A Short MSS1/2 Description

Some of you may have heard of such thing as Mi***soft screen capture codecs (aka WM Screen decoder). Here I’ll try to present some known information about it.

First of all, this codec employs old-school arithmetic coder — traking high and low values instead of low and range, normalisation by one bit, and it’s used for decoding everything (1-bit numbers, 8-bit numbers and symbols with variable probabilities) without any additional context-dependency stuff.

This codec employs predicting pixel value by its 4 neighbours (up left, up, up right and left). Intraframes and interframes seem to be coded in same way, but intraframes may have first line(s) data coded explicitly as 8-bit values (or was that palette?).

Each frame is recursively partitioned, final partitions may be filled with some value, restored from predicted values or copied from somewhere else.

Partitioning is done in this way: decode mode, if its value is 2, do something on decoded rectangle, otherwise decode partitioning value and use it to split rectangle horizontally (when mode is zero) or vertically (mode=1) and perform the same operation on halves.

Do not expect decoder to be ready any time soon though. At least not if someone else makes it.

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