Hyperspace Delivery Boy [LGP] colorkeys are broken on >16bpp displays

[sulix]

This looks like a game bug, not an sdl12-compat bug, but since it works with the bundled SDL 1.2, who knows.

Hyperspace Delivery Boy's colour keys do not work at all under sdl12-compat, or under a freshly built SDL 1.2, but do work with the game's bundled version of SDL 1.2.13.

The game is setting the colour key to 0xF800F8, but the actual colour of the key in the images is 0xFFFF (they're 16-bit). This would work, but the game appears to be using SDL_ConvertSurface to convert the surfaces to the screen surface's colour depth, before calling SDL_MapRGB(screen_format, 0xF8, 0x00, 0xF8), so if the screen surface is >16-bit, the colour key no longer matches.

Hacking the game to use 16-bit (by modifying SDL_SetVideoMode()) fixes the issue, but it persists in using 32-bit by default, as it:

• Calls SDL_GetVideoInfo() to get the default video format.
• If the bpp of that format is >= 16, it calls SDL_SetVideoMode() with that bpp.
• Otherwise, it calls SDL_SetVideoMode() with 16bpp.

The bundled version of SDL 1.2, however, does seem to return a 32-bit format from SDL_GetVideoInfo() as well, so maybe there's something else going on.

My feeling is that the closest thing to a "correct" hack here is to add a quirk which overrides the bit depth to 16, but I think what's going on with the bundled SDL 1.2 is possibly a bit more subtle.

[sulix]

Dropped my quick hack to force 16bpp here:
sulix@forcebpp

If I can't find a better solution, I'll clean this up, add a quirk, and send it in.

[sulix]

Okay, I've sent out #321 to report a max BPP of 16 to the game, which works around the issue.

The root cause is, as suspected, the game relying 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 (F81F) became
  (F800F8) instead of (FF00FF).
• 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:
  SDL_ConvertSurface does not convert color keys consistently 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.)