PNG term and anti-aliasing of lines

[Allin C. <cot...@wf...>]

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.)

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.

-- 
Allin Cottrell
Department of Economics
Wake Forest University, NC