Create an empthy surface in SDL

[Oromis]

(I asked this before on the beginnersforum, but I didn't get an answer)

How can I create an empthy surface in SDL? Is something like the following possible?

SDL_Surface *mySurface;
//create an empthy surface, setting the width and height
mySurface = CreateSurface (400,500); 

//perform some actions with the surface:
SDL_FillRect(mySurface, &mySurface->clip_rect, SDL_MapRGB(255,0,0); //draw red
apply_surface(20,20,someSurface,mySurface); //blit another surface to it

//blit surface to the screen
apply_surface(0,0,mySurface,screen);

If my question isn't clear, please tell me.

[guestgulkan]

mySurface = CreateSurface (400,500);

No.


Here is a beginners SDL tutorial:
http://gpwiki.org/index.php/SDL:Tutorials:Displaying_a_Bitmap

[Oromis]

Thanks for the reply.

Normally I don't post several topics on the same subjects, but I didn't get an answer. Next time I shall be more patient. :)

This tutial didn't help very much. I'm pretty sure that what I want is possible, but maybe you understand me wrong. I played around a bit with some code. Can you tell me why the following program won't run? I'm trying to create a white surface. But the SDL_FillRect() command crashes the program.

#include "SDL/SDL.h"

//screen attributes
const int SCREEN_HEIGHT = 500;
const int SCREEN_WIDTH  = 500;
const int SCREEN_BPP    = 32;

//screen
SDL_Surface *screen  = NULL;
//the surface
SDL_Surface *surface = NULL;

//function to draw a surface to the screen
void apply_surface(int x, int y, SDL_Surface* source, SDL_Surface* destination)
{
    SDL_Rect offsets;
    offsets.x=x;
    offsets.y=y;
    SDL_BlitSurface(source,NULL,destination,&offsets);
}

int main(int argc, char* args[])
{
    //Initialize SDL
    SDL_Init(SDL_INIT_EVERYTHING);
    
    //Set screen to video mode
    screen = SDL_SetVideoMode(SCREEN_WIDTH,SCREEN_HEIGHT,SCREEN_BPP,SDL_SWSURFACE);
    
    //==========================================================================
    //Here I'm trying to create an empthy surface:
    
    //allocate memory
    surface = new SDL_Surface;
    
    //set width and height
    surface->clip_rect.w = 300;
    surface->clip_rect.h = 300;
    
    //Draw the surface white (this comand crashes the program)
    SDL_FillRect(surface,&surface->clip_rect,SDL_MapRGB(surface->format,0xff,0xff,0xff));
    
    //apply the surface to the screen
    apply_surface( 100, 100, surface, screen);
    
    //flip screen
    SDL_Flip(screen);
    
    //whait before ending
    SDL_Delay(5000);
    
    //free surface
    SDL_FreeSurface(surface);
    
    //==========================================================================
    
    //quit SDL
    SDL_Quit();
    
    return 0;
}

(I know I should check for errors etc, but that's not really important now)

I tried the following too, and it doesn't crash the program. But the screen is still completely black:

//use the screen format
SDL_FillRect(surface,&surface->clip_rect,SDL_MapRGB(screen->format,0xff,0xff,0xff));

Thanks for your help.

[Disch]

There is an SDL_CreateRGBSurface that works almost exactly how you expected in your original post. I don't know why guestgulkan said "no" to that.

SDL_Surface* mySurface = SDL_CreateRGBSurface( flags, width, height, bitdepth, rmask, gmask, bmask, amask );

// Blit to/from mySurface at will

SDL_FreeSurface(mySurface);

performance is best when bitdepth and the masks match your output display format (that way no conversion has to take place) -- assuming this surface is going to be blitted to the display or another surface of the same format. To get that stuff you can probe the SDL_PixelFormat* of the surface created via SDL_SetVideoMode. To make this easier, you can wrap SDL_CreateRGBSurface in another function:

SDL_Surface* CreateSurface(Uint32 flags,int width,int height,const SDL_Surface* display)
{
  // 'display' is the surface whose format you want to match
  //  if this is really the display format, then use the surface returned from SDL_SetVideoMode

  const SDL_PixelFormat& fmt = *(display->format);
  return SDL_CreateRGBSurface(flags,width,height,
                  fmt.BitsPerPixel,
                  fmt.Rmask,fmt.Gmask,fmt.Bmask,fmt.Amask );
}

See SDL documentation for the 'flags' and further information. Get the downloadable version here:

http://www.libsdl.org/archives/sdldoc-html.zip

As the online docs don't seem to be reference docs, but merely tutorials and as such do not mention a lot of this. To figure out how to do something, you can just skim the function list in 'index.html' in the docs to see which function does what you want.

[guestgulkan]

I was saying no to the function name/parameters he gave.

[Oromis]

guestgulkan, you have understood me wrong; I knew that exact and function + parameters probably wouldn't exist, but I was looking for something similar. I'll try to be more clear next time.

Disch; great! That seems to be exactly what I need. And the wrapper makes things even more easy. Thanks!