How to set the origin at bottom left in 3d - c++

Question

I want to draw a cube of side 40 at bootom left corner. my glortho function is
glOrtho(0, // left
1000, // right
0, // bottom
1000, // top
0, // zNear
1000 // zFar
);
and lenght of x,y,z axis is up to 1000. so cube should be at bottom left and dimensions should be as i given. and what should be the gluLookAt(); function. I am not getting correct output. If there is any mistakes in the code, correct it and what function should add to the code.
#include <gl/glut.h>
#include <gl/gl.h>
#include <gl/glu.h>
#include <stdio.h>
#include <stdarg.h>
#include <math.h>
#define GL_GLEXT_PROTOTYPES
#ifdef __APPLE__
#else
#endif
void display();
void specialKeys();
double rotate_y=0;
double rotate_x=0;
void display(){
// Clear screen and Z-buffer
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
// Reset transformations
glLoadIdentity();
// Rotate when user changes rotate_x and rotate_y
glRotatef( rotate_x, 1.0, 0.0, 0.0 );
glRotatef( rotate_y, 0.0, 1.0, 0.0 );
// side - FRONT
glBegin(GL_POLYGON);
glColor3f( 1.0, 0.0, 0.0 );
glVertex3f( 0, 0, 0);
glVertex3f( 40,0,0);
glVertex3f(40,40,0 );
glVertex3f(0,40,0 );
glEnd();
// side - BACK
glBegin(GL_POLYGON);
glColor3f( 1.0,0.0,1.0 );
glVertex3f( 0,0,40 );
glVertex3f( 0,40,40);
glVertex3f( 40,40,40 );
glVertex3f( 40,0,40 );
glEnd();
// side - RIGHT
glBegin(GL_POLYGON);
glColor3f( 0.0, 0.0, 1.0 );
glVertex3f( 40,40,0 );
glVertex3f( 40,0,0 );
glVertex3f( 40,0,40 );
glVertex3f( 40,40,40 );
glEnd();
// side - LEFT
glBegin(GL_POLYGON);
glColor3f( 0.0, 1.0, 0.0 );
glVertex3f( 0,0,0 );
glVertex3f( 0,40,0 );
glVertex3f( 0,40,40 );
glVertex3f( 0,0,40 );
glEnd();
// side - TOP
glBegin(GL_POLYGON);
glColor3f( 0.0,0.0,1.0 );
glVertex3f( 0,40,0);
glVertex3f( 0,40,40 );
glVertex3f( 40,40,40 );
glVertex3f( 40,40,0 );
glEnd();
// side - BOTTOM
glBegin(GL_POLYGON);
glColor3f( 1.0, 0.5, 0.0 );
glVertex3f( 0,0,0 );
glVertex3f( 40,0,0 );
glVertex3f( 40,0,40 );
glVertex3f( 0,0,40);
glEnd();
glFlush();
glutSwapBuffers();
}
void init()
{
glClearColor(0.5,0.5,0.0, 0.0);
glColor3f(1,0,0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//gluOrtho2D(-1.0,1.0,-1.0,1.0);
glOrtho(0, // left
1000, // right
0, // bottom
1000, // top
0, // zNear
1001 // zFar
);
gluLookAt(0.0, 0.0, 0.0, 0.0, 0.0, -1000.0, 0.0, 1000.0, 0.0);
}
void specialKeys( int key, int x, int y ) {
// Right arrow - increase rotation by 5 degree
if (key == GLUT_KEY_RIGHT)
rotate_y += 5;
// Left arrow - decrease rotation by 5 degree
else if (key == GLUT_KEY_LEFT)
rotate_y -= 5;
else if (key == GLUT_KEY_UP)
rotate_x += 5;
else if (key == GLUT_KEY_DOWN)
rotate_x -= 5;
// Request display update
glutPostRedisplay();
}
int main(int argc, char* argv[]){
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(1000, 1000);
glutInitWindowPosition(10, 10);
// Create window
glutCreateWindow("Awesome Cube");
// Enable Z-buffer depth test
glEnable(GL_DEPTH_TEST);
glutDisplayFunc(display);
glutSpecialFunc(specialKeys);
init();
glutMainLoop();
return 0;
}

Answer 1

You screwed up your transformations. In Init() you set the current matrix mode to GL_PROJECTION and load some ortho matrix. Then you multiply the lookAt matrix onto this. This is wrong in principle, as the lookAt matrix should be applied to the GL_MODELVIEW stack. (The lookAt parameters you chose actually result in an identity lookAt matrix, so that call has no effect, but that is only a side note).
However, the real error is in display(). There you have glLoadIdentity() which will just overwrite your previous matrix with an identity matrix, so you lose the Ortho transform you did set up, since you still have GL_PROJECTION matrix stack active.
The correct way would be something like:
void init()
{
// ... your other stuff
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho( /* your ortho params */ );
glMatrixMode(GL_MODELVIEW); // switch back to the modelView matrix stack
}
void display()
{
glLoadIdentity();
gluLookAt( /* your Lookat parameters */ );
glRotate/Scale/Translate(...); // your local transformations
// ...
}
Note that all of that stuff is completely deprecated and has been removed from the core profile of modern OpenGL versions. When learing OpenGL nowadays, you should consider not learning that old cruft from 20 years ago.