SDL: Unable to flip multiple surfaces - c

Question

I've been trying to flip surfaces and have been successful if I'm only flipping a single surface (the same surface I got back from SDL_SetVideoMode). If I try to flip the surface I get back from SDL_DisplayFormat, nothing happens. I've attached demo code that demonstrates my problem:
#include <stdio.h>
#include <stdlib.h>
#include "SDL/SDL.h"
void main()
{
int i;
SDL_Surface *mysurface1;
SDL_Surface *mysurface2;
char *pxl;
SDL_Init( SDL_INIT_EVERYTHING );
mysurface1 = SDL_SetVideoMode( 640, 480, 8, SDL_DOUBLEBUF|SDL_HWSURFACE );
for (i = 0; i < 20; i++)
{
pxl = (char *)mysurface1->pixels + i*mysurface1->pitch + i;
*pxl = 100; // Red Line
}
SDL_Flip(mysurface1); // Works, we see a red line
sleep(5);
printf("Sleeping for 5...\n");
mysurface2 = SDL_DisplayFormat(mysurface1);
for (i = 0; i < 20; i++)
{
pxl = (char *)mysurface2->pixels + i*mysurface2->pitch + i;
*pxl = 255; // White line
}
SDL_Flip(mysurface2); // White line doesnt appear
printf("Done... No white line\n");
sleep(10);
}
Has anyone ever seen this before? Again, I think I tracked it down to surfaces that wont display if its a surface I got back from SDL_DisplayFormat. If I do it on the surface I get back from SDL_SetVideoMode, then I see the red line and everything works fine.

Answer 1

You can only flip the main display surface (the one created with SDL_SetVideoMode). In order to make your other surface visible, you need to blit it onto the main surface. Lookup SDL_BlitSurface for details on how to do that.

Answer 2

Pass the screen to the SDL_Flip function. The flip function modifies the value of screen->pixels so that it points to the surface that isn't visible on the screen.
However, this is only applicable to video devices such as SVGA and DGA. On X11, calling SDL_Flip(screen) is equivalent to calling SDL_UpdateRect(screen, 0, 0, 0, 0).
#include <stdio.h>
#include <stdlib.h>
#include "SDL/SDL.h"
void main()
{
int i;
SDL_Surface *screen;
char *pxl;
SDL_Init( SDL_INIT_EVERYTHING );
screen = SDL_SetVideoMode( 640, 480, 8, SDL_DOUBLEBUF|SDL_HWSURFACE );
printf("Drawing the red line ...\n");
printf("screen->pixels = %p\n", screen->pixels);
for (i = 0; i < 100; i++)
{
pxl = (char *)screen->pixels + i*screen->pitch + i;
*pxl = 100; // Red Line
}
printf("Flip screens\n");
SDL_Flip(screen); // Display the red line
printf("Drawing the white line ...\n");
printf("screen->pixels = %p\n", screen->pixels);
for (i = 0; i < 100; i++)
{
pxl = (char *)screen->pixels + i*screen->pitch + i;
*pxl = 255; // White line
}
sleep(3);
printf("Flip screens\n");
SDL_Flip(screen); // Display the white line
sleep(10);
}
On my Linux notebook, this prints:
Drawing the red line ...
screen->pixels = 0xb6c8c008
Flip screens
Drawing the white line ...
screen->pixels = 0xb6c8c008
Flip screens
The value of screen->pixels is the same, but this is only because on X11 the flip operation is a no-operation. On a video device such as SVGA or DGA, the two values would be different.

Answer 3

First, it seems SDL_Flip() only works on surfaces that correspond to the screen or a window, like those created by SDL_SetVideoMode(). Your other surface is off-screen; it doesn't make much sense to double-buffer it (or flip it), and it most likely isn't double-buffered anyway. Being an off screen surface, it won't appear until you blit it to your display surface with SDL_BlitSurface() or a similar function -- then, the changes will be visible next time you flip the display surface.
Essentially, mysurface2 isn't actually on your display until you put it there, by blitting it onto a surface that is on your display. If you replace the following:
SDL_Flip(mysurface2); // White line doesnt appear
With this:
SDL_BlitSurface(mysurface2,NULL,mysurface1,NULL);
SDL_Flip(mysurface1);
...then your code will probably work as you expect.