Alpha channel and SRT: use cases

We would like to remind you about the ability to transmit the alpha channel using SRT Streamer. This useful application has two main areas of use:

• remote titling
• transmission of chroma key processing results.

Remote titling

There are situations where titles are prepared in one location and overlaid on video in another location. For example, our clients use Titler Live to create titles and then receive an NDI stream with an alpha channel. This stream is captured using SRT Streamer and transmitted to the studio where the titles are applied to the main video stream.

The transmission of the chroma key processed result

Our client was tasked with filming invited experts on a chroma key screen in different cities and transmitting this stream to the central studio.
In the central studio, the experts’ images are placed in a virtual studio.
The question arises of where the chroma keying (separating people’s images from the green screen) should be done.
There are two options:

• do it in the central studio on the signal that has undergone encoding-decoding (for transmission over the Internet).
• do it directly on-site on the original signal and transmit the stream with an alpha channel over the Internet.

Obviously, chroma key processing done directly on-site during shooting yields better results for several reasons:

• first, this approach allows for quick adaptation of lighting for optimal results.
• second, when using hardware chroma key processing built into some SDI cards and cameras, it becomes possible to apply optimal processing algorithms and effectively separate the green screen, resulting in a high-quality stream with an alpha channel.

All of this combined allows for achieving the best results when working with chroma key.
Therefore, we recommend performing chroma key processing directly on the original signal, that is, at the shooting location.

Based on this, we proposed the following workflow to the client:

• chroma key processing is done on-site, generating a signal with an alpha channel (NDI or SDI).
• using SRT Streamer, this signal with the alpha channel is transmitted to the central studio via the SRT protocol, where an SRT MiniServer is used.
• from the SRT MiniServer, the signal, along with the alpha channel, is fed into a mixer for further processing.

We hope that this article has been useful and informative. We would be glad if you take the opportunity to download the update on the page. Thank you for your attention.