[R-SIG-Mac] font placement in Quartz

[Simon Urbanek]

On Nov 19, 2008, at 17:33 , Carl Witthoft wrote:

> Sorry if this is an  old issue; it's certainly of very low priority.
>
> I was playing around w/ font colors for, e.g. the main title in a  
> plot, and noticed that overwriting some text with the identical  
> text, but in white, leaves a small but noticeable 'border' outlining  
> each letter.  I played around a bit with overwriting plotted lines  
> in the chart and didn't seem to see this effect.  Is this a known  
> limitation on rendering fonts or some such?
>

It is the expected consequence of anti-aliasing. Since the edges of  
the text are not fully opaque (more precisely the pixels that are not  
entirely filled by the text glyph), they won't entirely mask what's  
underneath and thus any previous content (including perviously drawn  
text) will shine through (c.f. plotting text with alpha < 1). This is  
an intended behavior for semi-opaque drawing although less intuitive  
when the opacity is implicitly adjusted by anti-aliasing. You can  
avoid this effect by disabling anti-aliasing (at the cost of inferior  
visual quality).

Cheers,
Simon