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Thread: Was CGA mode 5 secretly intended for anaglyph 3D?

  1. #21

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    @kdr: Okay, so I don't just want to nod and pretend I understand, so I'll have to check you on that, and with no disrespect intended, you're not making this real easy to follow along for someone who doesn't already know what they're looking at, and exactly what they're looking for, at which point, why even explain, they already know. But I don't, and you've delivered a dazzling but not very didactic description. First, are we even, literally, on the same page? Is this the diagram you're talking about?

    It took me a long time to even find anything that says MUX, which I didn't think was even there at first, and the simple hint that those two lines are labelled and start in the bottom left hand corner might have been helpful. I don't think the video RAM is even on that sheet.
    I initially thought the biggest IC on that page was the M6845, but that's wrong, that's elsewhere, and the MK36000 seems to be an EPROM, making that chip, I dunno, maybe the character ROM while I'm merely guessing?

    I can see U7 through U10, so now I'm pretty confident we are on the same page, but I don't see anything that's unambiguously labelled just C0 or just C1, especially not on any of the LS166 ICs, which maybe(?) you meant when you mentioned '166 shift registers, and I swear there's a comprehension-essential comma missing in your sentence somehow(?). This is getting unnecessarily hard.

    Since we are on the same page, I think, is there any chance you could maybe edit that picture and highlight what you mean?

    PS: Oh wait, there's C0 and C1 in the top right – but I still can't really parse what exactly you're saying or why you're saying it, unless it was to dazzle, in which case, you did. No offence.

  2. #22

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    Don't feel bad about not finding it easy to follow. I had to spend a long time poring over the CGA schematics (and the datasheets of the various ICs it uses) before I understood it.

  3. #23

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    Quote Originally Posted by reenigne View Post
    Don't feel bad about not finding it easy to follow. I had to spend a long time poring over the CGA schematics (and the datasheets of the various ICs it uses) before I understood it.
    Thanks for the kind words – and @kdr: Sorry if my comment sounded a bit harsh. I appreciate the help.

  4. #24
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    Quote Originally Posted by ropersonline View Post
    Thanks for the kind words – and @kdr: Sorry if my comment sounded a bit harsh. I appreciate the help.
    All good, I ought to have included a link to the appropriate PDF and page number at the very least.

    I too have spent more hours than is healthy staring at those CGA schematics. Convinced that there must be some way to hack and slash the registers into submission, surely there is a pleasant colour palette hiding in there somewhere! Alas I have yet to find it. CGA wins this round.

    Please don't get discouraged -- the CGA really does reward you for your study, because all of the secrets are right there in the schematics: aside from the 6845 CRTC, it's 100% simple discrete TTL logic gates. It's not like EGA or VGA where all the juicy bits are hidden inside opaque VLSI chips. We moan about crappy CGA graphics, but in the end I am amazed at how much functionality the designers were able to squeeze out of a handful of 74xx chips crammed onto a full-length ISA card.

  5. #25
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    Quote Originally Posted by ropersonline View Post
    Correct me if I'm wrong and please let us know the results.
    ftp://ftp.oldskool.org/pub/misc/Soft...h/chromadt.zip has an example picture I converted from a much better artist than myself. Wearing cheap chromadepth glasses, the effect is indeed there.

    However, a few hours of playing with chromadepth and seeing many examples, I've learned that you need very specific source material, or a creative artist, to make use of the effect. Also, you can't transition objects in the Z plane and still have the effect work, because that involves them changing color, and that's jarring. So traditional anaglyph is still better than chromadepth, as you can transition into/out of Z without the objects changing color.
    Offering a bounty for:
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  6. #26

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    Quote Originally Posted by Trixter View Post
    ftp://ftp.oldskool.org/pub/misc/Soft...h/chromadt.zip has an example picture I converted from a much better artist than myself. Wearing cheap chromadepth glasses, the effect is indeed there.
    Thanks for reporting back.
    Is .CGD an old image format? I don't even remember that one. GIMP doesn't recognise it.
    ...
    Oh, wow, that viewer executable is absolutely tiny at 192 bytes. Hand-coded in pure assembly, probably? So I'm guessing all this does is, it just dumps the contents of the file into the CGA's VRAM as is? For convenience, would you have that picture in any other format (PNG, etc.) or be able to recommend a modern program that can convert or open it?

    Quote Originally Posted by Trixter View Post
    However, a few hours of playing with chromadepth and seeing many examples, I've learned that you need very specific source material, or a creative artist, to make use of the effect.
    You need scene-work to make the dream work.

