X11 expertise on ancient HW sought... (4-plane visual (overlay) via X-server on MS-WIndows)

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Mouse, thanks very much for taking the time to comment.

On 06/06/2015 02:35 PM, Mouse wrote:
>> Am I correct to assume that the GPU must support 4bpp in order for it
>> even to be possible for the X-server to propagate a 4-plane visual to
>> a client?
> No.  However, it is substantially more difficult for an X server to
> present a visual that the hardware doesn't support, which is probably
> why the server you have doesn't do it.  Well-behaved clients must be
> prepared to handle whatever capabilities the server presents; the
> problem here is that the client you have handles the server you have by
> complaining and dying.
>
> Depending on what other capabilities the client is using, you might be
> able to get the overlay effect using colourmap hackery with
> comparatively small code changes.  What PseudoColor or DirectColor
> visuals are available?

If you are talking about hacking the x-client, no chance. At least, that would
probably entail unacceptable cost for the customer, who does not have the source.

>> If yes, how can I determine if a GPU supports 4bpp?
> Read the documentation, of course, or contact the manufacturer's
> support department.

Kind of hopeless in the case of manufacturers of mass-market hardware...

>> Nvidia is very sparing with the information in their specs for the
>> Quadro 400 GPU.
> You may be out of luck without repalcing the hardware, then.  (That's
> one of the prices of buying undocumented hardware....)

The Quadro 400 is coincidence. The test hardware was randomly chosen before all
this 4-plane stuff was known. The solution can be anything reasonable.

>> Assuming I can find a GPU that supports/offers 4bpp, does anyone know
>> an X-server product/project that can provide 4-plane overlays?
> You probably do not need a GPU.  The era when 4-bit visuals were common
> was full of dumb memory-mapped framebuffers; modern CPUs are fast
> enough that they can probably fake up a 4-bit overlay visual and still
> run at least as fast as the hardware your client software was designed
> to run against.
>
> I'm not sure how hard it would be to do.  My X server hackery has never
> included faking something the hardware doesn't support, so my
> experience is rather limited in that direction.  But I've done DDX
> layers for at least three widely disparate framebuffers, and I feel
> reasonably confident what you want could be done.

Depending on the cost, hacking the X-Server might be an option.
I have presented MS-Windows as a given here, but in fact I could also use Linux.
Do any possibilities involving Linux occur to you?

cheers,
Rob