SoftwareConverter

Turns a decoded frame into RGBA, on the CPU.

This is the tier 0 renderer path, and it ships rather than being a placeholder. It is the reference every GPU renderer is checked against, and it is the fallback when a GPU path fails. At 1080p it costs a few milliseconds of CPU per frame, which is too much for comfortable playback and fine for a screenshot, a thumbnail strip or a correctness test.

What it has to get right

Four things, and each one is visible when wrong:

• The row pitch. A plane's rows are almost never exactly width bytes apart. Assuming they are produces an image that skews diagonally, and it is the most common first bug in a video pipeline. The pitch comes from the frame rather than being computed.

• The matrix. BT.601 and BT.709 differ enough that using one for the other shifts every hue, worst on saturated reds.

• The range. Video is usually studio range, where 16 is black and 235 is white. Treating it as full range turns blacks grey and clips whites.

• The bit alignment. A ten bit component sits either in the low ten bits of its word or in the high ten, and the two are sixteen times apart. Guessing produces a picture that is entirely white or entirely black.

Chroma upsampling here is nearest neighbour, which is tier 0's deliberate limit: it is exactly what a GPU renderer does with a nearest-filtered texture, so the two can be compared pixel for pixel. Bilinear chroma is a tier 1 improvement, and it is the point at which the declared chroma location starts to change the picture; see chromaSampleShift for why nearest neighbour does not care.