Re: High Resolution With 16 colors?

[dempson@actrix.gen.nz (David Empson)]

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

>     I would like to ask why Steve Worniak designed Apple II to restrict only
> 4 colors on high resolution 280x192.

The colours in hi-res graphics mode on the Apple II are directly related
to the design of the NTSC television system (the scheme is emulated for
more advanced output like RGB).  The precise timing of a pixel's
horizontal placement determines whether it will appear as green, violet,
orange or blue.  Two adjacent pixels appear as white.

> Later, he fixed a bug to add two more colors.  He should add 16 colors
> instead of 6 or 8 colors on high resolution (Not Double High Resolution).

Not possible - getting more than four colours out of NTSC requires
higher resolution for the timing of the horizontal pixel placement.
This is achieved in double hi-res graphics mode, which outputs bits at
14 MHz instead of 7 MHz.

Hi-res graphics cannot achieve this speed, because it would require
reading from RAM at 2 MHz.  (Double hi-res can do this because it reads
main and auxiliary memory in parallel during the same clock cycle at 1
MHz.)

>     Is it because the single byte is very restricted?  No excuse to use two
> bytes to combine colors together.  What is the difference that he designed
> low resolution 40x48 to provide 16 colors first before high resolution?

Lo-res graphics mode outputs data at 14 MHz.  The higher dot clock
frequency allows more colours to be selected.  The dot pattern to
generate the colour is fetched from the character generator ROM for a
IIe (not sure about the II+) and is repeated twice for each block in
lo-res mode (not repeated in double lo-res mode).

>     Think about the earliest resolution on IBM PC.  IBM PC had only 160x100
> resolution with two colors.  It had 4x4 pixel font for 40/80 columns.
> Please correct me if I am wrong.

I don't know the details of anything which predates CGA.

CGA supports 4 colours (either of two sets) at 320x200, and two colours
(white in combination with any of the sixteen colours) at 640x200.

Don't forget that CGA was designed to be used with a digital RGB
monitor, while the Apple II (which predates it by at least three years)
was designed to work with a television.  Completely different beasts.

>     Why did IBM PC only show 4 colors (black, white, blue, violent) on
> 320x200 resolution and another 4 colors (black, white, yellow, green) on
> another 320x200 resoltuion.

Because the data representation of the image only had room for two bits
per pixel in 320x200 mode (one bit per pixel in 640x200 mode).

> Apple II's high resolution allows to combine 8 colors together.

Well, six colours, and don't forget that the effective colour resolution
for hi-res graphics mode is only 140x192.

If CGA supports a 160x200 mode, then it could have four bits per pixel
and 16 colours anywhere on the screen.

-- 
David Empson
dempson@actrix.gen.nz