Re: [GD-DEVEL] Drawing anti-aliased text with transparent background

HI Ethan,

On 3/18/07, Ethan A Merritt <[email protected]> wrote:
> On Saturday 17 March 2007 11:54, Pierre wrote:
> > On 3/17/07, Andrei Stebakov <[email protected]> wrote:
> > > My goal is to create a .png image that has some text on a transparent
> > > background.
> >
> > Here you go:
> >
> > and the code:
> > http://cvs.php.net/viewvc.cgi/gd/playground/text_on_transparent_background/text_aa_alpha.c?revision=1.2&view=markup
> >
> > Please remember that it will work for text but not other shapes. The
> > antialiased lines always blend on the destination pixel, for now.
>
> Aha. That explains something that had been bothering me about some
> plots created by gnuplot.  So if the user chooses TrueColor output and
> a transparent background,

Please note that in my example it is a not the transparent color has a
maximum alpha (gdAlphaTransparent).

> then we have to turn off anti-aliasing for
> vectors.  Otherwise there are ghost fringes, which I had noticed but
> not thought a lot about.
> I think this is a bug, and should be fixed.  The destination pixel in my case
> is white [usually] with full transparency.  When you antialias onto it,
> the transparency should be taken into account for either setting of
> gdImageAlphaBlending(), right?  But apparently it isn't.

It is the exact same problem as Andrei had with the text. It is not a
bug per se but a missing feature (an annoying missing feature). It
will be implemented in 2.1.0.

> As it stands the counter-intuitive observation is that blending two
> transparent colors produces an opaque result.  But only for anti-aliased lines.
> I'm also puzzled as to why this is true even for pure horizontal or vertical
> lines.  Why are those affected by anti-aliasing at all?

The AA functions blend only against the RGB part of the destination
pixel. The axis aligned lines are not drawn using the anti-aliased
function.

This problem is really a pain for anyone working with transparency.
2.1.0 will add all necessary modes to all drawing functions (with or
without anti-aliasing).

I'm writing a proposal for new GD surfaces and mode. One of the goals
is to solve many of the current confusions between the transparent
color index (which is format specific) and the transparency/opacity
channel in each color. I think it will reduce the confusions and make
GD behaviors more user friendly.

> I can provide demo output if you like.

If you have one at hand, please yes. Even if I see your problem, an
image is always welcome .

--Pierre