guessFor

The conventional reading when a container declares nothing, which is common.

Standard definition is BT.601 and high definition is BT.709, and every player applies that rule because using one default for both visibly wrongs half the world's video. Two refinements over a single 576-line split:

• the two standard definition families do not share primaries. 525-line content (NTSC, 480 lines and below) is SMPTE 170M; 625-line content (PAL and SECAM, up to 576 lines) is BT.470BG. Both use the same BT.601 matrix coefficients, which is why the matrix is the same for the pair and only the primaries move.

• a height that is zero or negative describes nothing, so it is answered with Unspecified rather than being swept into the high definition branch. A guess made from a size that does not exist is not a guess, it is noise.

This stays a heuristic over one input. A caller that knows the frame rate, the container or the origin of the media can do better, and should: nothing here overrides a real declaration, it only fills a silence.