Resizable OpenGL window

SDL Development
1

Hi!

Consider the following little program, running under Linux with a TNT2 card
and nVidia GLX:

---- snip ----
#include <SDL.h>
#include <GL/gl.h>

static void window_resized(int w, int h)
{
SDL_SetVideoMode(w, h, 16, SDL_OPENGL | SDL_RESIZABLE);
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1, 1, 0, -1, 1);

glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_LINES);
	glVertex2i(0, 0); glVertex2i(1, 1);
	glVertex2i(1, 0); glVertex2i(0, 1);
glEnd();
SDL_GL_SwapBuffers();

}

int main(void)
{
SDL_Surface *s;
SDL_Event e;
int done = 0;

SDL_Init(SDL_INIT_VIDEO);
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 5);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 5);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 5);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
s = SDL_SetVideoMode(320, 240, 16, SDL_OPENGL | SDL_RESIZABLE);
window_resized(320, 240);

while (!done) {
	SDL_WaitEvent(&e);
	switch (e.type) {
		case SDL_VIDEORESIZE:
			window_resized(e.resize.w, e.resize.h);
			break;
		case SDL_QUIT:
			done = 1;
			break;
	}
}

SDL_Quit();
return 0;

}
---- snap ----

It is supposed to show a window with a diagonal white cross that resizes when
you resize the window. My concern is with the SDL_SetVideoMode() call in
window_resized(). It is appearently necessary, otherwise OpenGL gets the
geometry wrong. Why is this?

Bye,
Christian–
/ Coding on PowerPC and proud of it
/ http://www.uni-mainz.de/~bauec002/

2

It is supposed to show a window with a diagonal white cross that resizes when
you resize the window. My concern is with the SDL_SetVideoMode() call in
window_resized(). It is appearently necessary, otherwise OpenGL gets the
geometry wrong. Why is this?

SDL just reports that the window geometry has been changed by the user.
Because you may be in the process of rendering, or more importantly, drawing
in 2D to the display surface, SDL doesn’t change the internal variables out
from under you. This means that you need to re-set the video mode to the
correct size (or a different preferred size) when you get a resize event.

See ya,
-Sam Lantinga, Lead Programmer, Loki Entertainment Software