Add 5.1 surround sound

port/linux/README.md

@@ -370,6 +370,7 @@ the setting for one start of the game. It has priority over the file.
 | `audio.volume` | `1.0` | `HALO_VOLUME` | The master volume. |
 | `audio.music_volume` | `1.0` | `HALO_MUSIC_VOLUME` | The music's volume, of the master volume. |
 | `audio.effects_volume` | `1.0` | `HALO_EFFECTS_VOLUME` | The volume of the other sounds (effects and speech), of the master volume. |
+| `audio.channels` | `"auto"` | `HALO_AUDIO_CHANNELS` | `"stereo"`, or `"5.1"` for surround sound as with Dolby Digital on the Xbox. `"auto"`: 5.1 if the sound device has 6 or more channels. Linux and Windows only. |
 | `audio.reverb` | `true` | `HALO_REVERB` | `true`: the sounds of the world reverberate as the place the player is in does: the sound environments of the maps (a corridor, a cave, a large hall, outdoors) set the reverberation, as the I3DL2 reverb of the Xbox did. A sound behind a wall or a door is muffled in it too. `false`: no reverberation (sounds behind a wall are still muffled). |
 | `audio.resampler` | `"sinc"` | `HALO_AUDIO_RESAMPLER` | How the sounds are converted to the output's rate (48 kHz): `"sinc"` with a band-limited low pass, which keeps out the images above the sound's own rate; `"linear"` between neighbouring samples, which lets some of them through and sounds brighter for it. Audio Setup's SOUND PROCESSING screen sets it, with `audio.reverb`. |
 | `audio.voice_chat` | `"push_to_talk"` | `HALO_VOICE_CHAT` | How you talk in voice chat: `"push_to_talk"` (while `controls.push_to_talk` is held; the microphone opens when you first press it), `"open_mic"` (when the microphone hears speech), or `"off"`. You hear the other players in every case. Refer to "Voice chat". |
@@ -1216,7 +1217,7 @@ crafted file) is refused, and the level starts over.
 | High-res HUD | The HUD is drawn from high-res assets: redraws at 8x the size of the maps' bitmaps (4x for the largest), in `port/assets/hud`. They cover the meters, counters, panels and their outlines, the motion sensor, reticles, waypoints and scopes, but no bitmap with English text. `tools/hud_assets.py` makes them from the SVG redraws (the weapons' reticles from those of the PC HUD sheet that match the Xbox's exactly, the sniper rifle's 2x and 10x set in Overpass), and the build embeds them in the executable. When the game uploads one of those bitmaps, `src/hud_hires.c` gives the high-res texture in its place, if the bitmap's pixels are those of the English maps: another language's maps keep their own. The game sizes and places the HUD from its tags as before. `display.high_res_hud = false` turns this off. |
 | High-res text | The menus' and HUD's text is drawn with Overpass (`port/assets/fonts`, SIL Open Font License) in place of the maps' bitmap fonts, which are Interstate. `src/text_hires.c` rasterizes each glyph with stb_truetype (`port/third_party/stb`) at the resolution the game draws at, into an atlas that a placeholder bitmap of the game stands for. The game lays the text out from its font tags as before: each font is as tall as its tag's capitals, and narrowed where the tag's letters are narrower than the Xbox maps' (the fonts of a Custom Edition `loc.map`), so that its letters do not run into each other. The menus' titles (the screens' headers and the main menu's items) are pictures of text in the maps, so they are drawn as the high-res HUD is: `tools/title_assets.py` sets each one again in OpenCE, Roger White's public-domain Newtown respaced to match the maps' commercial title typeface (`tools/title_font.py`), at 4x the bitmap's size over its own plate or glow, each letter placed where the map's letter is, in `port/assets/titles`. The postgame carnage report's title is set over a hand-made SVG redraw of its panel (`port/assets/titles/svg`) instead. The A, B, X and Y button icons beside the menus' key labels and in their text ("Press A to Join") are drawn from hand-made SVG redraws at 8x the size of the maps' bitmaps (`port/assets/buttons`, made by `tools/button_assets.py`), in the same sizes and places. `display.high_res_text = false` turns it off. |
 | Anti-aliasing | Off unless `display.anti_aliasing` is set (`src/d3d8_gl.c`, `src/xgpu_post.c`). FXAA (written in the port) and SMAA (`port/third_party/smaa`, MIT licensed, at its HIGH preset, compiled as GLSL) are passes over the 3D view of each window, after the lens flares and before the HUD and the menus (`render/render.c`). Their programs are built when the setting is chosen. Supersampling draws the render targets the size of the screen at two times the resolution in each direction, and the display blit scales them down. Multisampling draws the back buffer and its depth buffer into multisampled renderbuffers, and with them any target that is drawn together with one of them (a mirror's view, in the secondary target with the back buffer's depth buffer), so that the attachments of a framebuffer are all multisampled or none is. A target's pixels are resolved into its texture before something reads the texture (as a texture, or at the display blit). Visibility tests count samples, divided by the samples of a pixel. An alpha-tested surface (foliage, grates) covers the samples of a pixel in proportion to its alpha past the reference (`gl_SampleMask`, not on Android). |
-| Sound | Xbox DirectSound on SDL3 audio (`src/dsound_sdl.c`): PCM and Xbox ADPCM, mixed at 48 kHz, with volume, pitch, mix bins, distance, stereo pan, occlusion and obstruction. There is no Doppler effect, no cones and no reverb. |
+| Sound | Xbox DirectSound on SDL3 audio (`src/dsound_sdl.c`): PCM and Xbox ADPCM, mixed at 48 kHz in stereo or 5.1, with volume, pitch, mix bins, distance, pan, occlusion and obstruction. There is no Doppler effect and no cones. On 5.1 the game gets the AC-3 speaker setting of the Xbox and sends its 2D sounds to the center and back speakers, 3D sounds pan across the five speakers, and the reverb plays from the front and back pairs. The game sends nothing to the LFE channel. |
 | Input | XInput on SDL3 (`src/xinput_sdl.c`): keyboard, mouse, gamepads with rumble, and the debug keyboard for the console. |
 | Files | The Win32 file functions and the MSVC file functions on POSIX, with the translation of Xbox paths. |
 | Threads | Threads, events, mutexes, critical sections, interlocked operations and alertable waits. |