# Optimize first-frame rendering (/en/realtime-media/rtc/build/capture-and-render-video/optimize-frame-rendering/ios)

> For AI agents: see the complete documentation index at [llms.txt](/llms.txt).

First frame output time is the duration between when a user joins a channel and when they first see the remote video. A shorter first frame output time reduces perceived wait time by rendering video more quickly.

This guide describes two best practices to reduce video rendering time in Video Calling.

## Prerequisites

Complete the steps in the [SDK Quickstart](/en/realtime-media/rtc/get-started-sdk) to build a basic Video Calling app.

## Understand the tech

To reduce video rendering time, Agora provides the following solutions:

* **Preload and initialize before joining the channel**
  Complete time-consuming operations ahead of time, such as preloading the channel, configuring the rendering view, and enabling accelerated rendering for audio and video frames.

* **Join early, subscribe later**
  Join the channel in advance but delay subscribing to the audio and video stream. When the user triggers the join operation, subscribe to the host's stream and begin rendering immediately.

The following table compares both solutions:

| Characteristic       | Preload and initialize early                           | Join early, subscribe on demand                                        |
| -------------------- | ------------------------------------------------------ | ---------------------------------------------------------------------- |
| Applicable scenarios | Most audio and video use cases                         | Scenarios with very high requirements for first frame rendering speed  |
| Core implementation  | Initialize and configure video settings before joining | Join the channel early without subscribing; subscribe only when needed |
| Cost                 | Normal billing                                         | May incur additional channel usage fees                                |

The following figure shows the time to output the first frame before optimization and with each solution:

![Optimize video rendering](https://assets-docs.agora.io/images/video-sdk/optimize-video-rendering-tech.jpg)

## Implement fast first-frame rendering

This section describes the implementation logic for both solutions.

<Tabs defaultValue="preload">
  <TabsList>
    <TabsTrigger value="preload">
      Preload and initialize early
    </TabsTrigger>

    <TabsTrigger value="subscribe">
      Join early, subscribe on demand
    </TabsTrigger>
  </TabsList>

  <TabsContent value="preload">
    The following figure illustrates the essential steps:

    ### Sequence diagram for implementation

    ![Sequence diagram for optimized video rendering](https://assets-docs.agora.io/images/video-sdk/optimize-video-rendering-solution-1.svg)

    ### Set up an RTC SDK instance

    Creating and initializing the RTC engine takes time. To reduce the time to first frame, Agora recommends initializing the engine when the module loads rather than when your app first calls an SDK method.

    <CalloutContainer type="info">
      <CalloutDescription>
        Initialize the engine only once. Avoid creating and destroying it multiple times.
      </CalloutDescription>
    </CalloutContainer>

    ```swift
    class AgoraQuickStartViewController: UIViewController {
        var agoraKit: AgoraRtcEngineKit?

        override func viewDidLoad() {
            super.viewDidLoad()

            // Create and initialize the engine during view controller creation
            initializeAgoraEngine()
        }

        private func initializeAgoraEngine() {
            let config = AgoraRtcEngineConfig()
            config.appId = "Your App ID"
            config.channelProfile = .liveBroadcasting
            config.delegate = self // You'll need to implement AgoraRtcEngineDelegate

            agoraKit = AgoraRtcEngineKit.sharedEngine(with: config, delegate: self)
        }
    }
    ```

    ### Enable accelerated rendering

    Call [`enableInstantMediaRendering`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/enableinstantmediarendering%28%29) to reduce the time it takes to render the first video frame and play audio after joining a channel.

    * Call this method **before** joining a channel. Ideally, call it right after engine initialization.
    * Both host and audience must call this method to benefit from faster rendering.
    * To disable this feature, destroy the engine with `release`, then reinitialize it.

    ```swift
    // Enable accelerated rendering before joining the channel
    agoraKit?.enableInstantMediaRendering()
    ```

    ### Set a video scenario

    Use [`setVideoScenario`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/setvideoscenario%28_:%29) to optimize performance for your specific use case. The SDK applies strategies tailored to the selected scenario.

    For example, for a one-on-one call, use `APPLICATION_SCENARIO_1V1`.

