Launch apps and control things with C-Play

This page covers three topics:

1. Control things with REST and WebSockets - send commands from C-Play to other systems
2. Launch OBS through REST + C-Troll - start an OBS instance that is ready for WebSocket control
3. OBS WebSocket examples - switch OBS profile, scene collection, and scene from C-Play

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REST in C-play

C-Play can be controlled by external systems through its HTTP Web API, but C-Play can also be the controller. The REST Commands Editor and REST layers can send commands to other programs when a show starts, when a slide is triggered, or when you press Trigger in the editor.

Typical uses are:

• Start or stop another program through C-Troll.
• Switch scenes, profiles, or scene collections in OBS Studio.
• Fire a command to a projector, lighting controller, media server, or show-control system.
• Run a REST layer from a C-Play presentation so external state changes happen at the same time as a slide.

Open Settings -> REST Commands Editor to create reusable commands. Commands are saved in data/predefined-rest-commands.json and can be selected later when configuring a REST layer.

Supported command types

REST commands support the normal HTTP methods GET, POST, PUT, and DELETE. They also support WS and WSS for WebSocket targets.

For HTTP commands, parameters are sent as form-style name/value pairs. For GET and DELETE, they are appended to the URL as query parameters. For POST and PUT, they are sent in the request body.

For WebSocket commands, the parameter list is converted to a JSON message. This is useful for simple WebSocket receivers, and C-Play also has special handling for OBS WebSocket v5.

OBS WebSocket commands

OBS Studio 28 and newer includes obs-websocket by default. The default WebSocket URL is:

ws://localhost:4455

When C-Play sees an OBS-style WebSocket command, it negotiates the obswebsocket.json subprotocol, identifies with OBS, and then sends an OBS request. In the REST Commands Editor, select method WS, enable OBS: Connecting to OBS, then choose one of the helper actions or enter a custom OBS request name.

The helper currently fills the fields for these common requests:

Action	OBS request	Request data field
Set Profile	SetCurrentProfile	profileName
Set Scene	SetCurrentProgramScene	sceneName
Set Scene Collection	SetCurrentSceneCollection	sceneCollectionName

If OBS WebSocket authentication is enabled, add a parameter named password with the OBS WebSocket password. C-Play will generate the authentication response during the OBS handshake. The helper can fetch OBS profiles, scenes, and scene collections when authentication is disabled; with authentication enabled, type the target name manually.

For other OBS requests, set OBS request to the request name from the obs-websocket protocol documentation, then add request data rows. Values such as true, false, numbers, JSON objects, and JSON arrays are converted to their JSON value types before C-Play sends the request.

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Launch OBS Portable through C-Troll

C-Troll can expose a small HTTP API that starts and stops configured programs. C-Play can call that API through a REST command, so a presentation can start OBS before switching it to the right profile, scene collection, and scene.

The default C-Troll examples in data/predefined-rest-commands.json use:

Field	Value
URL	http://localhost:7001/program/start
Method	GET
program	C-Play
cluster	LOCAL
configuration	Master

For OBS, create a matching C-Troll program entry that points to obs64.exe and set the working directory to the OBS bin/64bit folder. Then create a C-Play REST command with the OBS program/configuration names used by C-Troll.

Example command:

Field	Value
Title	Start OBS NDI through C-Troll
URL	http://localhost:7001/program/start
Method	GET
Ignore Status	enabled
program	OBS
cluster	LOCAL
configuration	NDI Browser

Launch OBS with WebSocket support

OBS WebSocket can be configured in the OBS UI under Tools -> WebSocket Server Settings, but it can also be overridden from the command line. This is especially useful when C-Troll launches OBS because the port can be part of the launch configuration.

Example Windows launch parameters:

obs64.exe --portable --multi --collection "C-Play NDI" --profile "C-Play" --scene "Browser" --websocket_port 4455 --websocket_ipv4_only

Useful parameters for this workflow:

Parameter	Purpose
--portable or -p	Use the OBS directory as a portable OBS profile/configuration root.
--multi or -m	Allow more than one OBS instance without showing the multiple-instance warning.
--collection "name"	Start with a specific scene collection.
--profile "name"	Start with a specific profile.
--scene "name"	Start with a specific scene.
--websocket_port 4455	Override the OBS WebSocket port for this launch.
--websocket_ipv4_only	Force obs-websocket to bind IPv4 only.
--websocket_password "password"	Override the WebSocket password. Use carefully, because command-line passwords can be visible to other local tools.

For the full OBS list, see the OBS launch parameters. For WebSocket-specific options, see the obs-websocket README.

Multiple OBS instances

If you need multiple independent NDI outputs, the easiest setup is usually multiple OBS directories, each running in portable mode. Give each directory its own scene collection, output name, WebSocket port, and C-Troll configuration.

For example:

OBS directory	WebSocket URL	NDI output
D:\OBS-Browser	ws://localhost:4455	OBS Browser
D:\OBS-VDO-Ninja	ws://localhost:4456	OBS VDO Ninja

This keeps the settings separate and avoids one OBS instance overwriting another instance’s profile, scene collection, plugin, or WebSocket settings.

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OBS WebSocket examples

Switch OBS scene

Field	Value
URL	ws://localhost:4455
Method	WS
OBS	Connecting to OBS enabled
OBS request	SetCurrentProgramScene
Request data	sceneName = Browser

Stored in data/predefined-rest-commands.json, the same command looks like:

{
	"enabled": true,
	"ignoreStatus": false,
	"method": "WS",
	"parameters": [
		{ "name": "requestType", "value": "SetCurrentProgramScene" },
		{ "name": "requestData.sceneName", "value": "Browser" }
	],
	"title": "OBS - Set scene Browser",
	"url": "ws://localhost:4455"
}

Switch OBS profile

{
	"enabled": true,
	"ignoreStatus": false,
	"method": "WS",
	"parameters": [
		{ "name": "requestType", "value": "SetCurrentProfile" },
		{ "name": "requestData.profileName", "value": "C-Play" }
	],
	"title": "OBS - Set profile C-Play",
	"url": "ws://localhost:4455"
}

Switch OBS scene collection

{
	"enabled": true,
	"ignoreStatus": false,
	"method": "WS",
	"parameters": [
		{ "name": "requestType", "value": "SetCurrentSceneCollection" },
		{ "name": "requestData.sceneCollectionName", "value": "C-Play NDI" }
	],
	"title": "OBS - Set scene collection C-Play NDI",
	"url": "ws://localhost:4455"
}

These commands can be triggered manually from the editor, selected as presets in a REST layer, or combined with a C-Troll launch command so C-Play starts OBS and then switches it into the right state.