Is necessary use amplify ui to use FaceLiveness?

You are correct in your understanding that the Amazon Rekognition Face Liveness feature requires the use of AWS Amplify, specifically for the video capture and liveness check process. While you can create a session ID and query results using your own API (.NET in your case), the actual face liveness detection process, including video capture and user interaction, must be implemented using the AWS Amplify SDK.

The AWS Amplify SDK, particularly the FaceLivenessDetector component, is essential for performing the StartFaceLivenessSession operation. This component handles the video capture, provides the user interface for guiding the user through the liveness check, and manages the interaction with the Face Liveness service.

It's important to note that you cannot use the Face Liveness feature without the Amplify SDK. The SDK is designed to provide a seamless integration of the face liveness detection process, including the necessary user interface components and the backend communication with the Amazon Rekognition service.

While you can implement parts of the process (like creating a session and retrieving results) using your own API, the core functionality of capturing the video and performing the liveness check requires the use of Amplify. This ensures proper implementation of security features, user guidance, and real-time analysis that are crucial for the face liveness detection process.

In summary, while you can handle some aspects of the integration using your own API, the use of AWS Amplify SDK is mandatory for the critical parts of the Face Liveness feature, particularly for starting the session and capturing the video.
Sources
Face Liveness FAQ - Amazon Rekognition
Programming the Amazon Rekognition Face Liveness APIs - Amazon Rekognition