Re: Mockingboard output through IIgs internal speaker?

[mjmahon@aol.com (Michael J. Mahon)]

Jeff Blakeney wrote:

<snip>

>That said I'll confuse the issue further by mentioning the fact that
>some people did do monochrome hi-res graphics that won't display
>properly if the colour burst is still on.  In fact, from at least the
>Apple II+ on up, it has been possible to do 560x192 monochrome
>graphics (with limitations but we're used to those :-) using only the
>hi-res screen.  By plotting a single dot on the screen it is only
>using half of that pixel on screen and by setting the high bit of the
>byte that that pixel is sitting in, you can shift that one dot to the
>other half of the pixel.  I remember typing in a demo program from a
>magazine that demonstrated this method many years ago but have never
>had the desire to try to put it into use.  :-)

I've used it from Applesoft for plotting simulated orbits with fewer
"jaggies".  The "trick" is to compute the x-coordinate as if it were
0..559, then set the color based on whether it's even or odd, and
plot at the x-coordinate / 2.  If two such lines ever cross within
a 7/14-pixel byte, then the last one plotted will determine the
color set bit, and earlier-plotted pixels may be shifted, but on
a sparse display, it works quite well.

Another technique, a bit harder, and subject to "vertical interlace"
flicker is to page flip between hi-res page 1 and 2 on the VBL
signal, and to plot all the even points (in an "even" color) on one
page and all the odd points (in an "odd" color) on the other page.

This approach works even for dense screens, and can easily
support 80-columns in software.  (Of course, unless you have
a medium- to long-persistence monochrome display, the flicker
from the vertical "interlace" will be a problem.)

-michael

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