glLoadIdentity和glPushMatrix/glPopMatrix,为什么不只使用前者呢？

Question

将变换应用于对象时,我们使用glPushMatrix/glPopMatrix.但为什么我们不使用glLoadIdentity呢？

从而

    glPushMatrix()
    ..apply tranformations
    ...draw object
    glPopMatrix()
    glPushMatrix()
    ..apply tranformations
    ...draw object
    glPopMatrix()

这应该是怎么做的吧？

可以变成

    glLoadIdentity()
    ..apply tranformations
    ...draw object
    glLoadIdentity()
    ..apply tranformations
    ...draw object

Answer 1

因为你无法做到这一点:

glPushMatrix()
..apply tranformations
glPushMatrix()
..apply ADDITIONAL transformations
..draw object with combined transforms
glPopMatrix()
...draw object without the additional transforms.
glPopMatrix()