Trying to Scale OpenGL Program with Width (but not height)? - c++

Question

I am fiddling with graphics, and trying to find a way to have the shape I am drawing (in this case, a triangle) scale with window sizing. However, I only want the width to update, and the height to remain the same.
I have done some research, and tried using glutGet(GLUT_SCREEN_WIDTH) as a multiplier or similar (e.g. (GLUT_SCREEN_WIDTH /100) - 250) to the vertices of my shape, but I feel I may be missing a key idea. Should I instead be applying the scaling operation to the viewport, not the shape's points? Whenever I scale the points, they don't seem to scale with the window. Code below.
#define GLUT_DISABLE_ATEXIT_HACK
#include <GL/glut.h>
#include <GL/gl.h>
//#include <assert.h>
void init (void)
{
glClearColor (1.0, 1.0, 0.0, 0.0); /* Set background to yellow */
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}
void display(void)
{
glClear (GL_COLOR_BUFFER_BIT);
glColor3f (0.0, 0.0, 1.0);
glBegin(GL_TRIANGLES);
glVertex2d (0.0, 0.0);
glVertex2d(1.0, 0.0);
glVertex2d (0.5, 0.866);
glEnd();
glFlush (); //Display immediately
}
void keyEscape( unsigned char key, int x, int y )
{
switch ( key )
{
case 113: // 'Q' key for escape
int windowID = glutCreateWindow ("triangle");
glutDestroyWindow (windowID);
exit (0);
break;
}
glutPostRedisplay();
}
void mouseEscape( int button, int state, int x, int y )
{
if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN)
{
int windowID = glutCreateWindow ("triangle");
glutDestroyWindow (windowID);
exit (0);
glutPostRedisplay();
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (250, 250);
glutInitWindowPosition ((glutGet(GLUT_SCREEN_WIDTH)-250)/2, (glutGet(GLUT_SCREEN_HEIGHT)-250)/2);
glutCreateWindow ("triangle");
init ();
glutKeyboardFunc(keyEscape);
glutMouseFunc(mouseEscape);
glutDisplayFunc(display);
glutMainLoop();
return 0;
}

Answer 1

Step 1: Get rid of the Init function. Projection setup is part of the drawing process.
Step 2: Use the Window width as input for the left/right parameter of glOrtho
Like this:
void display(void)
{
int const win_width = glutGet(GLUT_WINDOW_WIDTH);
int const win_height = glutGet(GLUT_WINDOW_HEIGHT);
float const win_aspect = (float)win_width / (float)win_height;
glClearColor (1.0, 1.0, 0.0, 0.0); /* Set background to yellow */
glClear (GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, win_aspect, 0.0, 1.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor3f (0.0, 0.0, 1.0);
glBegin(GL_TRIANGLES);
glVertex2d (0.0, 0.0);
glVertex2d(1.0, 0.0);
glVertex2d (0.5, 0.866);
glEnd();
glFlush (); // Tell OpenGL to process what we submitted so far
}
BTW: You should switch to a double buffered mode and use glutSwapBuffers instead of glFlush/glFinish; on some systems single buffered mode doesn't work (well). Today the only reliable method is double buffering.