Add RFC: Add notion of protocol

Author: tytan652

text/0045-add-notion-of-protocol.md

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+# Summary
+- Add notion of service protocol in libobs and OBS Studio
+- Outputs for streaming are registered with their compatible protocol and codecs
+- Only codecs compatible with the output and the service are shown
+- Audio encoder can be choosen if various compatible
+- Will be extended with RFC 39
+
+# Motivation
+Create a better management of outputs, encoders and services with an approach focused on protocols.
+
+# Design
+
+## Outputs API
+Only outputs with the `OBS_OUTPUT_SERVICE` flag are concerned.
+
+Adding to the API three flags:
+- `OBS_OUTPUT_VCODEC_AGNOSTIC`: indicate that the protocol is video codec agnostic.
+- `OBS_OUTPUT_ACODEC_AGNOSTIC`: indicate that the protocol is audio codec agnostic.
+- `OBS_OUTPUT_AVCODEC_AGNOSTIC`: combination of the two latter.
+
+Adding to `obs_output_info`:
+- `const char *protocols`: protocols separated with a semi-colon (ex: `"RTMP;RTMPS"`), required to register the output.
+- `const char *protocols_prefixes`: URL protocols prefixes separated with a semi-colon `"rtmp://;rtmps://"`, optional.
+  - Only one prefix per protocol in `protocols`.
+  - Those prefixes should be in the same order as `protocols`.
+  - Meant to be used for protocol auto-detection.
+- Two attributes already existent will be required if non-'codec agnostic':
+  - `const char *encoded_video_codecs`: video codecs supported by the protocol separated with a semi-colon, required if `OBS_OUTPUT_VCODEC_AGNOSTIC` is not set.
+  - `const char *encoded_audio_codecs`: audio codecs supported by the protocol separated with a semi-colon, required if `OBS_OUTPUT_ACODEC_AGNOSTIC` is not set.
+
+Asside from getters for new attributes, adding to the API those functions:
+- `bool obs_output_is_protocol_registered(const char *protocol)`: return true if an output with the protocol is registered.
+- `bool obs_enum_output_service_types(const char *protocol, size_t idx, const char **id)`: enumerate all outputs types compatible with the given protocol.
+- `const char *obs_output_get_prefix_protocol(const char *prefix)`: return the protocol bound to the prefix.
+
+### About protocols
+RTMP and RTMPS will on be considered compatible only with H264 and AAC.
+
+HLS will have no prefix set and not "auto-detectable".
+
+HLS, SRT and RIST are codec agnostic, since they use MPEG/TS container.
+
+FTL will be considered compatible only with H264 and opus because of the deprecation of the protocol in OBS Studio.
+
+## Services API
+Since a streaming service may not accept all the codec usable by a protocol, adding a way to set supported codecs is required.
+
+Adding to `obs_service_info`:
+- `const char *(*get_protocol)(void *data)`: return the protocol used by the service. It will allow multi-protocol services in the future.
+- `const char *supported_video_codecs`: video codecs supported by the service separated with a semi-colon. Optional, fallback to protocol supported codecs if not set.
+- `const char *supported_audio_codecs`: audio codecs supported by the serivce separated with a semi-colon. Optional, fallback to protocol supported codecs if not set.
+
+### About `rtmp-services`
+
+This plugin will only be compatible with H264 and AAC (and opus if FTL) and changed with RFC 39 afterward.
+
+So streaming will be only H264 and AAC or opus.
+
+## UI
+
+### Audio encoders
+If "Custom..." service is selected, the UI should allow to choose between compatible audio encoders. Just a combobox to choose which defaults to AAC.
+
+Otherwise AAC will be used or opus if FTL.
+
+### Settings loading order
+
+Service settings need to be loaded first and output ones afterward to show only compatible encoder.
+
+### About service JSON file
+
+- A field `"protocol"` will be added for HLS/FTL services, other protocol will be detected through prefixes, further changes will be done in RFC 39.
+  - Services still mono-protocol, RTMP+RTMPS combo is the only exception.
+- Services that ask for a protocol or only codecs that are not registered will not be shown.
+- `"output"` in `"recommended"` is no longer required (kept for backward compatibility) and `const char *(*get_output_type)(void *data)` in `obs_service_info` will be no longer used.
+
+### Service and Output settings interactions
+NOTE: Since Services API is not usable by third-party for now. The scenario where there is no simple encoder preset for the service does not need to be considered for now.
+
+Changing protocol by changing the service should update the Output page with only compatible encoders.
+
+If the actual codec is not supported by the service, the codec will be changed and the user warned about the change.
+
+## What if there is various registered outputs for one protocol ?
+
+The improbale situation where a plugin register a output for an already registered protocol could happen, so managing this possibility is required.
+
+- In `obs_service_info`: a `const char *(*get_preferred_output_type)(void *data)` could be considered and be a replacement to `const char *(*get_output_type)(void *data)`.
+- A function in the UI to preffer first-party outputs can be considered.