    ```swift
    // Set the video scenario
    agoraKit?.setVideoScenario(.application1V1Scenario)
    ```

    ### Preload a channel

    Joining a channel involves acquiring server resources and establishing a connection. Call [`preloadChannel`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/preloadchannel%28bytoken\:channelid\:uid:%29) to handle resource acquisition early and reduce join time.

    * The `token`, `channelId`, and `uid` must match the values used in `joinChannel`.
    * Call `preloadChannel` as soon as you retrieve the required info.
    * Don’t call `joinChannel` immediately after `preloadChannel`.

    ```swift
    private func prepareChannelInfo() -> Int {
        uid = getUid()
        channelId = getChannelInfo()
        token = getTokenFromServer(channelId: channelId, uid: uid)

        // Preload the channel
        agoraKit?.preloadChannel(byToken: token, channelId: channelId, uid: uid)

        return 0 // Return success code, adjust as needed
    }
    ```

    ### Set up the rendering view

    Setting the rendering view early ensures the first frame displays properly. If the view is not ready, the first frame might be skipped.

    If your app knows the remote user ID (For example, from Signaling), set the view immediately. Otherwise, use the `onUserJoined` callback.

    * **Set the remote view early:**

    ```swift
    func onShowChannels(channelId: String, remoteUid: UInt) {
        let canvas = AgoraRtcVideoCanvas()
        canvas.uid = remoteUid
        canvas.renderMode = .fit
        canvas.view = nil
        agoraKit?.setupRemoteVideo(canvas)
    }

    func onEIDUserJoined(uid: UInt, elapsed: Int) {
        // Already set - no additional setup needed
    }
    ```

    * **Set the view when the user joins:**

    ```swift
    // Event handler class
    extension AgoraQuickStartViewController: AgoraRtcEngineDelegate {
        func rtcEngine(_ engine: AgoraRtcEngineKit, didJoinedOfUid uid: UInt, elapsed: Int) {
            // Forward to UI logic
            DispatchQueue.main.async {
                self.onEIDUserJoined(uid: uid, elapsed: elapsed)
            }
        }
    }

    func onEIDUserJoined(uid: UInt, elapsed: Int) {
        let canvas = AgoraRtcVideoCanvas()
        canvas.uid = uid
        canvas.renderMode = .fit
        canvas.view = surfaceView
        agoraKit?.setupRemoteVideo(canvas)
    }
    ```

    ### Monitor rendering performance

    Use [`startMediaRenderingTracing`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/startmediarenderingtracing%28%29) to monitor first-frame rendering metrics. Results are reported via [`videoRenderingTracingResultOfUid`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginedelegate/rtcengine%28_\:videorenderingtracingresultofuid\:currentevent\:tracinginfo:%29).

    <CalloutContainer type="info">
      <CalloutDescription>
        Call this method when the user initiates joining. For example, on a **Join** button tap. This gives accurate first-frame timing.
      </CalloutDescription>
    </CalloutContainer>

    ```swift
    private func onJoinClicked() {
        agoraKit?.startMediaRenderingTracing()
        agoraKit?.joinChannel(byToken: token, channelId: channelId, info: nil, uid: uid, joinSuccess: nil)
    }
    ```

    ### Join a channel

    Call [`joinChannel`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/joinchannel%28bytoken\:channelid\:uid\:mediaoptions\:joinsuccess:%29) to enter the channel. To speed up first-frame playback, avoid delays like fetching a token in this method.

    If you can’t retrieve a token early, consider using a [wildcard token](/en/realtime-media/rtc/build/authenticate-users/deploy-token-server#generate-wildcard-tokens).

    ```swift
    private func prepareChannelInfo() -> Int {
        uid = getUid()
        channelId = getChannelInfo()
        token = getTokenFromServer(channelId: channelId, uid: uid)
        return 0 // Return success code
    }

    private func joinChannel() -> Int {
        let options = AgoraRtcChannelMediaOptions()
        let result = agoraKit?.joinChannel(byToken: token, channelId: channelId, uid: uid, mediaOptions: options)
        return result == 0 ? 0 : -1
    }
    ```

    ### Optimize callback performance

    The SDK runs callbacks like `onJoinChannelSuccess` on the same thread. If one callback is slow, it can delay others—including rendering events.

