[SDL] Font color gradients

[Sami Näätänen]

On Thursday 17 April 2008, Justin Davis wrote:
> I myself have actually been doing a lot of thorough research on this
> topic for my project OpenRPGMaker and I've found that a quick and
> dirty method of doing it would be to use TTF_RenderText_Solid() then
> make a new SDL_Surface of the same width and height (bpp can be
> whatever you want) and then go through each RGB pixel values and
> compare them with the text's RGB color. If they are the same then
> place the color value of the current x and y coordinate on you
> gradient (using the modulus operator on the coordinate to allow
> tiling of the gradient) onto the newly created surface, otherwise if
> they are not the same then simply place a pre-chosen color to place
> instead and then use
> SDL_SetColorKey(newsurface, SDL_SRCCOLORKEY,
> (Uint32)chosen_transparent_color) on the surface after it's finished
> rendering. The original surface can then be freed and the new one can
> be used in its place.
>
> On Sat, 2008-04-05 at 15:02 -0700, Mason Wheeler wrote:
> > Interesting.  Is there a way to create an alpha-only surface?  (No
> > color information, only an alpha channel.)  I could render the font
> > onto that and then do a bit of creative blitting...

I would do it like this:
1. Create the gradient surface. (this could be tiled or )
2. Create the text with SDL_ttf with alpha channel.
3. Replace every pixels RGB values of the SDL_ttf created surface with 
the gradients corresponding RGB value leaving the alpha value as is.

This has the added benefit of creating the gradient only once.

I would make my custom gradient text blitter, because it would be 
faster.

pseudo:

Get and set functions are left as an exercise. :)
Getting only Red, Green or Blue component is not very sensible, but I 
used that to make things clearer. 

This uses way too much functions, but it makes things clearer. (Again)

int
gradient_text_blitter( SDL_Surface* text_a,
		     SDL_Surface* gradient_rgb,
		     SDL_Surface* dst, int x, int y ) {
for(int i=0; i<text_a->w; i++)
  for(int j=0; i<text_a->h; i++) {
    Uint32 a = getA( text_a, i, j );
    if( a > 0 ) {
      Uint32 r = getR( gradient_rgb, i, j );
      Uint32 g = getG( gradient_rgb, i, j );
      Uint32 b = getB( gradient_rgb, i, j );
      alphaBlendPixelTo( dst, x+i, y+j, r, g, b, a );
    }
  }
}
void
alphaBlendPixelTo( SDL_Surface* dst, int x, int y,
		 Uint32 r, Uint32 g, Uint32 b, Uint32 a ) {
  if( x >= dst->w || y>= dst->h )
    return;

  Uint32 rgb = ((getR( dst, x, y )*(255-a)+r*a)/255;
  rgb = rgb<<8 + ((getG( dst, x, y )*(255-a)+g*a)/255;
  rgb = rgb<<8 + ((getB( dst, x, y )*(255-a)+b*a)/255;
  setPixelTo( dst, x, y, rgb );
}

> > ----- Original Message ----
> > From: David Olsen <jolynsbass at gmail.com>
> > To: A list for developers using the SDL library. (includes
> > SDL-announce) <sdl at lists.libsdl.org> Sent: Saturday, April 5, 2008
> > 2:36:51 PM
> > Subject: Re: [SDL] Font color gradients
> >
> > I would think that using the output from SDL_TTF(Blended - the one
> > that gives you a per-pixel alpha image), you could blit the text
> > onto a predefined surface which contains a color gradient, and you
> > should get a resulting image of the sort you want. (Oh, first you
> > must have SRCALPHA turned on in both surfaces, or it won't act as a
> > mask. Look at the documentation of the SDL_SetAlpha function) I
> > might have the surface order reversed... you might need to blit the
> > colored gradient surface onto the rendered text surface. But a
> > little experimentation along those lines should get you set up.
> > -Dave
> >
> >
> > ----- Original Message -----
> > From: "Mason Wheeler" <masonwheeler at yahoo.com>
> > To: <sdl at lists.libsdl.org>
> > Sent: Saturday, April 05, 2008 7:12 AM
> > Subject: [SDL] Font color gradients
> >
> > > I've been looking through SDL_TTF, and it looks like the only
> > > option available is to render a font in a single, solid color. 
> > > How would I do gradients and other custom colorations?  The ideal
> > > way would be to use the glyph shapes as "cutouts" in an existing
> > > graphic from an SDL_Surface, which would allow custom colors to
> > > be defined beforehand.  If this isn't feasible, I'd like to be
> > > able to define multiple foreground colors and create a gradient
> > > from them.
> > >
> > > How would I go about doing something like this?
> > >