[SDL] It seems SDL exchanges R & B channels

[Jonathan Dearborn]

Hey,

I'm not sure what's wrong with your pixel loop, but be careful about the types you use.  'unsigned int' should instead be SDL's 'Uint32' type if your particular surface is 32-bit.  Also, you're not guaranteed to have gotten the 32 bits that you asked for.  You could check for that or instead use a function that handles pixels for all of the standard bit depths (email me back if you'd like my copy of that).  Here's the line I use for setting a 32-bit pixel in case it helps:
*((Uint32 *)surface->pixels + y*surface->pitch/4 + x) = color;
If you're just trying to fill the surface with color, consider using SDL_FillRect(screen, NULL, SDL_MapRGB(screen->format, 255, 0, 0));

See ya,
Jonny D






> To: sdl at libsdl.org
> From: alex.chenhl at gmail.com
> Date: Tue, 26 Feb 2008 16:03:13 +0800
> Subject: [SDL] It seems SDL exchanges R & B channels
> 
> 
> Dear all,
> 
> In the test code, I wanna show a red window but it turn out to be blue.  
> What's wrong?
> 
> #include <stdlib.h>
> #if defined(_MSC_VER)
> #include "SDL.h"
> #else
> #include "SDL/SDL.h"
> #endif
> 
> SDL_Surface *screen;
> 
> void render()
> {
>      // Lock surface if needed
>      if (SDL_LockSurface(screen) < 0)
>          return;
> 
>      // Ask SDL for the time in milliseconds
>      int tick = SDL_GetTicks();
> 
>      // Declare a couple of variables
>      int i, j, b, yofs, ofs;
>      unsigned char *pc;
>      unsigned long *pl;
> 
>      // Draw to screen
>      Uint32 color = SDL_MapRGB(screen->format, 255, 0, 0);
>      printf("color:0x%08x\n", color);
> 
>      yofs = 0;
>      for (i = 0; i < 480; i++)
>      {
>          ofs = yofs;
>          for (j = 0; j < 640; j++)
>          {
>              ((unsigned int*)screen->pixels)[ofs++] = color;
>          }
>          yofs += screen->pitch / 4;
>      }
> 
>      // Unlock if needed
>      if (SDL_MUSTLOCK(screen))
>          SDL_UnlockSurface(screen);
> 
>      // Tell SDL to update the whole screen
>      SDL_UpdateRect(screen, 0, 0, 640, 480);
> }
> 
> // Entry point
> int main(int argc, char *argv[])
> {
>      // Initialize SDL's subsystems - in this case, only video.
>      if ( SDL_Init(SDL_INIT_VIDEO) < 0 )
>      {
>          fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
>          exit(1);
>      }
> 
>      // Register SDL_Quit to be called at exit; makes sure things are
>      // cleaned up when we quit.
>      atexit(SDL_Quit);
> 
>      // Attempt to create a 640x480 window with 32bit pixels.
>      screen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);
> 
>      // If we fail, return error.
>      if ( screen == NULL )
>      {
>          fprintf(stderr, "Unable to set 640x480 video: %s\n",  
> SDL_GetError());
>          exit(1);
>      }
> 
>      // Main loop: loop forever.
>      while (1)
>      {
>          // Render stuff
>          render();
> 
>          // Poll for events, and handle the ones we care about.
>          SDL_Event event;
>          while (SDL_PollEvent(&event))
>          {
>              switch (event.type)
>              {
>              case SDL_KEYDOWN:
>                  break;
>              case SDL_KEYUP:
>                  // If escape is pressed, return (and thus, quit)
>                  if (event.key.keysym.sym == SDLK_ESCAPE)
>                      return 0;
>                  break;
>              case SDL_QUIT:
>                  return(0);
>              }
>          }
>      }
>      return 0;
> }
> 
> 
> I compile and run the program on i686 GNU/linux Red Hat Enterprise Linux  
> ES release 4, the installed SDL version is SDL-1.2.7-8, the GUI  
> environment is VNC+icewm.
> 
> 
> -- 
> Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

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