Question

I’ve stumbled on an issue in OpenGL that I can’t explain, unless I somehow misunderstood how glPushMatrix() / glPopMatrix() work.

Shouldn’t the two pieces of code below have exactly the same effect?

  glPushMatrix();
  glVertex3f(0,0,0);
  glTranslatef(c->e->coords[0], c->e->coords[1], c->e->coords[2]);
  glPopMatrix();

vs

  glVertex3f(c->e->coords[0], c->e->coords[1], c->e->coords[2]);

In my application, only when I use the second one do I see anything. I tried with both pieces of code in the exact same place in the code, changing nothing else, and the one with push/pop matrix doesn’t draw anything on screen.

Answer 1

You can’t call glPushMatrix()/PopMatrix() inside a glBegin()/glEnd() block.

glBegin():

Only a subset of GL commands can be used between glBegin and glEnd.
The commands are glVertex, glColor, glSecondaryColor, glIndex,
glNormal, glFogCoord, glTexCoord, glMultiTexCoord, glVertexAttrib,
glEvalCoord, glEvalPoint, glArrayElement, glMaterial, and glEdgeFlag.
Also, it is acceptable to use glCallList or glCallLists to execute
display lists that include only the preceding commands. If any other
GL command is executed between glBegin and glEnd, the error flag is
set and the command is ignored.

Recommendation: Use vertex arrays or vertex buffer objects instead of classic immediate-mode glBegin()/glEnd() blocks.