Measures a source once and says which conversion conditions hold.
Deliberately cheap, because "convert only when necessary" is worth nothing if
finding out costs what converting would. Nothing here decodes a frame or
reads a byte of media data on an indexed container: the container's own box
headers answer where the index sits, and the packet walk runs metadata-only
over a bounded prefix.
Measures a source once and says which conversion conditions hold.
Deliberately cheap, because "convert only when necessary" is worth nothing if finding out costs what converting would. Nothing here decodes a frame or reads a byte of media data on an indexed container: the container's own box headers answer where the index sits, and the packet walk runs metadata-only over a bounded prefix.