OpenGL - how to fit a window on resize - c++

Question

I have a shape that is centered on the window screen. For now on window resize it remains the same (same width and height).
What is the best way to fit it to the window on resize, and also keep it's aspect ratio?
const int WIDTH = 490;
const int HEIGHT = 600;
/**
* render scene
*/
void renderScene(void)
{
//clear all data on scene
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//set white bg color
glClearColor(1, 1, 1, 1);
glColor3ub(153, 153, 255);
glBegin(GL_POLYGON);
glVertex2f(60, 310);
glVertex2f(245, 50);
glVertex2f(430, 310);
glVertex2f(245, 530);
glEnd();
glutSwapBuffers();
}
/**
* reshape scene
*/
void reshapeScene(GLint width, GLint height)
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glViewport(0, 0, width, height);
gluOrtho2D(0, width, height, 0);
glMatrixMode(GL_MODELVIEW);
glutPostRedisplay();
}
/**
* entry point
*/
int main(int argc, char **argv)
{
//set window properties
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
glutInitWindowSize(WIDTH, HEIGHT);
glutInitWindowPosition((glutGet(GLUT_SCREEN_WIDTH)-WIDTH)/2, (glutGet(GLUT_SCREEN_HEIGHT)-HEIGHT)/2);
glutCreateWindow("Test Window");
//paste opengl version in console
printf((const char*)glGetString(GL_VERSION));
//register callbacks
glutDisplayFunc(renderScene);
glutReshapeFunc(reshapeScene);
//start animation
glutMainLoop();
return 0;
}
Thanks

Answer 1

At the top:
const int WIDTH = 490;
const int HEIGHT = 600;
const float ASPECT = float(WIDTH)/HEIGHT; // desired aspect ratio
Then reshapeScene:
void reshapeScene(GLint width, GLint height)
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
int w = height * ASPECT; // w is width adjusted for aspect ratio
int left = (width - w) / 2;
glViewport(left, 0, w, height); // fix up the viewport to maintain aspect ratio
gluOrtho2D(0, WIDTH, HEIGHT, 0); // only the window is changing, not the camera
glMatrixMode(GL_MODELVIEW);
glutPostRedisplay();
}
Because you are not constraining the window resize to a particular aspect ratio, you need to pick one of the two dimensions (height, here) to drive the viewport reshape and calculate an adjusted width based on that height and the desired aspect ratio. See adjusted glViewport call above.
Also, gluOrtho2D is your camera, essentially. Since you aren't moving the camera or the object so much, you don't need to change that (so it just uses the initial WIDTH and HEIGHT).