[SDL] pixels getting set to NULL

[Warren Schwader]

ok I understand now why the color key doesn't get used when there is per
pixel alpha (my #2 item), but more imporant than that is still why the null
pointer when passing amask = 0 in SDL_CreateRGBSurface?
Note that I think the code should have used f->Amask for my #1 item.  Sorry
for the confusion.
--Warren Schwader

----- Original Message ----- 
From: "Warren Schwader" <warrens at sierratel.com>
To: <sdl at libsdl.org>
Sent: Tuesday, December 09, 2003 2:19 AM
Subject: Re: [SDL] pixels getting set to NULL


> Sam and Alan,
>
> Despite the advice and possible reasons, I cannot seem to correct this
> problem.
> So I decided to write the most concise program that I could that
> demonstrated the problem.
> Can you give it the once over please and see if you can tell what I'm
doing
> wrong?
> Here is the code (VC++5 Win2000) that produces it in a simple main
function:
>
> Please try these two things:
> #1 run it as it is.  An access violation occurs because dialogSurface's
> pixels
>         is set to 0 when trying to draw "doneImageSurface" the 2nd time
> #2 change "amask" to "f->Amask" when creating "dialogSurface"
> (SDL_CreateRGBSurface)
>       and run again.  This time the pixels are fine but the color key of
the
> dialogSurface doesn't work
>
>
//--------------------------------------------------------------------------
> ------------------------
> #include "sdl.h"
> #include "sdl_image.h"
> #include <memory.h>
>
> SDL_Surface* mainSurface;
> SDL_Event event;
>
> int main(int argc, char* argv[])
> {
> // initialize and create the main surface
>  SDL_Init(SDL_INIT_EVERYTHING);
>  mainSurface = SDL_SetVideoMode(800, 600, 32, SDL_SWSURFACE);
>
>
> // load a non-rectanglular image.  The pixel at 0, 0 will become the
> ColorKey
> // my image was 300 wide by 270 tall.
> // Make sure the transparent part of the image is not black so you can see
> it when drawn.
>  SDL_Surface* dialogImageSurface1 = IMG_Load("dialogimage.png");
>  SDL_Surface* dialogImageSurface = SDL_DisplayFormat(dialogImageSurface1);
>  SDL_FreeSurface(dialogImageSurface1);
>
> // load a button image to display on the dialog surface.
> // The pixel at 0, 0 will become the ColorKey for the button
>  SDL_Surface* doneImageSurface1 = IMG_Load("done.png");
>  SDL_Surface* doneImageSurface = SDL_DisplayFormat(doneImageSurface1);
>  SDL_FreeSurface(doneImageSurface1);
>  if (SDL_MUSTLOCK(doneImageSurface)) SDL_LockSurface(doneImageSurface);
>  Uint32 doneColor = 0;
>  memcpy(&doneColor, (char*)doneImageSurface->pixels,
> doneImageSurface->format->BytesPerPixel);
>  SDL_SetColorKey(doneImageSurface, SDL_SRCCOLORKEY | SDL_RLEACCEL,
> doneColor);
>  if (SDL_MUSTLOCK(doneImageSurface)) SDL_UnlockSurface(doneImageSurface);
>
>
> // create an RGB surface for the dialog
>  SDL_PixelFormat* f = mainSurface->format;
>  #if SDL_BYTEORDER == SDL_BIG_ENDIAN
>       Uint32 amask = 0x000000ff;
>  #else
>       Uint32 amask = 0xff000000;
>  #endif
>  SDL_Surface* dialogSurface = SDL_CreateRGBSurface(SDL_SWSURFACE,
>            dialogImageSurface->w, dialogImageSurface->h,
>            f->BitsPerPixel, f->Rmask, f->Gmask, f->Bmask,
>            amask);    //  f->Amask);
>
>
> // pre-draw the dialog once so it can be blitted faster later than if we
> // blitted the dialog image and all the buttons and text every time it
> needed repainting
>  SDL_BlitSurface(dialogImageSurface, NULL, dialogSurface, NULL);
