Re: Apple II VGA Video Generator Card Project

bieling@xxxxxxxx wrote:
Michael J. Mahon ha escrito:


Jorge Chamorro Bieling wrote:

Michael J. Mahon <mjmahon@xxxxxxx> wrote:



While the time to put a character in a buffer is constant regardless
of the frame buffer size, drawing a line is proportional to the size
of the frame buffer, so all line drawing will take longer.


You mean
1. drawing a longer line takes longer
or
2. drawing the same lenght line takes longer

I mean that a line that is half the width of the screen takes
longer, because it has 1) more pixels, and 2) each pixel has
more bits.

If we are looking at vectors, then the time for a constant speed
processor to draw them goes up as the square root of the frame
buffer size. (Of course, the time to fill an area goes up
directly with frame buffer size.)


Yes, at "constant speed" for any given depth

1.- It takes n times longer to draw n*x pixels as it takes to draw x
pixels. (lines)
2.- It takes n*n times longer to draw of n*x*n*x pixels as it takes to
draw x*x pixels. (areas)

But "takes longer" is not a measure of drawing speed.
The space, measured in pixels, is missing in this equation :
(drawing speed) = (pixels drawn) / (time to draw them).

And the frame buffer size is not, nor direct nor indirectly, in this
equation.

The size of the frame buffer is a full screen of pixels.

The size of an Apple II HGR frame buffer is 8KB, for example.

If you provide an enhanced resolution mode that has, say,
one byte per pixel, and 480,000 pixels (800x600), then
the frame buffer is 480KB, or sixty times larger than the
HGR frame buffer, and one should expect that lines of the
same length (on the screen) will take about SQRT(60) times
longer to draw, since that many more pixels are drawn.

So a drawing only "takes longer"

1.- When you are drawing more pixels (so obvious!).
2.- For frame buffers whose *depth* is bigger not whose *size* is
bigger.

The VGA card DOES NOT slow drawing speed at all... :-)

Remember, I was talking about *enhanced* (not compatible) modes
that provide either more resolution or more color depth or both.
You apparently think I was talking about using a VGA monitor to
display the normal Apple II video modes.

I am merely stating the obvious a bit more precisely, and pointing
out that you will not be happy with a 1MHz processor drawing at
typical VGA resolutions.

There is a reason that the Apple II has relatively low resolution
graphics, and that the 2.8MHz IIgs made only modest improvements
with SHR.

People need to understand that adding a high resolution VGA mode
to an Apple II does not make it a modern processor.

It would be nice, I concede, to be able to attach an Apple II to
a VGA monitor, with a stable but blocky display. It would be much
less useful to try to do something interesting with that monitor
in a high resolution mode.

-michael

Music synthesis for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."
.

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