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Thread: EGA on monochrome, I thought it had 4 level greyscale?

  1. #1

    Default EGA on monochrome, I thought it had 4 level greyscale?

    Am I remembering this wrong, or did the universe skip a groove again? I can't find any information about it, but I could have sworn that a 64k EGA on a TTL monochrome display was capable of 4 level greyscale 640x350 graphics, since intensity without video is supposed to deliver "dim".... as evidenced by attribute 0x78's behavior on a hercules card the displays are capable of it. There's TWO bits free for video (video and intensity) not one.

    This capability would be useful for a ... project I'm working on. I'm restricting myself to the hardware available to a 1984 AT class machine, but really want high resolution greyscale.

    I don't have a TTL mono display to test with right now, but for now I'm working in mode 0x10 as a placeholder.
    Last edited by deathshadow; January 22nd, 2019 at 05:20 PM.
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  2. #2
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    Depends on the monitor. A traditional MDA-style monitor is capable of displaying only 2 levels.

    There were a few monochrome monitors specifically made for EGA use (there were also monochrome VGA monitors). For those, in theory, you can have 64 levels of gray, though you'd be hard put to see the finer gradations and you wouldn't be working in the special EGA 0x10 monochrome mode, but rather, one of the color modes.

  3. #3

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    Yeah, I think the trick here is that native EGA output is digital, so it's going to depend to a great extent on how the monitor chooses to handle it.

    All by itself, as Chuck notes, it would hypothetically be capable of displaying 64 levels, though how many would be discernably unique is the question. At any rate you could certainly get a useful 16-level gradient out of it, I think.
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  4. #4

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    So, EGA mode F is actually 640x350x3 grays...
    I thought the only card supporting 3-grays graphics was Tecmar Graphics Master.

    There was that Wyse WY-700 card, supporting 4-grays, but on some special monitor, not IBM 5151.

  5. #5

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    So three bits for every 2 pixels to get “3” color graphics?

  6. #6

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    Quote Originally Posted by rmay635703 View Post
    So three bits for every 2 pixels to get “3” color graphics?
    No.
    At least not in EGA mode F - in this case it's definitely 2 bpp, and the resulting 4 combinations are: black, normal, intensified, blinking.

    Video modes with bpp other than 1/2/4/8/16/32 would be difficult to program, so probably the only exception was 24 bpp.

  7. #7

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    Quote Originally Posted by Xacalite View Post
    Video modes with bpp other than 1/2/4/8/16/32 would be difficult to program, so probably the only exception was 24 bpp.
    Not really - or, at least, not with a planar display like the EGA. If you're using chunky pixels (ala VGA 256-color modes,) then yes, it's more complicated than is worth bothering with, but with a planar display you only need to add additional bitplanes. The Amiga, for example, supports any bitdepth from 1-6 bpp.
    Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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  8. #8

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    Quote Originally Posted by commodorejohn View Post
    Not really - or, at least, not with a planar display like the EGA. If you're using chunky pixels (ala VGA 256-color modes,) then yes, it's more complicated than is worth bothering with, but with a planar display you only need to add additional bitplanes. The Amiga, for example, supports any bitdepth from 1-6 bpp.
    You're right, now I recall those Sanyo MBC-55x machines, with 8-color graphics, using 3 bitplanes: R, G, B.

  9. #9

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    Quote Originally Posted by Chuck(G) View Post
    Depends on the monitor. A traditional MDA-style monitor is capable of displaying only 2 levels.
    No, four minimum if you count black. connect a GB112 card to a standard TTL monochrome display, disable blink, in text mode.

    https://www.seasip.info/VintagePC/Images/hpattr0.png

    See how the darker background (bit 7 on the attr) doesn't match the text brightness in the top area? Yeah, that. You can even program a GB112 (hercules plus) or Herc in-colour to output 4 levels of gray on a TTL.

    I just thought EGA could do it in mode 0x10. (not mode F, that's 2 colour)... as evidenced by the spec saying that with only 64k of RAM mode 0x10 is in fact 4 colour, not 16. Though I see no way to determine or set that via the BIOS... which seems really stupid!

    If you can weasel 4 levels of grey in text mode out of a hercules plus card, and there's a four colour high res mode for 64k EGA's.... I don't see why the two can't cross the streams; and in fact I remember it being capable of such.

    I just won't be able to test that until I actually get a TTL mono display I guess.

    ... and NO I'm NOT talking about mode 0x0F. I'm talking about the 64k RAM 4 colour behavior of 0x10... which I also can't test as I only have clone EGA cards with 256 on the board and there seems to be no BIOS difference in how they're set.

    -- edit -- what am I doing?!? I have Ferraro's book right there on the shelf; if it exists, it should be in there!
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  10. #10
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    I think you're probably seeing an artifact. Namely, the pinout of a monochrome display indicates 2 levels of white+ black:

    Monochrome pinout

    All levels are TTL, so either 0 or 1. It is true that the background can be white, gray or black, but that's not the same thing. For any pixel, you get black, dark white and bright white. While it's true that some monitors may display "bright black", that's not the way it's supposed to work.

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