>  SDL_UpdateRect(dialogSurface, 0, 0, dialogSurface->w, dialogSurface->h);
>
> // reads the pixel at 0, 0 and use that color as the color key for the
> dialogSurface
> // so that we can have dialogs of non-rectanglular shape
>  if (SDL_MUSTLOCK(dialogSurface)) SDL_LockSurface(dialogSurface);
>  Uint32 color = 0;
>  memcpy(&color, (char*)dialogSurface->pixels,
> dialogSurface->format->BytesPerPixel);
>  SDL_SetColorKey(dialogSurface, SDL_SRCCOLORKEY | SDL_RLEACCEL, color);
>  if (SDL_MUSTLOCK(dialogSurface)) SDL_UnlockSurface(dialogSurface);
>
>  SDL_Rect rect;
>  rect.x = 100;
>  rect.y = 100;
>  rect.w = doneImageSurface->w;
>  rect.h = doneImageSurface->h;
>  SDL_BlitSurface(doneImageSurface, NULL, dialogSurface, &rect);
>  SDL_UpdateRect(dialogSurface, 0, 0, dialogSurface->w, dialogSurface->h);
>
>
> // finally blit the dialog surface to the mainSurface
>  if (SDL_MUSTLOCK(mainSurface)) SDL_LockSurface(mainSurface);
>  SDL_BlitSurface(dialogSurface, NULL, mainSurface, NULL);
>  SDL_UpdateRect(mainSurface, 0, 0, 800, 600);
>  if (SDL_MUSTLOCK(mainSurface)) SDL_UnlockSurface(mainSurface);
>
> // next, assume the user moused over a dialog button so we want to draw
the
> button again
> // onto the dialogSurface (not the mainSurface)
> // THIS IS WHERE PIXELS IS SET TO 0 FOR dialogSurface
>  void* dialogPixels = dialogSurface->pixels;  // pixels is 0 here
>  SDL_BlitSurface(doneImageSurface, NULL, dialogSurface, &rect);  // access
> violation
>  SDL_UpdateRect(dialogSurface, 0, 0, dialogSurface->w, dialogSurface->h);
>
>  if (SDL_MUSTLOCK(mainSurface)) SDL_LockSurface(mainSurface);
>  SDL_BlitSurface(dialogSurface, NULL, mainSurface, NULL);
>  SDL_UpdateRect(mainSurface, 0, 0, 800, 600);
>  if (SDL_MUSTLOCK(mainSurface)) SDL_UnlockSurface(mainSurface);
>
>
> // wait for a command to close the app
>  while (true)
>  {
>      SDL_WaitEvent(&event);
>      if (event.type == SDL_QUIT)
>          break;
>  }
>  SDL_Quit();
>  return(0);
> }
>
//--------------------------------------------------------------------------
> ------------------------
>
> Thanks,
> Warren Schwader
>
>
> ----- Original Message ----- 
> From: "Sam Lantinga" <slouken at devolution.com>
> To: <sdl at libsdl.org>
> Sent: Sunday, December 07, 2003 11:03 AM
> Subject: Re: [SDL] pixels getting set to NULL
>
>
> > > Hi Alan, thanks for the reply.  What you say is likely true, but in my
> case I amusing only software surfaces.  I should have stated that.  I
should
> have also stated that I am using per surface alpha too. I can try to Lock
> the sw surface and see what effect it has.
> >
> > > The interesting thing is that if I don't set the color key on the
> dialog's surface then everything works OK but then I'd be stuck with
square
> dialogs.  Another interesting thing is when I change a dialog item on my
> non-square dialog, it makes the color keyed pixels appear in a color other
> than what would be there if the image wasn't color keyed.  Strange.
> >
> > You always need to use the SDL_MUSTLOCK() macro to see whether you need
to
> > lock the surface before using it.  If the pixels are NULL, that's a good
> clue. :)
> >
> > See ya,
> > -Sam Lantinga, Software Engineer, Blizzard Entertainment
> >
> > _______________________________________________
> > SDL mailing list
> > SDL at libsdl.org
> > http://www.libsdl.org/mailman/listinfo/sdl
>
>
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