If the Compression Type integer of a Channel Block is 0x5A425200020000 (Zebra), a Zebra Compression Stream immediately follows the Compression Type integer, after which the end-of-channel-block marker follows the Zebra Compression Stream.
Zebra is a simple lossless compression scheme for compressing floating point or integer numbers. It consists of splitting up number samples into bytes, and storing the corresponding bytes of the different samples together: the most significant byte of all the samples are stored together, the next most significant byte of the samples are stored together, etc.
Each such group of bytes is called a Byte Channel. If the samples are
Byte Channels are stored in Big Endian format: most significant Byte Channel first, next most significant Byte Channel next, etc.; the least significant byte of all the image channel samples are the last Byte Channel, the second least significant byte is the second-to-last Byte Channel, etc.
The Zebra Compression Stream begins with a
Note: The Zebra Compression Stream is self-contained and may be used in any file format (not just in an XRH file).
The first four bytes of a Zebra Compression Stream store the value 0x535A4200 (ASCII string SZB\0) which marks the
Start of Zebra Stream Marker: SZB\0
The last 4 bytes of a Zebra Compression Stream store the value 0x455A4200 (ASCII string EZB\0) which marks the
End of Zebra Stream Marker: EZB\0
The data bytes of a Byte Channel are compressed using ZST o (a lossless compressor, also called Zstandard).
Each Byte Channel consists of a
The first four bytes of a Byte Channel store the value 0x53424300 (ASCII string SBC\0) which marks the
Start of Byte Channel Marker: SBC\0
The last 4 bytes of a Byte Channel store the value 0x45424300 (ASCII string EBC\0) which marks the
End of Byte Channel Marker: EBC\0
The ZST code stream of each Byte Channel is a compressed byte stream that when uncompressed provides the sample bytes of the Byte Channel in raster order.
For example, to reconstruct the 5th sample of a Channel Block: the 5th data byte of the first Byte Channel becomes the high order byte of the 5th sample of the Channel Block, the 5th data byte of the second Byte Channel becomes the next most significant byte of the 5th sample of the Channel Block, etc.
If all of the bytes of a Byte Channel are the same, then a Default Value (one byte) may be stored that specifies the value of all the bytes of the Byte Channel. In that case, zero is stored in Size of ZST Code Stream (see
If Filter Type equals one (1), the samples are filtered before being split up into Byte Channels, to map floating point numbers into integer sequences.
Otherwise, if Filter Type equals zero (0), there is no mapping of floating point numbers to integer sequences. That is useful if the samples are already integers, for example ancillary data.
For Filter Type equals one (1), each sample must be converted (mapped) from floating point to unsigned integer. To perform this mapping, first copy the floating point bit sequence of each sample directly into integer memory (bitwise bijection), then toggle the leading sign bit if positive, or toggle all bits if negative. The mapped samples (unsigned integers) are then split up into Byte Channels, and each Byte Channel is sent to the ZST encoder.
For data retrieval of Filter Type 1, after uncompressing, the Byte Channels are merged (shuffled) back into unsigned integers which are then inverse mapped back to the original floating point numbers.
For Filter Type equals zero (0), the samples are considered to be unsigned integers already and are split up into Byte Channels, and each Byte Channel is sent to the ZST encoder.
For data retrieval of Filter Type 0, after uncompressing, the Byte Channels are merged (shuffled) back into unsigned integers which are the retrieved samples.
Filter Type 0 is useful for samples that are already integers, or to apply your own custom filter.
ZST compression is used instead of ZLIB, because ZST allows for compression of larger buffers.
Filter Type 1 as described above is used because it creates a monotonic sequence of the floating point values as unsigned integers. This is described in Mapping to Integer, Section 3.2 in Peter Lindstrom & Martin Isenburg, Fast and Efficient Compression of Floating-Point Data, which uses a different type of compression but uses and explains this filter.