Re: Resolution vs hinting

On Fri, Mar 05, 2004 at 03:50:50PM -0500, James H. Cloos Jr. wrote:
| At what point does it no longer make sense to bother with hints
| and to use just the outline as a vector when rendering text?
| 
| Is 600 dpi at 24 bits per pixel sRGB enough?  300 dpi at 48?

This is a difficult question.  At the least you need to consider some
optics, display physics, perceptual psychology, and typeface aesthetics.
It's hard to find people who know about all those things.  I'm certainly
not one of them, but I can give you a few leads to follow.

Because the distance from the viewer to the display is crucial, you
should be thinking about pixels per degree, not pixels per inch.  People
with 20/20 vision can discriminate features that subtend about one
minute of arc (1/60 degree), so that's a rough upper limit on ideal
pixel size.

As I type this, I'm sitting about 20 inches away from a display that's
15.75 inches and 1600 pixels wide (102 dpi).  If I did the math
correctly, it subtends about 43 degrees and therefore each pixel
subtends about 1.6 minutes of arc.  Based on the rough approximation
above, if I maintain the same viewing distance, I'd need at least 160
dpi to reach my limit of visual acuity.

This is only part of the solution, though.  Just because a pixel smaller
than 1 minute of arc is too small for the average person to see reliably
doesn't mean that it's small enough for typography.  You also need
enough resolution to *position* the individual glyphs in an
aesthetically acceptable way for kerning and for line layout.

I can't offer you any conclusions there, but I can suggest an
experiment.  First render a line of text at a large size, then step back
and view it from a distance that yields about 1/2 minute of arc per
pixel.  Next render a line of text at the same size, but hint and snap
the glyph outlines to coordinates that correspond to multiples of 1
minute of arc.  Step back and compare the two lines.  The second one
might not be acceptably similar to the first, so try the process again
with sizes and distances that let you snap to 1/2 minute of arc.  Repeat
until the second line looks aesthetically close enough to the first.
That'll give you the angular resolution you need.

I'll wrap up with a pragmatic observation.  Laser printout at 300 dpi is
good enough for the vast majority of day-to-day purposes.  If viewing
distance is around 15 inches, that works out to 0.7 minutes of arc per
pixel.  And laser printout is bitonal, so it seems clear that for text
angular resolution is way more important than color resolution.

Hopefully someone who knows more about this than I do will be able to
offer you better answers.

Allen