glPushMatrix（）和glPopMatrix（）如何保持场景相同？ [英] How do glPushMatrix() and glPopMatrix() keep the scene the same?

问题描述

我在网上找到了一些代码，它会在屏幕上移动一个框，然后在框打到屏幕末尾后重置。

这是代码：

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.
Here is the 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 > 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();
}

现在，我理解的方式 glPushMatrix（） 和 glPopMatrix（） 是 glPushMatrix（）在堆栈上放置（或推送）一个新矩阵，以便你做一些事情，所以你弹出它后，你有一个干净的平板。这就是为什么如果我忽略 glEnd（）之后的 glPopMatrix（），我的方块似乎加速而不是移动

Now, the way I understand glPushMatrix() and glPopMatrix() is that glPushMatrix() 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 glPopMatrix() after glEnd(), my square seems to accelerate rather than move at a constant velocity.

然而，如何改变我在 glPushMatrix（） glPopMatrix（）保留？当我使用 glPushMatrix（）并改变顶层矩阵时，它可视化的变化，但是当我使用 glPopMatrix（），不是所有的改变都走了吗？当我再次恢复到一个干净的石板，我的箱子是怎么移动在屏幕上的？

How is it, however, that the changes I make inside of glPushMatrix() and glPopMatrix() are kept? When I use glPushMatrix() and make a change to the top matrix, it visualizes the changes, but when i use glPopMatrix(), 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?

如果我在进行更改后再次弹出矩阵，那么该记录的状态如何记录？

How is the state of that translation recorded if i just pop the matrix off again after making the change?

推荐答案

这只是用于转换坐标的矩阵，由 glPopMatrix 恢复。不是整个framebuffer。 framebuffer包含四边形的渲染，以后改变矩阵不会影响已经渲染的任何东西。

It's only the matrix, used to transform coordinates, that is restored by glPopMatrix. Not the whole framebuffer. The framebuffer contains the rendering of the quadrilateral, changing the matrix afterward doesn't affect anything already rendered.

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