What doesn't work: greyscale in software :-) from Tom Stepleton on 2018-03-11 (lisalist1)

From: Tom Stepleton <stepleton_at_email.domain.hidden>
Date: Sun, 11 Mar 2018 17:23:50 +0000

(I'm not sure anyone was wondering about this, and I probably could have figured it out by looking over the schematics or the system documentation and thinking carefully about timing. It was easy to write a tiny assembly program and just give it a go, however.)

Anyway: I was not able achieve a useful greyscale effect on the Lisa's display by changing the values in the video contrast register as fast as I could.

I could cause different scanlines on the display to appear in different shades, with some (but not a whole lot of) control over how it looked. But I wasn't able to get the intensity of scan lines to change in mid-flight across the screen, at least not that I could tell.

It was still fun to experiment. If you'd like to try playing with it yourself, you might find the CONSET2 routine in the Lisa Boot ROM listing on Bitsavers to be instructive. *I recommend strongly against it if you or anyone around your Lisa has photosensitive epilepsy, however.* You can very easily make the screen flash at frequencies that could be dangerous.

It would be interesting to hear from knowledgeable folks if they think I'm wrong in concluding that changing the beam intensity "mid-flight" is not really possible.

--Tom

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Received on 2018-03-11 13:24:27