DRAFT |
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,
const SDL_Rect* srcrect,
SDL_Surface* dst,
SDL_Rect* dstrect)
Function Parameters
src |
the SDL_Surface structure to be copied from |
srcrect |
the SDL_Rect structure representing the rectangle to be copied, or NULL to copy the entire surface |
dst |
the SDL_Surface structure that is the blit target |
dstrect |
the SDL_Rect structure representing the rectangle that is copied into |
Return Value
Returns 0 if the blit is successful or a negative error code on failure; call SDL_GetError() for more information.
Code Examples
You can add your code example here
Remarks
You should call SDL_BlitSurface() unless you know exactly how SDL blitting works internally and how to use the other blit functions.
This is the public blit function, and it performs rectangle validation and clipping before passing it to SDL_LowerBlit().
The blit function should not be called on a locked surface.
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).
This section needs to be modified to reflect the concepts in the new version. alpha and colorkey have been replaced in 2.0. The blit semantics for surfaces with and without alpha and colorkey are defined as follows:
RGBA->RGB:
- SDL_SRCALPHA set:
- alpha-blend (using alpha-channel). SDL_SRCCOLORKEY ignored.
- copy RGB. if SDL_SRCCOLORKEY set, only copy the pixels matching the RGB values of the source color 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.
- copy RGB, set destination alpha to source per-surface alpha value.
- if SDL_SRCCOLORKEY set, only copy the pixels matching the source color 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.
- copy all of RGBA to the destination. if SDL_SRCCOLORKEY set, only copy the pixels matching the RGB values of the source color key, ignoring alpha in the comparison.
RGB->RGB:
- SDL_SRCALPHA set:
- alpha-blend (using the source per-surface alpha value).
- copy RGB.
- if SDL_SRCCOLORKEY set, only copy the pixels matching the source color key.
- SDL_SRCALPHA set:
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The results of blitting operations vary greatly depending on whether SDL_SRCALPHA is set or not. See SDL_SetAlpha (This does not have a page. Does it still exist?) 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 color key else { if (source surface has SDL_SRCCOLORKEY set) blit using the color 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 color 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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