Alternating images - retinal persistance

The Oric video chip is not an easy beast to master, so any trick or method that allows to achieve nice visual results is welcome. Don't hesitate to comment (nicely) other people tricks and pictures :)
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JMB_STARSOFT
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Alternating images - retinal persistance

Post by JMB_STARSOFT »

Hi Oric Graphics Gurus,

(A very...) Long time ago, I coded for fun applications (adaptation of Vexed, Neko, Hanoi towers etc...) for a PDA from Texas Instrument, the Avigo. It was a strict 2 'colour' LCD device : only black and white. However someone found a clever way to display gray images on this device : by switching quickly the display between 2 prepared images, so having in a area an alternating change between white and black become gray by the help of retinal persistence.

Technically
(1) Oric with the help of little hardware (or emulation option) is able to vsync,
so an image switch at the right moment is possible, to avoid image tearing
(2) ULA does not allow to change the address for the screen memory (to be exact it can but only between address for hires and text mode),
so image alternance must be made by software (ASM),
and regarding the cycles available during vsync, an alternance of a full screen in Hires is impossible
== they will be serious limitations anyway.

But at least did someone already experiment such visual persistence trick, at least in some part of the hires screen or with limitation in text mode ?
I wonder if it Is acceptable visually (50hz -> so the 2 images seen alternately at 25hz) ?

If yes, this may be another trick to explore to push again a little bit further the graphics on our beloved computers.

Regards !
JMB
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Symoon
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Re: Alternating images - retinal persistance

Post by Symoon »

Long ago I tried to make orange colour by alterning the display of red and yellow colour, but it was horrible, and when stable it actually only gave an area coloured half in red, and half in yellow ;)
But Fabrice and Chema did some interesting things, which I never tried sadly.
Here's the discussion, hope it can help:
https://forum.defence-force.org/viewtopic.php?t=944
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Dbug
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Re: Alternating images - retinal persistance

Post by Dbug »

I know for sure that Jonathan did some tests, on the top half of the screen by switching between HIRES and TEXT, that technically did work, but because of the very contrasted colors on the Oric that was very eye strain inducing.

Myself I did a tech demo of small spaceship floating around, redefining the character set every frame to alternate 0101 to 1010 to give some additional shades of gray, that kind of worked, but still would not play for hours with that.

You can find the demo in attachment.
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iss
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Re: Alternating images - retinal persistance

Post by iss »

The most colorful demo is @euphoric's 27 colors hexagons (50Hz tap attached).
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