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

[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: and "xprop -root" tells me: 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