macOS OpenGL: simple problem with 2 overlapping squares

[Soulstorm]

I am taking my first steps on OpenGL, and I have the following code written using GLUT Library:

#include <Carbon/Carbon.h>
#include <GLUT/glut.h>
#include <OpenGL/OpenGL.h>
#include <iostream>
#include <cmath>


static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;

///////////////////////////////////////////////////////////
// Called to draw scene
void RenderScene(void)
{
	glEnable(GL_DEPTH_TEST);
	glPolygonMode(GL_BACK,GL_FILL);
	//glEnable(GL_CULL_FACE);
	// Clear the window and the depth buffer
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	
	glColor3f(1.0f,1.0f,0.0f);
	
	glPushMatrix();
	glRotatef(xRot, 1.0f, 0.0f, 0.0f);
	glRotatef(yRot, 0.0f, 1.0f, 0.0f);
	
	glBegin(GL_QUADS);
	
	glVertex3f(0,0,0);
	glVertex3f(50,0,0);
	glVertex3f(50,50,0);
	glVertex3f(0,50,0);
	
	glEnd();
	
	glColor3f(1,0,0);
	
	glBegin(GL_QUADS);
	
	glVertex3f(50,50,0);
	glVertex3f(50,50,50);
	glVertex3f(50,0,50);
	glVertex3f(50,0,0);
	
	glEnd();
	
	
	glPopMatrix();
	glFlush();
}



// This function does any needed initialization on the rendering
// context. 
void SetupRC()
{
	// Black background
	glClearColor(0.0f, 0.0f, 0.0f, 1.0f );
	
	// Set drawing color to green
	glColor3f(0.0f, 1.0f, 0.0f);
	
	// Set color shading model to flat
	glShadeModel(GL_FLAT);
	
	// Clock wise wound polygons are front facing, this is reversed
	// because we are using triangle fans
	glFrontFace(GL_CW);
	
	glEnable(GL_DEPTH_TEST);
	
}


///////////////////////////////////////////////////////////
// Called by GLUT library when the window has chanaged size
void ChangeSize(int w, int h)
{
    GLfloat nRange = 100.0f;
	
    // Prevent a divide by zero
    if(h == 0)
        h = 1;
	
    // Set Viewport to window dimensions
    glViewport(0, 0, w, h);
	
	
    // Reset coordinate system
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
	
    // Establish clipping volume (left, right, bottom, top, near, far)
    if (w <= h) 
        glOrtho (-nRange, nRange, nRange*h/w, -nRange*h/w, -nRange*2.0f, nRange*2.0f);
    else 
        glOrtho (-nRange*w/h, nRange*w/h, nRange, -nRange, -nRange*2.0f, nRange*2.0f);
	
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


void SpecialKeys(int key, int x, int y)
{
	if(key == GLUT_KEY_UP)
		xRot-= 5.0f;
	
	if(key == GLUT_KEY_DOWN)
		xRot += 5.0f;
	
	if(key == GLUT_KEY_LEFT)
		yRot -= 5.0f;
	
	if(key == GLUT_KEY_RIGHT)
		yRot += 5.0f;
	
	if(key > 356.0f)
		xRot = 0.0f;
	
	if(key < -1.0f)
		xRot = 355.0f;
	
	if(key > 356.0f)
		yRot = 0.0f;
	
	if(key < -1.0f)
		yRot = 355.0f;
	
	// Refresh the Window
	glutPostRedisplay();
}

///////////////////////////////////////////////////////////
// Program entry point
int main(int argc, char* argv[])
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_RGB | GLUT_RGB);
	glutInitWindowSize(800,600);
	glutCreateWindow("OpenGL Test");
	
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);
	glutSpecialFunc(SpecialKeys);
	glutMainLoop();
	
	SetupRC();
	return 0;
}

The code was supposed to show 2 squares, connected to each other like an open book. So far so good. But when I try to rotate the image, the red square always overlaps the yellow one. Why is this happening?

 

[lazydog]

I think you need to attach a depth buffer to the window.

Try:-

glutInitDisplayMode(GLUT_RGB | GLUT_RGB | GLUT_DEPTH);

b e n

 

[Soulstorm]

It was a stupid problem after all... Thanks a lot!

 

[Soulstorm]

I have another problem. I want to make a 3D cube, but I still have the same problem. The depth testing is enabled this time. Here is my code.

