SDL_TTF and transparency

SDL Development
1

I’m migrating an application that used GL to do most of it’s rendering
to SDL and I’m having trouble getting transparency to work. In the
example I have below I’m trying to render a font, give the surface that
returns a particular alpha value, and then blit it to the screen. This
doesn’t seem to work, I still see the font even though it should be
completely transparent. With GL I was simply setting the GL color with
glColor4ub( color->red, color->green, color->blue, color->alpha ) and
then doing any rendering that was necessary. Is what I’m attempting to
do similar to what I was doing in GL? Is there a better way to do
transparency when I want the entire surface to be at a specific alpha level?

#include
#include <SDL/SDL.h>
#include <SDL/SDL_ttf.h>

using namespace std;

int main() {
if (SDL_Init(SDL_INIT_VIDEO) < 0) {
cout << "Unable to init SDL: " << SDL_GetError() << endl;
exit(1);
}

if ( TTF_Init() < 0 ) {
    cout <<  "Couldn't initialize TTF: " << SDL_GetError() << endl;
}

SDL_Surface *screen = SDL_SetVideoMode(500, 500, 32, SDL_SWSURFACE);

if (screen == NULL) {
    cout <<  "Error in SDL_SetVideoMode(): " << SDL_GetError() << endl;
    exit(1);
}

SDL_WM_SetCaption("FontTest", NULL);

SDL_Color color;
color.r = 255;
color.g = 255;
color.b = 255;

TTF_Font *theFont = TTF_OpenFont("/aeon/game/fonts/times.ttf", 40);
TTF_SetFontStyle(theFont, 0);

SDL_Surface *temp = TTF_RenderText_Blended(theFont,"Foo Bar",color);
SDL_Surface *font = SDL_DisplayFormatAlpha(temp);

SDL_SetAlpha(font,SDL_SRCALPHA,0);

SDL_Rect destination;
destination.x = 100;
destination.y = 100;
destination.w = 100;
destination.h = 100;

SDL_BlitSurface(font,NULL,screen,&destination);
SDL_UpdateRect(screen, 0, 0, 500, 500);

sleep(3);

}

2

Joe Keen wrote:

I’m migrating an application that used GL to do most of it’s rendering
to SDL and I’m having trouble getting transparency to work. In the
example I have below I’m trying to render a font, give the surface that
returns a particular alpha value, and then blit it to the screen.

Blended (antialiased) fonts use per-pixel blending. You can’t mix that with
per-surface blending in SDL 1.2. The per-pixel blending will always be used if
it’s available. This is written in the SDL_SetAlpha() docs.

3

Ilya Olevsky wrote:

Joe Keen wrote:

I’m migrating an application that used GL to do most of it’s rendering
to SDL and I’m having trouble getting transparency to work. In the
example I have below I’m trying to render a font, give the surface that
returns a particular alpha value, and then blit it to the screen.

Blended (antialiased) fonts use per-pixel blending. You can’t mix that with
per-surface blending in SDL 1.2. The per-pixel blending will always be used if
it’s available. This is written in the SDL_SetAlpha() docs.

So, if I’m understanding right, I’d do something along the lines of
replacing the SDL_SetAlpha(font,SDL_SRCALPHA,0) line with the following code

Uint32 colorkey=SDL_MapRGBA(font->format, color.r, color.g, color.b,0);
SDL_SetColorKey(font, SDL_RLEACCEL | SDL_SRCCOLORKEY, colorkey);

That seems to give me no different result. Do you have an example that
would show how to give fonts an alpha value?

4

So, if I’m understanding right, I’d do something along the lines of
replacing the SDL_SetAlpha(font,SDL_SRCALPHA,0) line with the following code

Uint32 colorkey=SDL_MapRGBA(font->format, color.r, color.g, color.b,0);
SDL_SetColorKey(font, SDL_RLEACCEL | SDL_SRCCOLORKEY, colorkey);

That seems to give me no different result. Do you have an example that
would show how to give fonts an alpha value?

Ok as I understand, you’re trying to set opacity (per-surface alpha) of a font
surface that was created with TTF_RenderText_Blended(). This can’t be done in
SDL because this function creates a surface with per-pixel alpha blending to
make a real time antialiased font. You would have to use TTF_RenderText_Solid()
or TTF_RenderText_Shaded() and then you can use per-surface alpha. Those methods
won’t give you a font that gets antialiased in real time though.

And yes, this is something that’s trivial to do in OpenGL or D3D, but SDL is a
software renderer so it has its limitations. There’s another recent topic about
the same thing you can check out: “Alpha blending transparent PNGs” which covers
an alternate method to accomplish what you want to do.