SDL_BlitSurface的参数是两个PNG时，如何保护其透明度

原文 https://bugzilla.libsdl.org/show_bug.cgi?id=68#c2 Description Davide Coppola 2006-01-23 04:32:53 EST I have tried to blit a PNG with an alpha channel on a transparent surface created using format data from the image, the result of SDL_BlitSurface() is just nothing! I have solved my problem using memcpy() if the image has the same dimension of the surface and using getpixel()/putpixel() if the image is smaller than the surface (or if I need a partial blit). this source shows the problem: http://mars.sourceforge.net/SDL/trans_bug/trans_test1.c this are my solutions: http://mars.sourceforge.net/SDL/trans_bug/trans_test2.c and this is the package with all the sources and data: http://mars.sourceforge.net/SDL/trans_bug.tar.gz I hope this could help you. Comment 1 Ryan C. Gordon 2006-01-27 11:23:21 EST Setting Sam as "QA Contact" on all bugs (even resolved ones) so he'll definitely be in the loop to any further discussion here about SDL. --ryan. Comment 2 Sam Lantinga 2006-03-20 02:40:09 EST This is not technically a bug. According to the documentation: * 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. SDL_DisplayFormatAlpha() turns on SDL_SRCALPHA by default (to enable blending) What I did to fix this in your test program was just add SDL_SetAlpha(img, 0, 0) before the blit onto window. Of course a manual memcpy works just as well. :) Thanks for the great test case! Comment 3 Davide Coppola 2006-03-20 04:20:11 EST (In reply to comment #2) I confirm that your solution works well, and I'm glad of this ;) Just a note for someone interested: if you want to blit img on screen after you have set SDL_SetAlpha(img, 0, 0), you have to set SDL_SetAlpha(img, SDL_SRCALPHA, 0). Best regards.

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文章最后说到，在blit前加入

SDL_SetAlpha(img, 0, 0)

即可。

实测有效.