Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!newstop!sundc!potomac!saturn!xanthian
From: xanthian@saturn.ADS.COM (Metafont Consultant Account)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: (Non) Square Pixels?
Message-ID: <11032@saturn.ADS.COM>
Date: 27 Feb 90 13:58:21 GMT
References: <4687@lmrc.uucp> <3119@cello.UUCP> <943@tardis.Tymnet.COM>
Organization: Advanced Decision Systems, Mt. View, CA (415) 960-7300
Lines: 91

In article <943@tardis.Tymnet.COM> jms@tardis.Tymnet.COM (Joe Smith) writes:

>The shape of the dots of phosphor and the number of phosphor dots
>per square inch do not really affect the shape of the pixels.


This discussion would be a lot clearer, and blessedly shorter, if the
originating posting for the thread, or any other posting along the
way, had bothered to state that what we would like to be square is the
imaginary rectangle formed by connecting the _centers_ of the (almost
inevitably _round_) phosphor dots or dot clusters in a two by two
array of addressable pixels on an (ideal) screen, not particularly the
dots or dot clusters themselves being square.

When the physical vertical distance between two vertically adjacent
pixel centers is the same as the physical horizontal distance between
two horizontally adjacent pixel centers, then the math to draw a
circle on the screen that looks perfectly round on the screen is
simple and requires no "aspect ratio" corrections.  When the ratio of
horizontal to vertical center to center distances is something other
than 1.0:1.0, then drawing a circle that looks round on the screen
involves turning on a different set of pixels than for the square
pixel case, and the difference is called an aspect ratio correction.

Complicating the process of trying to achieve a "square aspect ratio"
(i.e., a 1.0:1.0 one), are many realities.

First, physical chemical phosphor dot placement on the inside of the
screen face, or shadow mask grid design sitting just inside the
phosphors, are considerations that require certain aimings of the crt
beam to achieve the desired color by hitting the right chemical
composition dot, or the right shadow mask grid opening.  If the
phosphor dot centers or the mask grid are not on square aspect ratio
centers, you probably can't get a square aspect ratio in the display,
though clever design can get you close.

Second, "bottle" technology and availability in the low price market
is driven by television needs, not by computing needs, so a consumer
oriented computer, which has to compete on price since few customers
would even understand the technical spec sheet, is probably going to
have to live with the shape of the crt glass as demanded by TV, while
a "techie" oriented computer workstation can pay the higher price for
special run "bottles".  Again, the TV 3:4 bottle aspect ratio
influences how close the computer manufacturer can come to a square
aspect ratio for pixel center offsets.

Third, compatibility with an NTSC or a PAL standard again constrains
you by telling you how many lines have to fit one way on the screen,
albeit you can choose to use only a subset of them.  Again, a
constraint away from the "ideal" aspect ratio results.

Fourth, given all these constraints, programming and architecture for
the result is a lot simpler if the number of pixels in either
direction either is a power of two, or is chosen to contain as large a
power of two as possible, so that scan lines can begin and end on byte
boundaries, screen saves fit 512 byte disk sectors, and so on.  Again,
this may not allow hitting a square aspect ratio for pixel center
offsets, when combined with all the other constraints.

The standard solution for decades now has been to do an aspect ratio
correction before deciding what picture pixels to illuminate, so that
circles come out round on the screen, whenever that is feasible.

Some nice things don't work out well though; bit images don't rescale
nicely by aspect ratios that aren't powers of two, so doing a 90
degree flip of a font character is likely to change its vertical to
horizontal size ratio in its own coordinate system, because copying
pixels is much more satisfactory than resampling them for this task.

Similarly, scanned images are likely to look like accordians as they
are subjected to quarter turns, because the cpu cycles needed to
maintain the aspect ratios by resampling would slow the turns to a
crawl, and the color change artifacts caused by resampling would be
even more objectionable than the aspect changes.  Trust me, I've coded
this one.

The world is full of compromises; aspect ratios "should" be square;
they rarely are.

--
Again, my opinions, not the account furnishers'.

xanthian@well.sf.ca.us (Kent Paul Dolan)
xanthian@ads.com - expiring soon; please use Well address for replies.
Kent, the (bionic) man from xanth, now available as a build-a-xanthian
kit at better toy stores near you.  Warning - some parts proven fragile.

Just another "overqualified" (read over 40 and previously well paid)
unemployed/unemployable computer graphics programming guru.

-> METAFONT, TeX, graphics programming done on spec -- (415) 964-4486 <-