New subject: [Bug 58662] 16-bit bitmap rendering

3 Sep 2025


      http://bugs.winehq.org/show_bug.cgi?id=58662
Bug ID: 58662
           Summary: 16-bit bitmap rendering
           Product: Wine
           Version: 10.14
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: opengl
          Assignee: wine-bugs@winehq.org
          Reporter: zowie+wine@vandillen.io
      Distribution: ---
Created attachment 79243
  --> http://bugs.winehq.org/attachment.cgi?id=79243
Glitchy 16-bit render of Civilization 3. Without OpenGL Civ 3 does not have
visible glitches.
I've added a new test earlier today for 16-bit bitmap rendering with wGL [1].
In conclusion, it's pretty broken. The solution to this problem isn't trivial,
so I'm opening this issue for discussion.
On Windows, when wglChoosePixelFormat is called with the flag
`PFD_DRAW_TO_BITMAP` and `cColorBits = 16`, you'll be given a pixel format with
r5g5b5a1 (except that the alpha bit is somewhat quirky). This gives you a
software rendered OpenGL context for OpenGL 1.1 [2].
On Wine, asking for a 16-bit pixel format gets you nothing, because all 16-bit
pixel formats with the DRAW_TO_BITMAP are required to have an alpha bit in Wine
and none of the pixel formats on my Nvidia driver actually have an alpha bit
(there's only r5g6b5 and r16). The alpha bit requirement was introduced
recently in MR !8392 [3]. It's worth noting that not getting a pixel format —
although it's wrong — actually works pretty well for Civilization 3 (see
“relevant software” section).
Removing the check for an alpha bit, Wine will be able to find a 16 bit pixel
format again, but you'll get unusably bad renders (screenshots in the
attachments).
Some observations on the screenshots:
- You can see alternating vertical black/blue and orange/white/yellow lines
every horizontal pixel.
- Blue pixels appear in the black lines, but they're rare. I only see blue
pixels in the text in the SimGolf screenshot.
- The render buffer doesn't seem to be getting cleared (pretty apparent in
Civilization 3: the old squares were sticking around)
- Zooming in, you can see that there aren't only vertical lines, but also
horizontal lines (see zoomed-in screenshot).
My theory is that there's two problems:
- Wine is probably putting 32-bit-per-pixel renders into the 16-bit-per-pixel
bitmaps. This explains the vertical & horizontal lines. I'm guessing there's
also some memory corruption as a consequence: It's presumably overwriting twice
as much bitmap memory as it should.
- The right pixel format just doesn't exist. The format r5g5b5a1 is not
something that is actually implemented on hardware (at least not anymore), it's
just a quirky Windows software renderer thing.
The second point is a bit troublesome because I don't see any easy workarounds.
The most obvious solution is to make an OpenGL 1.1 software renderer of our
own. Alternatively, it might be possible to simulate the behaviour by using a
32-bits-per-pixel OpenGL context. Whenever glFinish is called, we could
download the render to a "secret" Wine-internal bitmap that's also 32 bpp and
then overwrite the 16 bpp bitmap based on the pixels of the 32 bpp bitmap. I'm
not sure how reliable that is.
Looking at the OpenGL 1.1 reference [4], I think making an OpenGL 1.1 software
renderer could be a fun project, but the other solution definitely seems like
less effort. I've only been involved in Wine for seven days though, so I'll
defer the decision to senior maintainers.
[1] https://gitlab.winehq.org/wine/wine/-/merge_requests/8888
[2]
https://www.khronos.org/opengl/wiki/Platform_specifics:_Windows#PFD_DRAW_TO_...
[3] https://gitlab.winehq.org/wine/wine/-/merge_requests/8392
[4] https://www.talisman.org/opengl-1.1/Reference.html
---
Relevant software (screenshots in attachments)
- Sid Meier's SimGolf (demo: https://archive.org/details/Simgolfdemo)
  In SimGolf, a 16-bit DRAW_TO_BITMAP OpenGL context seems to be used for
drawing the terrain. Currently, the terrain renders as black.
- Sid Meier's Civilization 3
  In Civilization 3, it seems that 16-bit bitmap rendering is used to draw an
outline for the movement range of a selected unit (that's just a guess. It
draws a square when a unit is selected but I haven't seen it without glitches).
It's notable that Civilization 3 currently works fine. Because
wglChoosePixelFormat doesn't find a 16-bit pixel format, the `null_gl`
implementation is being used right now. That works out pretty well because the
movement range outline isn't something you miss when you don't know it's
supposed to be there.
---
Relevant bugs:
Sid Meier's SimGolf does not render properly |
https://bugs.winehq.org/show_bug.cgi?id=50876
Civilization III Complete shows black terrain (Wine compiled with OSMesa
support) | https://bugs.winehq.org/show_bug.cgi?id=41930
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[Bug 58662] New: 16-bit bitmap rendering