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Archiver > TMG > 2005-06 > 1117913476


From: Dennis Lee Bieber <>
Subject: RE: [TMG] Picture sizes & font point size in HTML output from TMG (6.0)
Date: Sat, 04 Jun 2005 12:31:16 -0700
References: <003601c56888$8fc74790$2a0c1fac@jfc><5.1.1.6.2.20050603163952.00aa1cc0@mail.umich.edu><003601c56888$8fc74790$2a0c1fac@jfc><5.1.1.6.2.20050604120516.00a668d0@mail.umich.edu>
In-Reply-To: <5.1.1.6.2.20050604120516.00a668d0@mail.umich.edu>


On or about 6/4/2005 09:17 AM a carrier pigeon from David S. Flower delivered:

> pixels, DPI ... have always been confusing to me.

Pixel Picture Element smallest representable piece of
information in the image

PPI Pixels Per Inch Number of Picture Elements per
inch of rendered display

LPI Lines Per Inch From half-tone print industry,
number of half-tone "cells" per inch of rendered display

DPI Dots Per Inch Hardware capability, smallest
physical mark the device produces

For color image scanners: DPI == PPI (one scanner dot
produces full color range in one Pixel)

For full color mode monitor displays: DPI == PPI (this ignores
the phosphor dot pitch factor -- I'm referring to the video driver
resolution, not the phosphor on the glass, since you need three phosphor
dots (RGB) at minimum to produce a pixel)

For color printers: DPI <> PPI <> LPI Color
printer "dots" are bistate: On or OFF, no middle levels. As a result,
producing any shade outside of (I'm using pure CMY for this example) [white
[paper]], Cyan, Magenta, Yellow, [Red [M+Y]], [Green [C+Y]], [Blue [M+C]],
[Black [C+M+Y]] requires using dither patterns where some dots are off, and
others are on. To produce a 256(7)-level grey scale requires being able to
go from no-dots-on to 256-dots-on. Such requires the use of a 16x16 square
of printer dots. Divide the printer DPI (most photo printers are 720-2880
depending on mode) by 16 to obtain the effective half-tone LPI. Common
practice is for the source image PPI to be 2x the LPI (1.5x to 2.5x).
Anything less may show artifacting (abrupt edges) and anything more is data
that the rendering engine has to throw away when setting up the print.

300PPI is commonly considered "photo-grade" (National Geographic,
for example). This translates to a 150LPI, or 2400DPI printer (using CMYK).

Photo inkjets can produce high quality at somewhat lower LPI/PPI
because of extensions: Some claim to dynamically vary the dot size (a
Dye-Sublimation printer with 256 "pulse" levels means a 300DPI is the same
as 300PPI). Others have added "half-shades" (6 and 7 color printers
CcMmYK(k) ), so again can produce a half intense smooth area at a lower LPI
(instead of using a box of half on, half off, they can use a single
half-intense dot).

The end result of all this is that an image meant for print use is
not suited for use on a web page.

If you intend the web page for some minimal equipment level
(800x600 resolution), you should probably limit your images to 720hx520v
pixels (720x480 is common for digital video at NTSC format). 720x520 allows
for 80 pixels to be used by the browser status bar, menus, scroll bars...
when the browser is full screen on an 800x600 display (it would only be
quarter screen on my current display 1600x1200).

This same image, if sent to a photograde printer, would optimize
at 2.4x1.73 inches.


>Thanks again,
>
> Dave Flower
>
>
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--
> ============================================================ <
> | Wulfraed Dennis Lee Bieber KD6MOG <
> | Bestiaria Support Staff <
> ============================================================ <
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