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Add a SDL12COMPAT_MAX_BPP hint, and use it to restrict Hyperspace Delivery Boy to 16bpp (fixes #317) #321

Merged
merged 2 commits into from
Sep 14, 2023
Merged

Add a SDL12COMPAT_MAX_BPP hint, and use it to restrict Hyperspace Delivery Boy to 16bpp (fixes #317) #321

merged 2 commits into from
Sep 14, 2023

Commits on Sep 14, 2023

  1. Add a hint to clamp the reported screen bit depth 

    Some games (e.g. Hyperspace Delivery Boy) only work on a 16-bit display,
but request the highest bit-depth available. Add a hint which makes all
queries for a mode (including the current mode, and the implicit format
chosen by providng SDL_SetVideoMode() a bpp of 0) report a bit depth
less than or equal to the value of SDL12COMPAT_FORCE_BPP.

For example:
SDL12COMPAT_FORCE_BPP=16 ./hdb

There are a few interesting "features" of this implementation:
- All video modes are now clamped to 32bit, which is the default value
  of the hint. This seems like something we'd probably want anyway, as
  many SDL 1.2 apps would have trouble with >32bit video modes anway
  (and we never properly supported them).
- This is our first proper integer hint, so add an SDL_GetHintInt()
  function.
- Split the BPP -> format conversion into its own helper function.
- This doesn't clamp the bit depth of user-created surfaces, or of the
  screen if the application specifically requests a bit depth. It only
  changes defaults and reported modes.
    @sulix
    sulix committed Sep 14, 2023
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  2. Quirks: Hyperspace Delivery Boy should run in 16bpp mode 

    The LGP port of Hyperspace Delivery Boy has broken colour keys if run in
32-bpp mode (see bug libsdl-org#317). This is because it relies heavily on the
imprecise RGB565->RGB888 conversion in earlier SDL 1.2 versions, when
running in 32-bpp mode.

The game's assets are all in 565 format, and the game converts these to
the screen's format on load. It then sets a colour key. This presents a
problem, because:
- The generic BlitNToN implementation in SDL 1.2 just shifted the
  values, so the resulting image was not at full range. Magenta became
  (F800F8).
- Early versions of SDL 1.2 fell back to the BlitNToN blitter very
  frequently:
  libsdl-org/SDL-1.2@6f4a75d
- So, Hyperspace Delivery Boy calls SDL_MapRGB(0xF8, 0, 0xF8) to get the
  colour key, then sets it on the converted surface.
- In SDL 2.0, the blitters now properly do a full-range conversion, so
  the magenta becomes (FF00FF), which now doesn't match the hardcoded
  (F800F8).
- That being said, in general, it's not guaranteed that SDL_MapRGB()
  will do the same format conversion as SDL_CovertSurface(), so the
  "correct" way of handling this is to set the colour key before
  converting, which works (albeit slowly) in SDL2:
  libsdl-org/SDL#1854
- Since the conversion behaviour is different even between SDL 1.2
  versions, it's not worth trying to imitate it here, so we just force
  the game to run in 16-bpp mode, which works fine.
- (And the game's README recommends it, too.)
    @sulix
    sulix committed Sep 14, 2023
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