glFrustum, GL_MODELVIEW versus GL_PROJECTION [duplicate] - opengl

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OpenGL, problems with GL_MODELVIEW GL_PROJECTION
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Closed 6 years ago.
I am trying to learn that but I really don't understand.
Using:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3f(-0.5, -0.5, 0.5);
glVertex3f(0.5, -0.5, 0.5);
glVertex3f(0.5, 0.5, 0.5);
glVertex3f(-0.5, 0.5, 0.5);
glEnd();
That's the only thing I do in the draw routine. I am drawing a face and it's all OK.
Now I want to see the same face closer and farther from the observer, so I change the Z coordinate. The problem is that the size of the face as it appears on the screen does not change.
I did a try setting a frustum with glFrustum(-15, 15, -15, 15, 0.1, 15), but then everything disappears (the face isn't in frustum view?).
All I want is to draw a face, setting a frustum and moving the face along the Z coordinate seeing as the face getting bigger or smaller. I know there are GL_MODELVIEW and GL_PROJECTION, but I don't understand what they are, maybe this is the trouble.

Answer 1

In OpenGL, by convention, the camera looks towards the negative direction of the Z-axis, i.e. the Z-axis points out of the screen. Your quad is located at Z = 0.5, effectively behind the camera. So to see at least something you have to move the quad to the negative coordinates:
glBegin(GL_QUADS);
glVertex3f(-0.5, -0.5, -0.5);
glVertex3f(0.5, -0.5, -0.5);
glVertex3f(0.5, 0.5, -0.5);
glVertex3f(-0.5, 0.5, -0.5);
glEnd();
Alternatively you can rotate the camera:
glMatrixMode(GL_PROJECTION);
glFrustum(-15, 15, -15, 15, 0.1, 15);
glMatrixMode(GL_MODELVIEW);
static const GLfloat m[] = { -1, 0, 0, 0, 0, 1, 0, 0, 0, 0, -1, 0, 0, 0, 0, 1 };
glLoadMatrixf(m);
That being said, you are better to ditch the legacy OpenGL fixed-pipeline functionality and teach yourself the programmable pipeline.