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Re: Does overwriting a line write it twice?

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In article <m5hnjs$nml$1@reader1.panix.com>, 
john@please.see.sig.for.email.com says...

> Suppose you mave something like
>   r1 g1 b1 setrgbcolor
>   w1       setlinewidth
>   x1 y1    moveto
>   x2 y2    lineto
>   r2 g2 b2 setrgbcolor
>   w2       setlinewidth  %%% with w2<w1
>   x1 y1    moveto        %%% ----------
>   x2 y2    lineto
> So you're drawing a different color narrower line 
> inside a wider line.
>    Does this line get written twice? 

Yes (assuming x1, x2, y1, y2 don't change value)


> In particular,
> if printed on an inkjet, is ink laid down along the
> entire w1-wider line, and then ink laid down again
> on top of that along the w2-narrower line?

No. The actual print head doesn't understand any print language at all. 
What happens is that a RIP (Raster Image Processor) interprets the input 
(PostScript in this case) and renders it to a bitmap. The bit depth and 
resolution of the bitmap depend on the printing capabilities of the 
printer, but for inkjets this is usally 1 bpp. So you end up with 4x1bpp 
images, one per ink.

The raster samples are used, more or less directly, to determine when 
the ink gets laid down.

So internally the line gets written twice, but that just means the page 
buffer pixels are written and then overwritten. The final raster only 
includes one colour at each pixel.

Its very common in PostScript and PDF to have partially obscured 
elements on the page.


>    If so, would a ps2pdf (and then printing the pdf)
> stop that? Or how would you stop that from happening?

It won't happen. Also, you cna do exactly the same construction in PDF 
as you have done in PostScript, and a PostScript to PDF converter would 
almost certainly preserve that construct. Doing aything else would be 
very difficult.


> That is, just lay down ink once, in color r2,g2,b2
> inside the w2-narrower line, and in color r1,g1,b1
> along the outside edges between w2 and w1.
>    gv'ing a ps file with those instructions looks
> like the narrow-inside-wide line is only written once,
> but I can't slow down the computer enough to be sure.

For a screen rendering its entierly possible that it *is* written twice, 
there often isn't any point in rendering to an intermediate bitmap and 
then squirting that to the display, better to use the display as the 
page buffer.

But for screen display it does depend very much on the application, 
Ghostscript for instance will rener to an intermediate bitmap and then 
blit that to the display. But it does 'sync' the output, so certain 
drivers will draw each element to the bitmap, then refresh the screen 
with the current bitmap. This would have the effect of drawing the line 
twice if you looked really carefully (and were running under a debugger 
so you could stop the applicatio, it'll be too fast otherwise)


			Ken

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Does overwriting a line write it twice? JohnF <john@please.see.sig.for.email.com> - 2014-12-01 12:40 +0000
  Re: Does overwriting a line write it twice? ken <ken@spamcop.net> - 2014-12-01 14:02 +0000
    Re: Does overwriting a line write it twice? JohnF <john@please.see.sig.for.email.com> - 2014-12-02 05:18 +0000
  Re: Does overwriting a line write it twice? jdaw1 <jdawiseman@gmail.com> - 2014-12-01 12:39 -0800
    Re: Does overwriting a line write it twice? JohnF <john@please.see.sig.for.email.com> - 2014-12-02 05:27 +0000

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