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Hi,

I'm seeing strange behaviour with his code:

#include <SDL.h>

int main (void) {
SDL_Surface *screen,*s1,*s2;
SDL_Event e;
SDL_Rect r;

SDL_Init (SDL_INIT_VIDEO);
screen = SDL_SetVideoMode (300, 300, 16, 0);

s1 = SDL_CreateRGBSurface (0, 50, 50, 24, 0x00f80000,
0x0007e000,
0x00001f00,
0x000000ff);
s2 = SDL_CreateRGBSurface (0, 50, 50, 24, 0x00f80000,
0x0007e000,
0x00001f00,
0x000000ff);
SDL_FillRect (s1, NULL, 0x00ffffff);
SDL_FillRect (s2, NULL, 0x004000ff);

r.x = 100;
r.y = 100;
SDL_BlitSurface (s1, NULL, screen, &r);
r.x = 125;
r.y = 125;
SDL_BlitSurface (s2, NULL, screen, &r);

SDL_UpdateRect (screen, 0, 0, 0, 0);

while (SDL_WaitEvent (&e)) {
if (e.type==SDL_QUIT) break;
}

SDL_Quit();
return 0;
}

This creates two 16 bit + alpha surfaces, and blits them to the screen,
overlapping them. The problem is that the surfaces are opaque but SDL is
alpha blending them a litlle. That is, I expect something like (incoming
ascii-art)

****
****
**++++
**++++
++++
++++

But SDL shows something like

****
****
**xx++
**xx++
++++
++++

where 'x' is a blend of '*' and '+' (very close to '+')

Why does that happen? I'm guessing it has to do with using 16 bit
surfaces, I have not seen that happening at 24 bpp... however I have
this problem in an app whereI have a lot of bitmaps in-memory and the
memory usage difference is important.

Thanks for your time,
Daniel


--
"Why program by hand in five days what you can spend five years of your
life automating."
-- Terrence Parr, ANTLR author
--
Except - Free Software developers for hire - http://except.com.ar

Daniel F Moisset wrote:


There is a comment in video/SDL_blit_A.c:BlitNtoNPixelAlpha() :
/* FIXME: for 8bpp source alpha, this doesn't get opaque values
quite right. ...

I think this function is used to do alpha blending in the case of your
surfaces. The compositioning used (>>8 instead of /255) doesn't handle
opaque alpha correctly.

-Alex.