to learn clean way of doing OpenGL if You just started to learning it.
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      1. POHow do glPushMatrix() and glPopMatrix() keep the scene the same?
      1. USpetraszd
    2. CO@petraszd: problem is there are still a lot of resources teaching OpenGL using that old style. We still have to give solutions for those problems until resources like NeHe and others upgrade.
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      1. POHow do glPushMatrix() and glPopMatrix() keep the scene the same?
      1. USMihai Maruseac
    3. CO@Mihai: NeHe's pretty much dead (not updating), so they're not going to upgrade.
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      1. USNicol Bolas
 

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