    <CalloutContainer type="info">
      <CalloutDescription>
        Don’t block the callback thread with network calls, file I/O, or heavy processing.
      </CalloutDescription>
    </CalloutContainer>

    #### Best practices

    * Avoid complex operations in `onJoinChannelSuccess`.
    * Don’t block `onUserJoined` or other rendering-related callbacks.
    * Use background threads for heavy logic.
  </TabsContent>

  <TabsContent value="subscribe">
    The following figure illustrates the essential steps:

    ### Sequence diagram for implementation

    ![Sequence diagram for optimized video rendering](https://assets-docs.agora.io/images/video-sdk/optimize-video-rendering-solution-2.svg)

    ### Set up the rendering view

    If you know the host’s user ID before joining the channel, call [`setupRemoteVideoEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/setupremotevideoex%28_\:connection:%29) as early as possible to set up the rendering view. This ensures the rendering pipeline is initialized in advance, helping avoid delays in displaying the first decoded frame.

    If the host’s user ID is not available beforehand, wait for the `onUserJoined` callback, then call [`setupRemoteVideoEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/setupremotevideoex%28_\:connection:%29).

    ```swift
    // Define necessary variables first
    let connection = AgoraRtcConnection(channelId: channelId, localUid: uid)
    let remoteUid: UInt = 12345 // Replace with actual remote user ID

    // Set the host's rendering view
    let canvas = AgoraRtcVideoCanvas()
    canvas.uid = remoteUid
    canvas.renderMode = .fit
    canvas.view = getView()
    agoraKit?.setupRemoteVideoEx(canvas, connection: connection)
    ```

    ### Join a channel without automatically subscribing

    Joining a channel typically takes the most time before the first video frame appears. For use cases like fast channel switching, delay subscribing to media streams to speed up rendering:

    1. Call [`joinChannelEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/joinchannelex%28bytoken\:connection\:delegate\:mediaoptions\:joinsuccess:%29) to join the channel.
    2. In [`AgoraRtcChannelMediaOptions`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcchannelmediaoptions), set `autoSubscribeAudio` and `autoSubscribeVideo` to `false`.
    3. Subscribe manually when the user is ready to view content.

       ```swift
       // Define connection and event handler
       let connection = AgoraRtcConnection(channelId: channelId, localUid: uid)
       let eventHandler = self

       // Set channel media options
       let options = AgoraRtcChannelMediaOptions()
       options.channelProfile = .liveBroadcasting
       options.clientRoleType = .audience
       options.autoSubscribeAudio = false
       options.autoSubscribeVideo = false

       // Join the channel without subscribing
       agoraKit?.joinChannelEx(byToken: token, connection: connection, delegate: self, mediaOptions: options, joinSuccess: nil)
       ```

    ### Subscribe to streams and start rendering

    When the user chooses to view content:

    1. Resume media subscription using [`muteRemoteVideoStreamEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/muteremotevideostreamex%28_\:mute\:connection:%29) and [`muteRemoteAudioStreamEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/muteremoteaudiostreamex%28_\:mute\:connection:%29).
    2. Call [`startMediaRenderingTracingEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/startmediarenderingtracingex%28_:%29) to log rendering metrics.
    3. The SDK reports results in the [`videoRenderingTracingResultOfUid`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginedelegate/rtcengine%28_\:videorenderingtracingresultofuid\:currentevent\:tracinginfo:%29) callback, which you can use for performance analysis.

       ```swift
       func switchToChannel() {
           // Start video rendering tracing
           agoraKit?.startMediaRenderingTracingEx(connection)

           // Resume remote media subscriptions
           agoraKit?.muteRemoteVideoStreamEx(remoteUid, mute: false, connection: connection)
           agoraKit?.muteRemoteAudioStreamEx(remoteUid, mute: false, connection: connection)
       }
       ```
  </TabsContent>
</Tabs>

## Troubleshooting

Refer to [Slow first-frame rendering of remote video when using the Agora RTC SDK](/en/api-reference/faq/quality/optimize_video_rendering).