|Extension TIFF Tag JPEGQTables|
TIFF Tag Reference
TIFF Tag JPEGQTables
Old-style JPEG compression field. TechNote2 invalidates this part of the specification.
No new TIFF writer code should ever attempt to use this tag. It is part of an invalidated compression scheme, old-style JPEG, that was always unclear to begin with, and next enjoyed many mutualy exclusive implementations. The following description is for TIFF reading purposes only.
This field was orginally intended to point to a list of offsets to the quantization tables, one per component. Each table consists of 64 BYTES (one for each DCT coefficient in the 8x8 block). The quantization tables are stored in zigzag order, and are compatible with the quantization tables usually found in a JPEG stream DQT marker.
The original specification strongly recommended that, within the TIFF file, each component be assigned separate tables, and labelled this field as mandatory whenever the JPEGProc field specifies a DCT-based process.
We've seen old-style JPEG in TIFF files where some or all Table offsets, contained the JPEGQTables, JPEGDCTables, and JPEGACTables tags are incorrect values beyond EOF. However, these files do always seem to contain a useful JPEGInterchangeFormat tag. Therefore, we recommend a carefull attempt to read the Tables tags only as a last resort, if no table data is found in a JPEGInterchangeFormat stream.