From: utzoo!decvax!decwrl!turtleva!ken
Newsgroups: net.graphics
Title: Re: resolution questions
Article-I.D.: turtleva.135
Posted: Wed Jan 26 22:29:41 1983
Received: Sun Jan 30 00:49:52 1983
References: ucf-cs.842

    The question was at what point one can get good print quality.  I
take it this means fonts, although lines were mentioned in the original
article.  I don't know too much about printers, but I have had a lot of
experience with video.
    What is your definition of quality? How do you measure it? What
sort of things are most disturbing to you about print at 512x512?
    The things most disturbing to me are the ragged/jagged edges,
otherwise known as aliasing (in the sampling theorem sense).  Certainly
increasing the resolution to 4Kx4K reduces the jaggedness, but does not
eliminate it completely.
    To eliminate the aliasing (jaggedness) completely, one must resort
to anti-aliasing techniques (aptly named!).  When you draw lines,
circles, or splines, you are sampling a continuous mathematical
function at regularly spaced points on a rectangular grid, so sampling
theory must be taken into account.  Sampling theory says that you damn
well better low-pass-filter your signal before you sample it, lest you
get aliasing.
    Aliasing occurs because you round the location of an edge to the
nearest pixel.  One can move the apparent location of an edge between
pixels by diminishing the intensity:  Anti-aliasing techniques trade
off amplitude (brightness) for phase (location), resulting in a shift
in the apparent position of an edge.
    I used antialiasing techniques to render fonts, lines, and polygons
when I was a member of the engineering staff of a broadcast quality
video art system (AVA) a few years ago.  The frame store resolution was
768x486x8, and we got great looking characters.  The quality was so
good, that even characters *3 pixels high* were readable, as long as
they were upper case and well separated.  6 pixel characters were good,
and 10 pixel characters were excellent, especially compared with 3.
    The more bits per pixel that one has, the more one can get rid of
the aliasing.  The biggest jump in quality was from 1 to 2 bits per
pixel, although we generally used 4 (for intensity; the other 4 were
used for tint).
    Anti-aliasing tends to smooth out edges at the expense of making
edges look softer.  Increasing the resolution will make edges sharper;
3 pixel high characters will still be as readable as at lower
resolutions.

				Ken Turkowski (turtlevax!ken)

[1] Crow, F.C. A comparison of antialiasing techniques. IEEE Comput.
    Gr.  Appl.  v.1 no.1 (Jan. 1981) pp. 40-48
[2] Turkowski, K. Anti-Aliasing through the use of coordinate transformations.
    ACM Tr. Graphics, v.1 no.3 (July 1982) pp.213-234