Re: PNG term and anti-aliasing of lines

[Ethan A M. <merritt@u.washington.edu>]

On Thursday 15 November 2007 18:09, Allin Cottrell wrote:
> Could anyone tell me, under what precise conditions do we get 
> anti-aliasing of (width 1) lines in PNG plots?  (With the 
> libgd-based driver, that is.)

Gnuplot asks for anti-aliased lines if and only if the linewidth is 1.
After that it's up to libgd.

The somewhat out-of-date libgd documentation says:
   Antialiased lines can be drawn on both truecolor and palette-based
   images. However, attempts to draw antialiased lines on highly complex
   palette-based backgrounds may not give satisfactory results, due to
   the limited number of colors available in the palette. Antialiased
   line-drawing on simple backgrounds should work well with palette-based
   images; otherwise create or fetch a truecolor image instead.

I can't think of any reason there would be a difference between
MSWin and linux, only between different versions of libgd.

		Ethan

 
> I ask because (a) on Linux I'm using reasonably current CVS 
> gnuplot with the latest libgd release, but I've never seen 
> anti-aliasing of lines in PNG output, while (b) with a similar 
> setup on Windows PNG output seems to be anti-aliased by default.
> 
> I've just recently twigged to the fact that you can get 
> anti-aliasing in PNG output on Linux if you add the "truecolor" 
> flag to the invocation of the png terminal.  Somehow, though, this 
> flag does not seem to be required to get anti-aliasing on Windows.  
> Is this a libgd quirk, perhaps?
> 
> The motivation for this question is, in part, that I'd like to be 
> able to choose between AA and non-AA PNG output in a 
> platform-independent manner.  Sometimes AA can be nice: it reduces 
> the artifacts you get on certain line segments when plotting "with 
> lines".  On the other hand, AA looks kinda funny when applied to 
> the straight lines that border a plot: they just become "fuzzy".
> So if a plot is mostly straight lines (e.g. "with impulses"), 
> having AA turned off would be preferable.
> 

-- 
Ethan A Merritt