How do this viewport() and gluOrtho2d work?

OpenGL OpenGL: Basic Coding
1

Hello everyone,

I’m new to openGL, and I’m currently reading the Red book. While practicing the example in the book, I was stuck with this code:


void reshape( int w, int h ) {
	glViewport( 0, 0, w, h );
	glMatrixMode( GL_PROJECTION );
	glLoadIdentity();
	float aspect_ratio = static_cast<float>( w )/h;
	if( w <= h ) {
		gluOrtho2D( -1.0, 1.0, -1.0 / aspect_ratio, 1.0 * aspect_ratio );
	}
	else {
		gluOrtho2D( -1.0 * aspect_ratio, 1.0 * aspect_ratio, -1.0, 1.0 );
	}
	glMatrixMode( GL_MODELVIEW );
	glLoadIdentity();
}

The confused part is the if-else statement. I don’t understand the side affect of aspect_ratio? Does the left, right, bottom up scale base on the window width and height? Could anyone help me explain it?

Thanks,

2

Think of it this way. No matter what resolution the user stretches are window to, we want to be able to display the entire eye-space region X=-1…1, Y=-1…1 in the visible part of the window without “stretching” it (assumes square pixels).

So if the window is “wide” (w>h), then we map Y=-1…1 over the entire height of the window (the smallest dimension)

And if the window is “tall” (h>w) then we map X=-1…1 over the entire width of the window (the smallest dimension).

The aspect ratio of the remaining dimension just scaled to match the aspect ratio of the window (again, as I said, it assumes pixels are square) so that eye-space doesn’t appear to be “stretched” to map it to the window.

3

Hi Dark Photon,
Many thanks. I kinda see it now.