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Computer Science

Raster Image Storage

Higher Precision Extended Range

XRH File Format 4.0

Zebra Compression

If the Compression Type integer of a Channel Block is 0x5A425200040000 (Zebra), the Channel Block Data is a Zebra Compression Stream, after which the end-of-channel-block marker follows the Zebra Compression Stream.


Figure 1:  Channel Block (image plane) of an XRH file, with Compression Type set to 0x5A425200040000, stores a Zebra Compression Stream as the Channel Block Data (yellow in this diagram).

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 32-bit numbers (4 bytes per sample), then there are 4 Byte Channels; if the samples are 64-bit numbers (8 bytes per sample), then there are 8 Byte Channels, etc.

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.


Zebra Compression Stream

The Zebra Compression Stream begins with a start-of-zebra marker (ASCII string SZB\0),

followed with a 64-bit Size integer that stores the total size (number of bytes) of the entire Zebra Compression Stream (including this Size integer and the start and end markers),

followed with a 64-bit Compression Type integer (0x5A425200040000),

followed with a 32-bit integer that stores the Image Width (must equal the File Header Image Width),

followed with a 32-bit integer that stores the Image Height (same as File Header Image Height),

followed with a 32-bit integer that stores the Sample Definition (must equal the Channel Block Sample Definition),

followed with a 32-bit IEEE floating point number that stores the bias that is added to make all numbers of the channel non-negative (zero if no bias is needed or if the samples will not be STOR_U_FLOAT24 encoded),

followed with 28 bytes that are reserved for future use and must store zero,

followed with Byte Channels, one byte channel per byte of Sample Width (which is defined in the lower 16 bits of Sample Definition), e.g., 4 Byte Channels for 32-bit samples, 8 Byte Channels for 64-bit samples, etc.,

followed with the end-of-zebra marker (ASCII string EZB\0) that marks the end of the Zebra stream.


Figure 2:  Zebra Compression Stream (shown yellow and light brown) as the Data of a Channel Block in an XRH 4.0 file. The Byte Channels are depicted light brown. The byte offset (number of bytes) from the beginning of the Zebra Compression Stream to the beginning of the Byte Channels is 64 bytes.

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.

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 (the last 4 bytes of the Zebra Compression Stream).

End of Zebra Stream Marker:  EZB\0


Sample Definition

Sample Definition is a 4-byte integer that stores the Sample Type in the high byte, Sample Padding in the next byte, and Sample Width in the lower two bytes.

Sample Type must be STOR_UINT, STOR_IEEE_FILT, or STOR_U_FLOAT24. See the Data Types page of this specification.

If Sample Type is STOR_UINT, then Sample Width must be multiples of 8, and Sample Padding must be either zero or the number of bits that causes sample stride to be multiples of 1, 4 or 8 bytes (stride is the number of bytes from the beginning of a sample to the beginning of the next sample).

If Sample Type is STOR_IEEE_FILT, then Sample Width must be 32 and Sample Padding must be zero.

If Sample Type is STOR_U_FLOAT24, then Sample Width must be 24 and Sample Padding must be 8.


Byte Channel

The data bytes of a Byte Channel are compressed using ZST (a lossless compressor, also called “Zstandard”).

Each Byte Channel consists of a start-of-byte-channel marker (ASCII string SBC\0),

followed with a 64-bit integer that specifies the size of the ZST code stream that stores the data bytes of the Byte Channel,

followed with that ZST code stream (the compressed data generated by ZST),

followed with an end-of-byte-channel marker (ASCII string EBC\0).


Figure 3:  Byte Channel.

The first four bytes of a Byte Channel store the value 0x53424300 (ASCII string SBC\0) which marks the start-of-byte-channel.

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.

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.


Default Value

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, one (1) is stored in Size of ZST Code Stream (see Figure 3 above) and the single-byte Default Value is stored in place of the ZST Code Stream (immediately after the Size of ZST Code Stream, without storing a ZST Code Stream), after which the end-of-byte-channel marker immediately follows the single-byte Default Value.


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XRH 4.0 File Format
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Introduction
Data Types
File Layout & Header
Comments
Custom Properties
Channel Names
Channels
Zebra Compression (this page)
SZMOD Compression
JP2K Compression
Color Transformations
Example Images