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From: dalloff@freenet.columbus.oh.us (Dave Althoff)
Newsgroups: comp.sys.apple2
Subject: Re: Why can't certain colors go to some pixels?
Date: 29 Apr 1996 01:08:23 -0400
Organization: The Greater Columbus FreeNet
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DR TE$TH & THE ELECTRIC MAYHEM (ronb@cc.usu.edu) wrote:
: It has been about 6 years since I even thought about apples, but while working
: on my ray tracer, I realized that when I was a kid, I had all of the facilities
: to do something similar on my ][E.  The thought occurred to me however, that
: the ][e did not allow you to write certain colors to certain pixel positions. 
: Does anyone out there have a technical explanation for why this was the case?

Easy!  You're talking about the seemingly incomprehensible rules for color
on the ][e 280x192 hi-res screen.

The Apple ][ hi-res screen uses a one-bit display, with seven pixels per
byte.  This allows the entire screen...all 53,760 pixels...to fit into
only 8,192 bytes of memory.  That's 192 rows of 40 bytes per row.  Then
the screen memory for the first page of hi-res graphics is inconveniently
located (if you've ever had an Applesoft BASIC program crash into the
hires page you know what I mean) located from $2000-$3FFF in memory.  The
reason for this is so that if you bought an Apple ][ and only bought 16k
of memory, you could still have one page of hi-res graphics.  With only
32k, you can have two pages of graphics and still use DOS!

But Apple went a step further.  Instead of merely providing one-bit
black-and-white graphics, Apple set the screen up to allow *four* colors
on the hi-res screen.  In order to do this, all of the even horizontal
positions were assigned one color, and the odd positions assigned another.
 That's two colors.  Then by using the eighth bit of the screen data byte
to contain color set information for that byte (a pallette choice, if you
will), the screen could accommodate four discrete colors, though only two
in any given byte.  That is a total of five colors:  black, green, violet,
orange, and blue.  There is no such thing as a white point; but there are
four ways to get a "white" point to appear on the screen:
a)  Plot a green point next to a violet point
b)  Plot an orange point next to a blue point
c)  Plot a green point next to a blue point (adjacent bytes)
d)  Plot a violet point next to an orange point (adjacent bytes)

It's a clever way to get four colors onto the screen, but it can be a
hassle if you're actually trying to use hi-res color.  If color is not
important to you and you are using the Applesoft drawing tools, set your
HCOLOR to 3 or 7 to plot on all points.

HI-RES COLOR TABLE:
HCOLOR=0  Black (green/violet color set)
HCOLOR=1  Green (odd pixels only)
HCOLOR=2  Violet (even pixels only)
HCOLOR=3  "white" (plot green and violet on all pixels)
HCOLOR=4  Black (orange/blue color set)
HCOLOR=5  Orange (odd pixels only)
HCOLOR=6  Blue (even pixels only)
HCOLOR=7  "white" (plot orange and blue on all pixels)

To show how the color sets work, try this:
]HGR
]HCOLOR=3
]HPLOT 1,0 TO 1,191 :REM Plots a vertical green line
]HCOLOR=4
]HPLOT 0,0 TO 0,191 :REM The green line turns orange!
]HCOLOR=3
]HPLOT 4,0 TO 4,191 :REM Orange line turns green; violet line appears
]HCOLOR=7
]HPLOT 7,0 TO 7,191 :REM Orange line should appear
]HPLOT 10,0 TO 10,191 : REM Blue line should appear
]HCOLOR=2 :REM This should set color to violet
]HPLOT 140,0 TO 140,191 :REM Verify correct color
]HPLOT 0,0 TO 0,191 : REM Green line will turn white
]HCOLOR=6
]HPLOT 8,0 TO 8,191:REM Orange line should turn white

That ought to help a little 8-)

--Dave Althoff, ][.
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