Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!portal!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: TT graphics modes
Keywords: sample&hold
Message-ID: <2735@atari.UUCP>
Date: 7 Nov 90 21:19:28 GMT
References: <1990Nov7.154503.9366@imada.dk>
Organization: Atari Corp., Sunnyvale CA
Lines: 55

micro@imada.dk (Klaus Pedersen) writes:
>Shifter mode called "Sample&Hold".
>(the article said something about HAM and lotsa colors!).

SAH mode has nothing to do with lots of colors.  What it lets you
do is draw filled polygons really fast.  You can draw two dots
on the screen and get a solid line between them, without having
to write the color value to all the pixels in between in memory.
Example below.

>  What happend to the 256 GRAYSCALE mode of the TT ? 

It's there.

>  What is the display reflech rate of the TT (TT-LOW/TT-HIGH) ?

That's "refresh rate" and the color modes are 60Hz no matter what
country you're in. TT HIGH rez is 1280x960 monochrome, and its
frame rate is 67hz, I think.

EXAMPLE OF SAH MODE:

In SAH mode, nonzero pixel values in memory appear as that color, and
zero pixel values in memory mean "continue with previous color."  That's
why it's also called "smear" mode: a colored dot will "smear" to the
right until it reaches another nonzero pixel value.

In the diagrams below, a period (".") is a zero dot and a star ("*") is a
one.  In addition, a zero ("0") represents a pixel value which has the
same color as the background.  Let's say you want a picture like this:

........
..****..
...**...
..****..
........

You can get it by drawing this:

........
..*...0.
...*.0..
..*...0.
........

This means you only had to write six pixels, not thirteen.  Of course,
this is just a sample: the savings is much more dramatic for bigger pictures.

(The trailing "0" in the second picture is what stops the smear from
going all the way to the right edge of the screen.  The left edge starts
out in the background color.)

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt