SDL_BlitSurface

Use this function to perform a fast blit from the source surface to the destination surface.

Syntax

int SDL_BlitSurface(SDL_Surface* src,
                    SDL_Rect*    srcrect,
                    SDL_Surface* dst,
                    SDL_Rect*    dstrect)

Function Parameters

Return Value

If the blit is successful it returns 0, otherwise it returns -1 on failure; call SDL_GetError() for more information.

See Remarks for details if the return value is -2.

Code Examples

You can add your code example here

Remarks

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This is the public blit function, and it performs rectangle validation and clipping before passing it to SDL_LowerBlit(). green

You should call SDL_BlitSurface() unless you know exactly how SDL blitting works internally and how to use the other blit functions.

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The blit function should not be called on a locked surface. green

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(i.e. when you use your own drawing functions you may need to lock a surface, but this is not the case with SDL_BlitSurface().)
Like most surface manipulation functions in SDL, it should not be used together with OpenGL.
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This assumes that the source and destination rectangles are the same size. If either srcrect or dstrect are NULL, the entire surface (src or dst) is copied. The final blit rectangles are saved in srcrect and dstrect after all clipping is performed.

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The width and height in srcrect determine the size of the copied rectangle. Only the position is used in the dstrect (the width and height are ignored). Blits with negative dstrect coordinates will be clipped properly.

If srcrect is NULL, the entire surface is copied. If dstrect is NULL, then the destination position (upper left corner) is (0, 0).

The final blit rectangle is saved in dstrect after all clipping is performed (srcrect is not modified).
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The blit semantics for surfaces with and without alpha and colorkey are defined as follows:

• \verbatim

  RGBA->RGB:

  • SDL_SRCALPHA set:
    • alpha-blend (using alpha-channel). SDL_SRCCOLORKEY ignored.
    SDL_SRCALPHA not set:
    • copy RGB. if SDL_SRCCOLORKEY set, only copy the pixels matching the RGB values of the source colour key, ignoring alpha in the comparison.

  RGB->RGBA:

  • SDL_SRCALPHA set:
    • alpha-blend (using the source per-surface alpha value); set destination alpha to opaque.
    SDL_SRCALPHA not set:
    • copy RGB, set destination alpha to source per-surface alpha value.
    both:
    • if SDL_SRCCOLORKEY set, only copy the pixels matching the source colour key.

  RGBA->RGBA:

  • SDL_SRCALPHA set:
    • alpha-blend (using the source alpha channel) the RGB values; leave destination alpha untouched. [Note: is this correct?] SDL_SRCCOLORKEY ignored.
    SDL_SRCALPHA not set:
    • copy all of RGBA to the destination. if SDL_SRCCOLORKEY set, only copy the pixels matching the RGB values of the source colour key, ignoring alpha in the comparison.

  RGB->RGB:

  • SDL_SRCALPHA set:
    • alpha-blend (using the source per-surface alpha value).
    SDL_SRCALPHA not set:
    • copy RGB.
    both:
    • if SDL_SRCCOLORKEY set, only copy the pixels matching the source colour key.
  \endverbatim

If either of the surfaces were in video memory, and the blit returns -2, the video memory was lost, so it should be reloaded with artwork and re-blitted:

• @code
  while ( SDL_BlitSurface(image, imgrect, screen, dstrect) == -2 ) {
  while ( SDL_LockSurface(image) < 0 )
  Sleep(10);
  -- Write image pixels to image->pixels --
  SDL_UnlockSurface(image);
  }
  @endcode

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This happens under DirectX 5.0 when the system switches away from your fullscreen application. The lock will also fail until you have access to the video memory again.

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The results of blitting operations vary greatly depending on whether SDL_SRCALPHA is set or not. See SDL_SetAlpha (green

) for an explanation of how this affects your results. Colorkeying and alpha attributes also interact with surface blitting, as the following pseudo-code should hopefully explain.

if (source surface has SDL_SRCALPHA set) {
    if (source surface has alpha channel (that is, format->Amask != 0))
        blit using per-pixel alpha, ignoring any colour key
    else {
        if (source surface has SDL_SRCCOLORKEY set)
            blit using the colour key AND the per-surface alpha value
        else
            blit using the per-surface alpha value
    }
} else {
    if (source surface has SDL_SRCCOLORKEY set)
        blit using the colour key
    else
        ordinary opaque rectangular blit
}

Note: the SDL blitter does not (yet) have the capability of scaling the blitted surfaces up or down like it is the case with other more sophisticated blitting mechanisms. You have to figure something out for yourself if you want to scale images (e.g. use SDL_gfx).

Note: when you're blitting between two alpha surfaces, normally the alpha of the destination acts as a mask. If you want to just do a "dumb copy" that doesn't blend, you have to turn off the SDL_SRCALPHA flag on the source surface. This is how it's supposed to work, but can be surprising when you're trying to combine one image with another and both have transparent backgrounds.
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Related Functions

• SDL_LowerBlit

• SDL_UpperBlit

CategoryAPI, CategorySurface