Resizing an SDL window and drawing in GL, view position moves

[officialhopsof]

I am a little unsure as to what exactly is going on with this, but basically I have the following code:

#include "SDL/SDL.h"
#include "Button.h"
#include "SDL/SDL_opengl.h"
#include <iostream>
 
using namespace std;
 
const int SCREENWIDTH  = 800;
const int SCREENHEIGHT = 600;
 
void buttonAction()
{
    cout<<"Button Pressed"<<endl;
}
 
void init_GL(int width, int height)
{
    glClearColor( 0, 0, 0, 0 );
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho( 0, width, height, 0, -1, 1 );
    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
}
 
int main( int argc, char* args[] )
{
    //The images
    SDL_Surface* hello  = NULL;
    SDL_Surface* screen = NULL;
    bool         quit   = false;
    SDL_Event    event;
 
    int screenWidth  = SCREENWIDTH;
    int screenHeight = SCREENHEIGHT;
 
    //Start SDL
    SDL_Init( SDL_INIT_EVERYTHING );
 
    //Set up screen
    screen = SDL_SetVideoMode( screenWidth, screenHeight, 32, SDL_OPENGL | SDL_RESIZABLE );
 
    //Start OpenGL
    init_GL(screenWidth, screenHeight);
 
 
    SDL_WM_SetCaption( "Button Test", NULL );
    Button button(30, 30, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", *&buttonAction);
 
    //While the user hasn't quit
    while( quit == false )
    {
 
 
            //If there's events to handle
            if( SDL_PollEvent( &event ) )
            {
                if(event.type == SDL_VIDEORESIZE)
                {
                    screenWidth  = event.resize.w;
                    screenHeight = event.resize.h;
 
                    glViewport(0, 0, screenWidth, screenHeight);
                    glMatrixMode( GL_PROJECTION );
                    glLoadIdentity();
                    glOrtho( 0, screenWidth, screenHeight, 0, -1, 1 );
                    glMatrixMode( GL_MODELVIEW );
                    glLoadIdentity();
                }
 
                //Handle button events
                button.handleEvents(event);
 
                //If the user has Xed out the window
                if( event.type == SDL_QUIT )
                {
                    //Quit the program
                    quit = true;
                }
            }
 
        glPushMatrix();
 
            //Fill the screen black
            glBegin( GL_QUADS );
                glColor4f( 0.0, 0.0, 0.0, 1.0 );
                glVertex3f( 0, 0, 0 );
                glVertex3f( screenWidth, 0, 0 );
                glVertex3f( screenWidth, screenHeight, 0 );
                glVertex3f( 0, screenHeight, 0 );
            glEnd();
 
            //Show the button
            button.draw();
 
        glPopMatrix();
 
        //Update the screen
        SDL_GL_SwapBuffers();
    }
 
    //Quit SDL
    SDL_Quit();
 
    return 0;
}

and when i first draw everything seems to be working fine, and looks like this:


but when i resize i see the following:


any idea as to what is causing my problem?

Thank you for your time!
Brandon Murphy

[tanzanite]

You define a clear color but never actually clear the screen. Add a "glClear(GL_COLOR_BUFFER_BIT)" somewhere.

edit:

//Fill the screen black
            glBegin( GL_QUADS );
                glColor4f( 0.0, 0.0, 0.0, 1.0 );
                glVertex3f( 0, 0, 0 );
                glVertex3f( screenWidth, 0, 0 );
                glVertex3f( screenWidth, screenHeight, 0 );
                glVertex3f( 0, screenHeight, 0 );
            glEnd();

Use the mentioned glClear(GL_COLOR_BUFFER_BIT) here instead.

[officialhopsof]

That fixed the issue of it drawing over itself, but the image still moves around the screen as I resize the window. I have the origin set to be the upper left corner (or so I believe, if i did it correctly).

Thanks!
Brandon

[kRogue]

You need to call glViewport when the window is resized.. Also, you really should call glClear() at the start of every frame.

[officialhopsof]

I do call glViewPort when the screen is resized. I have boiled my problem down a little more and have the following code now:

#include "SDL/SDL.h"
#include "SDL/SDL_opengl.h"
#include <iostream>
 
using namespace std;
 
const int SCREENWIDTH  = 800;
const int SCREENHEIGHT = 600;
 
void init_GL(int width, int height)
{
    glClearColor( 0, 0, 0, 0 );
    glViewport(0, 0, width, height);
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho( 0, width, height, 0, -1, 1 );
    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
}
 
int main( int argc, char* args[] )
{
    SDL_Surface* screen = NULL;
    bool         quit   = false;
    SDL_Event    event;
 
    int screenWidth  = SCREENWIDTH;
    int screenHeight = SCREENHEIGHT;
 
    SDL_Init( SDL_INIT_EVERYTHING );
 
    screen = SDL_SetVideoMode( screenWidth, screenHeight, 32, SDL_OPENGL | SDL_RESIZABLE | SDL_DOUBLEBUF);
 
    init_GL(screenWidth, screenHeight);
 
    SDL_WM_SetCaption( "Button Test", NULL );
 
    while( quit == false )
    {
            if( SDL_PollEvent( &event ) )
            {
                if(event.type == SDL_VIDEORESIZE) // The window is resized
                {
                    screenWidth  = event.resize.w;
                    screenHeight = event.resize.h;
 
                    glViewport(0, 0, screenWidth, screenHeight);
                    glMatrixMode( GL_PROJECTION );
                    glLoadIdentity();
                    glOrtho( 0, screenWidth, screenHeight, 0, -1, 1 );
                    glMatrixMode( GL_MODELVIEW );
                    glLoadIdentity();
                }
 
                if( event.type == SDL_QUIT )
                {
                    quit = true;
                }
            }
 
        glPushMatrix();
 
            glClear(GL_COLOR_BUFFER_BIT);
 
            glTranslatef( 50, 50, 0 );
 
            glBegin(GL_LINE_STRIP);
                glColor4f( 1.0, 1.0, 1.0, 1.0 );
 
                glVertex3f( 0,  0,  0 );
                glVertex3f( 50, 0,  0 );
                glVertex3f( 50, 50, 0 );
                glVertex3f( 0,  50, 0 );
                glVertex3f( 0,  0,  0 );
 
            glEnd();
 
        glPopMatrix();
 
        SDL_GL_SwapBuffers();
    }
 
    SDL_Quit();
 
    return 0;
}

if you run that you will notice that as i make the window shorter the square that I drew moves downward, yet I am not calling a translate method on it. when i make the window taller the square moves upward. Resizing the width of the window has no effect on the position of the square.

Thank you for your time,
Brandon Murphy