From: "Al Morgan" <muaddib AT proaxis DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Page flipping with allegro
Date: Tue, 17 Aug 1999 04:04:05 -0700
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

What is a fast way to do page flipping?  I made a fast program to teach
myself allegro, but page flipping goes REALLY slow:

As you can see, I'm using scroll_screen to switch between the top and bottom
of a 320x480 screen, with a 320x240 viewport, but normally the little guy
that is moving around goes too fast to control, but with my page flipping
routines, he slows down to a crawl.

#include <allegro.h>

#define MASK_COLOR_8 0

int CURRENT_SCREEN = 0;

void page_flip()
    {
    scroll_screen(0, CURRENT_SCREEN * 240);
    CURRENT_SCREEN = !CURRENT_SCREEN;
    clear(screen);
    }

class player
    {
    public:
        float x;
        float y;
        float speed;
        BITMAP *image;

        player()
            {
            PALETTE p;
            image = load_bmp("player.bmp", p);  // this is a small bmp of a
little guy
            set_palette(p);


            x = 60;
            y = 60;
            speed = 1;
            }
        void inline move()
            {
            if(key[KEY_UP] && y > 10)
                y -= speed;
            if(key[KEY_DOWN] && y < 190)
                y += speed;
            if(key[KEY_LEFT] && x > 10)
                x -= speed;
            if(key[KEY_RIGHT] && x < 270)
                x += speed;
            }
        void inline draw()
            {
            draw_sprite(screen, image, (int)x, (int)y + CURRENT_SCREEN *
240);
            }
    };

void main()
    {
    allegro_init();
    install_keyboard();
    install_mouse();

    set_gfx_mode(GFX_MODEX, 320, 240, 320, 480);

    set_clip(screen, 0, 0, 319, 479);

    clear(screen);

    player bob;

    while(!key[KEY_ENTER])
        {
        bob.move();
        bob.draw();
        page_flip();
        }
    }