#include <Carbon/Carbon.h>
#include <GLUT/glut.h>
#include <OpenGL/OpenGL.h>
#include <iostream>
#include <cmath>


static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;

///////////////////////////////////////////////////////////
// Called to draw scene
void RenderScene(void)
{
	// Angle of revolution around the nucleus
	//static float fElect1 = 0.0f;
	
	// Clear the window with current clearing color
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glPushMatrix();
	
	glRotatef(xRot, 1.0f, 0.0f, 0.0f);
    glRotatef(yRot, 0.0f, 1.0f, 0.0f);
	
	glBegin(GL_QUADS);
	
	glColor3f(1,0,0);
	//mprostini pleyra
	glVertex3f(0,0,0);
	glVertex3f(50,0,0);
	glVertex3f(50,50,0);
	glVertex3f(0,50,0);
	
	glColor3f(0,1,0);
	//deksia pleyra
	glVertex3f(50,0,0);
	glVertex3f(50,0,50);
	glVertex3f(50,50,50);
	glVertex3f(50,50,0);
	
	glColor3f(0,0,1);
	//panw pleyra
	glVertex3f(50,50,0);
	glVertex3f(50,50,50);
	glVertex3f(0,50,50);
	glVertex3f(0,50,0);
	
	glColor3f(1,1,1);
	//aristeri pleyra
	glVertex3f(0,0,50);
	glVertex3f(0,0,0);
	glVertex3f(0,50,0);
	glVertex3f(0,50,50);
	
	glColor3f(0,1,1);
	//katw pleyra
	glVertex3f(0,0,0);
	glVertex3f(50,0,0);
	glVertex3f(50,0,50);
	glVertex3f(0,0,50);
	
	glColor3f(1,1,0);
	//pisw pleyra
	glVertex3f(0,0,50);
	glVertex3f(50,0,50);
	glVertex3f(50,50,50);
	glVertex3f(0,50,50);
	
	glEnd();
	
	glPopMatrix();
	// Show the image
	glutSwapBuffers();
}

// This function does any needed initialization on the rendering
// context. 
void SetupRC()
{
	// Black background
	glClearColor(0.0f, 0.0f, 0.0f, 1.0f );
	
	// Set drawing color to green
	glColor3f(0.0f, 1.0f, 0.0f);
	
	// Set color shading model to flat
	glShadeModel(GL_FLAT);
	
	// Clock wise wound polygons are front facing, this is reversed
	// because we are using triangle fans
	glFrontFace(GL_CCW);
	glEnable(GL_CULL_FACE);
	glEnable(GL_DEPTH_TEST);
}


///////////////////////////////////////////////////////////
// Called by GLUT library when the window has chanaged size
void ChangeSize(int w, int h)
{
    GLfloat nRange = 100.0f;
	
    // Prevent a divide by zero
    if(h == 0)
        h = 1;
	
    // Set Viewport to window dimensions
    glViewport(0, 0, w, h);
	
	
    // Reset coordinate system
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
	
    // Establish clipping volume (left, right, bottom, top, near, far)
    if (w <= h) 
        glOrtho (-nRange, nRange, nRange*h/w, -nRange*h/w, -nRange*2.0f, nRange*2.0f);
    else 
        glOrtho (-nRange*w/h, nRange*w/h, nRange, -nRange, -nRange*2.0f, nRange*2.0f);
	
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


void SpecialKeys(int key, int x, int y)
{
	if(key == GLUT_KEY_UP)
		xRot-= 5.0f;
	
	if(key == GLUT_KEY_DOWN)
		xRot += 5.0f;
	
	if(key == GLUT_KEY_LEFT)
		yRot -= 5.0f;
	
	if(key == GLUT_KEY_RIGHT)
		yRot += 5.0f;
	
	if(key > 356.0f)
		xRot = 0.0f;
	
	if(key < -1.0f)
		xRot = 355.0f;
	
	if(key > 356.0f)
		yRot = 0.0f;
	
	if(key < -1.0f)
		yRot = 355.0f;
	
	// Refresh the Window
	glutPostRedisplay();
}

///////////////////////////////////////////////////////////
// Program entry point
int main(int argc, char* argv[])
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(800,600);
	glutCreateWindow("OpenGL Test");
	
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);
	glutSpecialFunc(SpecialKeys);


	glutMainLoop();
	
	SetupRC();
	return 0;
}

I am trying to rotate the cube using the arrow keys, but the sides are not drawing properly. Any ideas? Have I drawn them using the wrong direction? Sorry for the newbish questions, these are my first steps with OpenGL.

 

[lazydog]

Hi

You need to do a:-

	glEnable( GL_DEPTH_TEST ) ;

A good place would be in your RenderScene() function just after clearing the viewport.

b e n

 

[Soulstorm]

Hi

You need to do a:-

	glEnable( GL_DEPTH_TEST ) ;

A good place would be in your RenderScene() function just after clearing the viewport.

b e n

That worked, thanks a lot. But, I had put this command in my SetupRC() function. Why didn't it work? Why did I have to put it after I clear the viewport?

 

[lazydog]

…because SetupRC() isn't called until after your main loop has exited!

b e n