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Re: Colour machine with green screen monitor [Re: Duke] #104519 03/12/16 09:08 PM
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Pernod Offline OP
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Originally Posted By Duke
In case of a green screen it basically discards the red and blue components.

Hooked it up and it's not quite what I'd like. Any colours that don't contain any green are completely lost. On a simple 8 colour machine I get full green for green, yellow, cyan and white and black for red, blue, magenta. I was hoping for some greenscale, like watching a colour TV broadcast on a black and white TV.


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Re: Colour machine with green screen monitor [Re: Pernod] #104523 03/12/16 10:27 PM
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R. Belmont Online Content
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Yeah, that's not at all what a real composite greenscreen looks like. The green should be the usual RGB to grayscale formula but mapped to a green scale.

Pernod: update_row. You can't escape it.

Re: Colour machine with green screen monitor [Re: R. Belmont] #104527 03/13/16 12:29 PM
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Pernod Offline OP
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Originally Posted By R. Belmont
Pernod: update_row. You can't escape it.

Agreed, it's the only option for now.


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Re: Colour machine with green screen monitor [Re: Pernod] #104528 03/13/16 12:52 PM
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Yeah, that's not a good green-screen implementation. The proper formula for any kind of monochrome monitor should look like:

Luma = dot(IncomingRGB, [0.3 0.59, 0.11])
OutR = TintR * Luma
OutG = TintG * Luma
OutB = TintB * Luma

TintRGB would be 0, 1, 0 for a green-screen monitor, 1, 1, 1 for a black-and-white monitor, and something like 1, 0.75, 0 for an amber monitor.

Re: Colour machine with green screen monitor [Re: Pernod] #104530 03/13/16 01:37 PM
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I had a monochrome VGA monitor. It used only the green signal, and had a slight green tint to it. The screenshot earlier in the thread is pretty close to how it looked.

The correct implementation for a monochrome monitor will depend on how it makes the monochrome signal from the color signal.

Re: Colour machine with green screen monitor [Re: Pernod] #104531 03/13/16 01:48 PM
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Every monochrome VGA monitor I've ever seen/used did the grayscale formula like JD says. And every composite monitor (which is what we're discussing here) *definitely* did the grayscale formula.

Re: Colour machine with green screen monitor [Re: Pernod] #104532 03/13/16 03:08 PM
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The old greyscale Mac and Sun monitors only use the green signal - they signal to the computer that they don't support colour so it knows to go into monochrome output mode. I realise this is completely different to the greenscreens on the Apple II, BBC, etc. which took the "yellow" luma component from a composite video signal.

Re: Colour machine with green screen monitor [Re: Pernod] #104533 03/13/16 03:20 PM
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Yep, I've got a black and white IBM VGA monitor with only the green line on the connector. It's also interesting that most VGA compatible cards will remap the CRTC ports to 0x3bn when a monochrome monitor is attached so old software will think it's MDA rather than CGA.

Last edited by crazyc; 03/13/16 03:21 PM.
Re: Colour machine with green screen monitor [Re: Pernod] #104534 03/13/16 03:43 PM
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Huh, that's interesting. Guess we'll have to emulate that.

Re: Colour machine with green screen monitor [Re: Vas Crabb] #104613 03/18/16 07:07 PM
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Originally Posted By Vas Crabb
The old greyscale Mac and Sun monitors only use the green signal - they signal to the computer that they don't support colour so it knows to go into monochrome output mode. I realise this is completely different to the greenscreens on the Apple II, BBC, etc. which took the "yellow" luma component from a composite video signal.
That reminds me, many early Amstrad CPC games had a "Color or green screen" option on start up. Perhaps it changed certain colors so that they wouldn't be completely lost when using just the green signal?

I had always wondered about its usefulness since not much seemed to change and I also assumed that it just grayscaled the full RGB, but it would indeed make sense if the CPC's green monitor made use of that signal only.

EDIT: Okay, maybe it's not the case. Note how the second (100% red) row of bricks is still visible on the real monitor:



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