Glossary of terms

Offline processing

Offline processing represents a mode where Smelter is operating only on non-real time inputs and outputs e.g. combining 2 mp4 files into one. In this mode, the processing time is fully decoupled from the real time and progresses as fast as hardware allows it.

Live processing

Live processing represents a mode where Smelter is operating on any real time inputs or outputs. Processing will be synchronized to the real-time (wall) clock, meaning a 1-second segment of video will take approximately 1 second to process.

Timestamp

Unless stated otherwise all user-facing timestamps are defined in milliseconds and measure time from the queue start. Note that “time” in this context means time of the processed audio/video streams, so it might be different from the real (wall clock) time.

Example scenario:

• You are converting 5 minute MP4 file. Processing took 2 minutes to complete, but the current timestamp when the processing ended will be 5 minutes.
• You have a real-time “required” input that is delivered to slowly (e.g. network throughput, hardware bottleneck). As a result, the current timestamp will fall behind the wall clock time.

Input

Input is an entity that needs to be registered before use and can be used as a source of frames. For example, incoming RTP stream or MP4 file.

Offset

Input offset is a timestamp that represents start of the stream (relative to the queue start). If offset is not defined, then it is calculated based on arrival time of the first packet.

Required

Input marked as required will force a Smelter to block until data for the current timestamp is available.

In most cases:

• All inputs should be required in offline processing.
• None of the inputs should be required in live processing.

However, there are specific situations, when mixing those options can make sense.

Output

Output is an entity that needs to be registered before use and can be used as a destination for generated frames. For example, outgoing RTP stream or writing to disk as MP4 file.

Resource

Resource is an entity that needs to be registered before use, but it is neither an Input nor an Output. For example, shader source, font, or an image.