NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/sys/mac/programm/15353

Text File | 1992-09-11 | 3.4 KB | 68 lines

Newsgroups: comp.sys.mac.programmer
Path: sparky!uunet!europa.asd.contel.com!darwin.sura.net!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!destroyer!gumby!kzoo!k044477
From: [e-mail] (Jamie R. McCarthy)
Subject: Re: simulating page-flipping via cluts
Message-ID: <[e-mail]>
Organization: Kalamazoo College
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Fri, 11 Sep 1992 16:31:35 GMT
Lines: 57

[e-mail] writes:
>[e-mail] (Jamie R. McCarthy) writes:
>
>> If you're clut-flipping to simulate page-flipping,
>> [you need every single color]
>
>Two questions: How do you simulate page-flipping with clut-flipping??
>
>And second: Wouldn't this fail to accomplish anything at all on
>a direct color screen, or am I *completely* misunderstanding (a
>distinct possibility.)

The easy one first:  yes, clut-flipping does nothing on a direct-color
screen.  In fact, a direct-color screen doesn't even have a color table
to flip.

Clut flipping is a nasty little technique that lets you pretend you have
two video pages, each of which has half as many bits per pixel as the
real video page.  In short, you split each pixel in half, and alternate
between setting the clut so that one half is visible and the other half
not.  An example might help clarify.

Let's say we're working on an 8-bit-deep screen.  Conceptually, split
each pixel into two 4-bit halves, call them left and right.  To set the
pixel at x,y on the left video page to, say, color 7, you set the high
four bits of the actual pixel at x,y to 7, without disturbing the low
four bits.  To set the pixel on the right video page to color 4, you set
the low four bits to 4 without disturbing the high four bits.

Let's say the palette we're using is sixteen shades of gray.  (Note that
you only get sixteen colors, since each page gets only four bits per
pixel.  Note also that you can have different palettes for the left and
the right page, and that there's actually a good use for that:  there
are glasses that sync to your monitor that use LCD shutters to give a
decent 3-D effect.  But I digress.)  Then, to display the left page, you
set colors 0x00-0x0F to black, 0x10-0x1F to dark gray, ... , 0xF0-0xFF
to white.  To display the right page, you set 0x00, 0x10, ... 0xF0 to
black, 0x01, 0x11, ... 0xF1 to dark gray, and so on.

Note that you don't have to use hi-nybble/lo-nybble for left and right
pages;  you can split up the pixel any way you want.  It doesn't even
have to be an even split, and you can split it as many ways as there are
bits in the pixel in the first place.  If you don't mind having only two
colors and seeing the same eight frames over and over, you can do
eight-position full-screen animation at whatever your monitor's refresh
rate is, using just a tiny fraction of CPU power.

Note also that, if the palette(s) for your virtual pages all have all
their bits clear for white and all their bits set for black, you can
leave those two colors where they are, and the Palette Manager will be
usable.  (Though I'll concede that there's no good reason not to use
SetEntries instead of AnimatePalette when you're getting this
low-level...)
--
 Jamie McCarthy      Internet: [e-mail]      AppleLink: j.mccarthy
 "I may not understand what you say, but I'll defend to your death
  my right to deny it."                 - Albert, from 'Outrageously Pogo'