    Quote Originally Posted by Trixter View Post
    Also, you can't transition objects in the Z plane and still have the effect work, because that involves them changing color, and that's jarring. So traditional anaglyph is still better than chromadepth, as you can transition into/out of Z without the objects changing color.
    So I don't have chromadepth glasses, but I'm guessing it might look like a mouse scurrying across the kitchen table underneath the gingham tablecloth?
    (Or something moving underneath somebody's skin like in so many horror films... Eugh!)

  7. #27

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    Quote Originally Posted by Trixter View Post
    However, a few hours of playing with chromadepth and seeing many examples, I've learned that you need very specific source material, or a creative artist, to make use of the effect. Also, you can't transition objects in the Z plane and still have the effect work, because that involves them changing color, and that's jarring. So traditional anaglyph is still better than chromadepth, as you can transition into/out of Z without the objects changing color.
    That's about what I figured - interesting to hear about it, though. I'd never heard of the things until their mention in this thread.
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  8. #28
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    Quote Originally Posted by ropersonline View Post
    For convenience, would you have that picture in any other format (PNG, etc.)
    It was adapted from this: https://www.reddit.com/r/painting/co...int_and_black/
    Offering a bounty for:
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    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  9. #29

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    Quote Originally Posted by kdr View Post
    I wasn't being particularly precise...

    You can follow along by looking at Sheet 2 of the schematics for the CGA,...
    With this in mind, I will try to translate what you said into something that's somewhat easier to follow. Corrections welcome.

    Quote Originally Posted by kdr View Post
    ...which is where the video data is generated by combining 16 bits of data from video RAM...
    By which I assume you meant lines AT0 through AT7 and lines CC0 through CC7, originating from U35 and U34, respectively, towards the bottom left.

    Quote Originally Posted by kdr View Post
    ...with the MUX A and MUX B signals that control the current video mode.
    Which as we've established in a process akin to dentistry are the lines that start in the bottom left.

    But actually, no, I give up again for now, because you've really not made your point clear.

    Here's what I have so far. The 16 bits of data are yellow, MUX A and B are green. Only, contrary to what you said, the 16 bits and MUX lines are not combined. Sure, seven of the 16 bits (AT0 thru AT6) also reach those lower 74153 chips U9 and U10, but you said 16 bits of data are combined with the MUX signals, and I don't see that happening. Not on what's before me.

    Can you explain what you meant? Preferably in a way that does not assume familiarity with the point you're trying to make?

    I do have a hunch on what you might have meant, but going with that would be disregarding inaccuracies and making assumptions, and I think if you want to explain your point, then it's better that you explain it, because I can speculate all day, but I would still only be speculating, because your current explanation does not lead the person that doesn't already know the answer through the valley of the shadow of ignorance. Because you're not with them. So I fear idiocy.

    On that sheet 2 of 6, what is our starting point, where are we trying to go, and what are the steps, in order?

    Here again is the rest of what you said:

    Quote Originally Posted by kdr View Post
    C0 is the output of the '166 shift register U7 that handles the even bits from graphics mode (bits 0, 2, 4, 6, etc...) and C1 is the output of the '166 shift register U8 handling the odd bits. So each C0/C1 pair is (in sequence) bits 0+1, 2+3, 4+5, etc. C0 and C1 are then wired up to the '153 dual 4-to-1 mux at U9 that outputs the R and G bits; C0 goes into 2C2 and C1 goes into 1C2. When in graphics mode, the mux takes the inputs on xC2 and puts them on xY -- so 1C2 (which is the C1 bit of the current pixel) goes to 1Y (which is the R video output) and 2C2 (the C0 bit of the current pixel) goes to 2Y (the G video output).

    Which is all to say that, yes, reenigne is correct that C0 is Green and C1 is Red.

    Incidentally, this is where the ugly palette comes from: the other mux U10, which generates the B and I video bits, is fed from the SEL BLUE and BACKGROUND I signals in graphics mode. And these are 'global' signals, i.e. they don't come from the pixel data in video RAM but rather originate from the mode control and color select registers.... there is absolutely no way for the bits that come out of video RAM to influence the B or I video output when in graphics mode. That's why the 160x100 "16-colour graphics" mode is actually a text mode: it's the only way to hook up the data in video RAM to the B/I video output.

  10. #30

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    Okay, so maybe in furtherance of my hunch, would you or anyone at least be willing to confirm that:

    • the U7 and U8 ICs are SN54/74LS166 8-BIT SHIFT REGISTERS or equivalent, and that
    • U101 is a SN54/74LS174 HEX D FLIP-FLOP or equivalent, and that
    • U9 and U 10 are DM74LS153 Dual 1-of-4 Line Data Selectors/Multiplexers or equivalent?

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