[SDL] aalineRGBA problem

[Armin Steinhoff]

Tommi Kyntola <tommi.kyntola at ray.fi> schrieb am 13.06.2006 11:44:53:
> 
> 
> When the lines are not straight the antialiasing reduces the pixel
> colors just a tad away from white. And then your use of nonzero
> Amask in the SDL_CreateRGBSurface causes the SDL_gfx to go for
> alpha blending. And without alpha channel set in your image you
> get didley on the image.

Yes ... it is now working with the Amask set to zero :)

> 
> I suggest you skip the alpha stuff unless you need it and use
> SDL_DisplayFormat to get an image suitable for blitting.
> You're using ABGR, which on little endian machines happens to be what
> the opengl wants (big endiand RGBA), but using that to blit
> on screen in 2D is probably not what you're looking for, because I'm
> betting you're display surface has something other than ABGR
> (could it be ARGB?).

Sorry ... I don't understand fully the concept behind these RGBA masks. 

> It's been a while since I last played with SDL_gfx, so take all that
> with some salt, but your example would seem to work ok if you set the
> Amask to zero in the SDL_CreateRGBSurface call _or_ use SDL_setAlpha(image,0,255);
> to get something non-transparent on the surface.

Thank you for your help.

Best Regards

Armin


> 
> However, I distinctly recall that there was fishy with SDL_gfx
> and the way it handled the alpha channeling. Or to be more precice,
> I never got it to work the way I was looking for, but it's quite
> likely I overlooked something as I was never really that
> keen on getting it to work.
> 
> Maybe I'll check back with an example ...
> 
> cheers,
> 	Tommi Kyntola
> 
> Armin Steinhoff wrote:
> > Hello,
> > 
> > I have a little bit trouble with the call  'aalineRGBA' from the SDL_Gfx library. 
> > When I create  a surface with a bpp of 32bit, 'aalineRGBA' draws only vertical and horizontal lines (ticks of a meter).
> > Only the start and end point of all other lines are drawn. 
> > 
> > On a surface with 8 bpp all lines are drawn, but the color is wrong.
> > 
> > I have attached a short example. What's wrong in my code?  ( I'm working under SuSE LINUX 9.0. )
> > 
> > Thank you
> > 
> > Armin
> > 
> > ______________________________________________________________________
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> > 
> > 
> > ------------------------------------------------------------------------
> >  
> > #include <stdlib.h>
> > #include <math.h>
> > #include "SDL.h"
> > #include "SDL_image.h"
> > #include "SDL_gfxPrimitives.h"
> > 
> > void cpoint(float rad, short cx, short cy, short radius, short * px, short * py)
> > {
> >     // cx, cy are the center point of the circle
> >     // px, py are points of the circle
> >     *px = cx + (short)((float)radius * cosf(rad));
> >     *py = cy + (short)((float)radius * sinf(rad));
> > }
> > 
> > int draw_tick(SDL_Surface * surf, float rad, short cx, short cy, short radius, short tick_len,
> >               Uint8 r, Uint8 g, Uint8 b, Uint8 a )
> > {
> >     short spx, spy, epx, epy;
> >     
> >     cpoint(rad, cx, cy, radius, &spx, &spy);
> >     cpoint(rad, cx, cy, radius - tick_len, &epx, &epy);
> >     
> >     fprintf(stdout, "spx/spy %d,%d  epx/epy: %d/%d\n", spx, spy, epx, epy);     
> >     return(aalineRGBA(surf, spx, spy, epx, epy, r, g, b, a)); 
> > }
> > 
> > int main()
> > {
> > 
> >     SDL_Surface *display;
> >     SDL_Surface *image;
> >     int i, done, resp;
> >     float rad;
> >     
> >     SDL_Rect drect;
> >     SDL_Rect srect;
> >     SDL_Event event;
> >     
> >     // init video stuff
> >     if ( SDL_Init( SDL_INIT_VIDEO) < 0 )
> >     {
> >      fprintf(stderr, "SDL konnte nicht initialisiert werden:  %s\n",
> >        SDL_GetError());
> >      exit(1);
> >     }
> >     
> >     atexit(SDL_Quit);
> > 
> >     // init screen
> >     display = SDL_SetVideoMode( 1024,768, 0, SDL_SWSURFACE);
> >     if ( display == NULL )
> >     {
> >      fprintf(stderr, "SDL_SetVideoMode %s\n", SDL_GetError());
> >      exit(1);
> >     }
> >     
> >     image = SDL_CreateRGBSurface(SDL_SWSURFACE, 1024, 768, 32, 0xFF, 0xFF00, 0xFF0000, 0xFF000000);
> >     if ( image == NULL ) {
> >         fprintf(stderr, "SDL_CreateRGBSurface: %s\n",SDL_GetError());
> >         return(NULL);
> >     }
> >     //source rect
> >     srect.x = 0;
> >     srect.y = 0;
> >     srect.w = (image->w); 
> >     srect.h = image->h;   
> >     
> >     // destination rect
> >     drect.x = 0;
> >     drect.y = 0;
> >     drect.w = (image->w);
> >     drect.h = image->h;
> >     
> >     for (i=0; i < 360; i+=10)
> >     {
> >         rad = 6.28 * (float)i/360.0;
> >           
> >         resp = draw_tick(image, rad, 150, 150, 50, 10, 255, 255, 255, 255 );
> >         fprintf(stdout, "resp: %d\n", resp);     
> >           
> >         SDL_BlitSurface(image, &srect, display, &drect);
> >         
> >         SDL_UpdateRects(display,1,&drect);
> >     }      
> >  
> >     SDL_Delay(6000);
> >     
> >     SDL_FreeSurface(image);
> > }
> > 
> > 
> > ------------------------------------------------------------------------
> > 
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> > SDL at libsdl.org
> > http://www.libsdl.org/mailman/listinfo/sdl
> 
> 
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