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

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[also posted to comp.graphics.x today]

Hello Folks,

I'm trying to get an application that currently uses a local display on an
ancient DEC Alpha workstation with a (for the time) mid-to-high-end graphics
controller (ZLX-E2) to instead use an X-server running under MS-Windows.

The application is complaining that it cannot find a "4/5-bit visual". It almost
certainly wants to use this visual for an overlay, as the application displays
moving objects superimposed on a map.

On the original hardware, xdpyinfo tells me:
> [...]
> supported pixmap formats:
>     [...]
>     depth 4, bits_per_pixel 8, scanline_pad 32
>     [...]
> screen #0:
>    [...]
>   depths (4):    8, 12, 24, 4
>   [...]
>   number of visuals:    21
>   [lots of other visuals here, but no 4-plane except for the following]
>   visual:
>     visual id:    0x36
>     class:    PseudoColor
>     depth:    4 planes
>     available colormap entries:    16
>     red, green, blue masks:    0x0, 0x0, 0x0
>     significant bits in color specification:    4 bits

and "xprop -root" tells me:
> [...]
> SERVER_OVERLAY_VISUALS(SERVER_OVERLAY_VISUALS) = 0x36, 0x1, 0x0, 0x1

As you can see, there seems to be exactly one overlay, whose visual id (0x36)
corresponds to the single 4-plane visual listed by xdpyinfo.

When I use the above commands to retrieve the capabilities of the MS-Windows
X-server (Exceed, in this case), xdpyinfo does not list a 4-plane visual at all.
"xprop" lists lots of overlays, 24 in total, all of them 8-plane visuals.

The MS-Windows box is running Windows-7 (64bit) and has a Nvidia Quadro 400 GPU.
I used the "Nvidia Control Panel" to set "Enable overlay" to "on" in the "Manage
3D settings" section. Also, in the Exceed X-server configuration I enabled
"OpenGL", and within that enabled "Overlay Support" and "GLX 1.3 Support".

I conclude that the MS-Windows SW/HW system (X-server, MS-Win GPU driver, GPU)
cannot offer 4-plane visuals. However, I don't know what system component(s)
is/are the cause the problem.

I have tested VcXsrv, Reflection-X, Exceed (with 3D option), X-Win32 and even
the ancient DEC Pathworks X-server eXcursion with no success. I'm working on
getting an evaluation copy of PTC's MKSTools X/Server. Of the X-servers I've
tested, Exceed seems to offer the most configuration parameters.

I'm not even sure the Quadro 400 can handle 4bpp "visuals", or whatever
MS-Windows calls them. In fact, I wonder if any modern hardware offers 4bpp
capability. On my Linux box with a GeForce GT 430 I don't have any 4-plane
visuals, and xprop doesn't mention any overlays either.

I'm somewhat confused about where overlays fit into the X scheme. I have seen
lots of references to overlays in an OpenGL context, however the Alpha seems not
to have any OpenGL capability: GLX is not in the list of extentions printed by
xdpyinfo. Can someone clear this up for me?

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? If yes, how
can I determine if a GPU supports 4bpp? Nvidia is very sparing with the
information in their specs for the Quadro 400 GPU.

Assuming I can find a GPU that supports/offers 4bpp, does anyone know an
X-server product/project that can provide 4-plane overlays?

thanks,
Rob