For purposes of testing I tried throwing together some test code which draws a spinning triangle and a spinning square. In theory, hitting [a] would move the camera up (and so move all objects on screen down); except I can't seem to get this code to work, and I can't figure out why, even after comparing the code to various tutorials. Can anyone explain what I'm doing wrong? To clarify, it draws the objects correctly, except it seems that nothing I do can move the camera. Thanks.

#include "main.h"
#include "camera.h"

double xpos = 0;
double ypos = 0;
double zpos = 10;

void ShapeChange(int newWidth, int newHeight)
{
	if(newHeight == 0)
		newHeight = 1;

	// Set the viewport to be the entire window
	glViewport(0, 0, newWidth, newHeight);

	// Reset the coordinate system before modifying
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	float ratio = (float)newWidth/(float)newHeight;

	gluPerspective(45, ratio, 0.1, 100.0);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	gluLookAt(xpos, ypos, zpos, xpos, ypos, zpos-1, 0.0, 1.0, 0.0);
}

void RenderScene(void)
{
	static float rotPyramid = 0;
	static float rotCube = 0;

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glPushMatrix();
	glLoadIdentity();
	glTranslatef(-1.5f,0.0f,-10.0f);
	glRotatef(rotPyramid, 0.0f, 1.0f, 0.0f);
	//glRotatef(135.0f, 0.0f, 1.0f, 0.0f);
	glColor3f(0.5f, 0, 0);
	glBegin(GL_TRIANGLES);						// Drawing Using Triangles
		glVertex3f( 0.0f, 1.0f, 0.0f);				// Top
		glVertex3f(-1.0f,-1.0f, 0.0f);				// Bottom Left
		glVertex3f( 1.0f,-1.0f, 0.0f);				// Bottom Right
	glEnd();
	
	glLoadIdentity();
	glTranslatef(1.5f, 0.0f, -10.0f);
	glRotatef(rotCube, 1.0f, 0.0f, 0.0f);
	//glRotatef(45.0f, 1.0f, 0.0f, 0.0f);
	glColor3f(0, 0.5f, 0);
	glBegin(GL_QUADS);
		glVertex3f(-1.0, 1.0, 0.0f);
		glVertex3f(1.0, 1.0, 0.0f);
		glVertex3f(1.0, -1.0, 0.0f);
		glVertex3f(-1.0, -1.0, 0.0f);
	glEnd();
	
	glPopMatrix();
	glutSwapBuffers();

	rotPyramid += 25.0f;
	rotCube -= 1.0f;

}

void KeyProc(unsigned char key, int x, int y)
{
	if (key == 27) exit(0); // esc
	if (key == 97) // a
	{
		ypos++; 
		glLoadIdentity();
		gluLookAt(xpos, ypos, zpos, xpos, ypos, zpos-1, 0.0, 1.0, 0.0);
	}
}

void main(int argc, char **argv)
{
	//CCamera camera; // this won't work
	//CCamera* pCamera = new CCamera; // nor will this
	//CCamera* pCamera = CCamera::Instance(); // but this is 100% cool (doesn't actually do anything yet)

	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
	glutInitWindowPosition(100,100);
	glutInitWindowSize(640,480);
	glutCreateWindow("The Prinny Game... yeah");

	glutDisplayFunc(RenderScene);
	glutIdleFunc(RenderScene);
	glutReshapeFunc(ShapeChange);
	glutKeyboardFunc(KeyProc);

//	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
//	glClearDepth(1.0f);
	glEnable(GL_DEPTH_TEST);
//	glDepthFunc(GL_LEQUAL);
//	glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
//	glShadeModel(GL_SMOOTH);

	glutMainLoop();
}

Well, one problem is that you're calling LoadIdentity() before you render your scene. Thus, anything you did previously to the Modelview matrix (for instance, changing it using gluLookAt) will be undone.

Cheers,
--Brian

Quote:Original post by Nairb
Well, one problem is that you're calling LoadIdentity() before you render your scene. Thus, anything you did previously to the Modelview matrix (for instance, changing it using gluLookAt) will be undone.

Cheers,
--Brian

Isn't that what glPushmatrix() (and the later glPopMatrix()) is supposed to accomplish? Save the modelview, draw stuff, then restore the view?

Or have I fundamentally misinterpreted how this stuff is supposed to work? :(

You're a little off.

glPushMatrix saves the current matrix and glPopMatrix retrieves it (using a stack).

So here's what your code is doing:

push matrix to stack
clear matrix
translate/rotate
draw
pop previous matrix to stack

So you're saving the gluLookAt, then tossing it out just before you draw, then retrieving it just after.

I imagine you want to use glPushMatrix and glPopMatrix so that the rotations/translations of the individual objects is independent. To do that, put a push before the rotation and translation and a pop after the drawing of that object, and get rid of the glLoadIdentity in that function entirely.
In other words:

push matrix
translate/rotate
draw
pop matrix

push matrix
translate/rotate
draw
pop matrix

Quote:Original post by Nairb
You're a little off.

glPushMatrix saves the current matrix and glPopMatrix retrieves it (using a stack).

So here's what your code is doing:

push matrix to stack
clear matrix
translate/rotate
draw
pop previous matrix to stack

So you're saving the gluLookAt, then tossing it out just before you draw, then retrieving it just after.

I imagine you want to use glPushMatrix and glPopMatrix so that the rotations/translations of the individual objects is independent. To do that, put a push before the rotation and translation and a pop after the drawing of that object, and get rid of the glLoadIdentity in that function entirely.
In other words:

push matrix
translate/rotate
draw
pop matrix

push matrix
translate/rotate
draw
pop matrix

Ooohhhh, okay. Yup, pretty much a misinterpretation on my part of how the system works, right after I read your post I had an epiphany and saw how I was thinking wrong. I thought gluLookAt actually... damn, I don't know what I was thinking, I thought it moved objects already drawn or something.

Here's what I changed the relevant sections to, for reference.

void RenderScene(void){	static float rotPyramid = 0;	static float rotCube = 0;	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);		glPushMatrix();	glTranslatef(-1.5f,0.0f,-10.0f);	glRotatef(rotPyramid, 0.0f, 1.0f, 0.0f);	//glRotatef(135.0f, 0.0f, 1.0f, 0.0f);	glColor3f(0.5f, 0, 0);	glBegin(GL_TRIANGLES);						// Drawing Using Triangles		glVertex3f( 0.0f, 1.0f, 0.0f);				// Top		glVertex3f(-1.0f,-1.0f, 0.0f);				// Bottom Left		glVertex3f( 1.0f,-1.0f, 0.0f);				// Bottom Right	glEnd();	glPopMatrix();		glPushMatrix();	glTranslatef(1.5f, 0.0f, -10.0f);	glRotatef(rotCube, 1.0f, 0.0f, 0.0f);	//glRotatef(45.0f, 1.0f, 0.0f, 0.0f);	glColor3f(0, 0.5f, 0);	glBegin(GL_QUADS);		glVertex3f(-1.0, 1.0, 0.0f);		glVertex3f(1.0, 1.0, 0.0f);		glVertex3f(1.0, -1.0, 0.0f);		glVertex3f(-1.0, -1.0, 0.0f);	glEnd();	glPopMatrix();	glutSwapBuffers();	rotPyramid += 25.0f;	rotCube -= 1.0f;}void KeyProc(unsigned char key, int x, int y){	if (key == 27) exit(0); // esc	if (key == 97) // a	{		ypos++; 		glLoadIdentity();		gluLookAt(xpos, ypos, zpos, xpos, ypos, zpos-1, 0.0, 1.0, 0.0);	}}