TIFF Tag JPEGInterchangeFormat

最新推荐文章于 2021-02-25 11:44:59 发布

LongZh_CN

最新推荐文章于 2021-02-25 11:44:59 发布

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本文详细介绍了TIFF文件格式中与JPEG压缩相关的两个字段：JPEGInterchangeFormat及JPEGInterchangeFormatLength。这两个字段主要用于指示TIFF文件内是否包含JPEG格式的数据流及其长度。这些字段属于旧式的JPEG压缩方案，目前不推荐使用。

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TIFF Tag JPEGInterchangeFormat

IFD		Image
Code		513 (hex 0x0201)
Name		JPEGInterchangeFormat
LibTiff name		TIFFTAG_JPEGIFOFFSET
Type		LONG
Count		1
Default		None

Description

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 originally intended to indicate whether a JPEG interchange format bitstream is present in the TIFF file. If a JPEG interchange format bitstream is present, then this field should point to the Start of Image (SOI) marker code.

If this field is zero or not present, a JPEG interchange format bitstream is not present.

In practice, many different uses of the JPEGInterchangeFormat and JPEGInterchangeFormatLength tags are observed in old-style JPEG compressed TIFFs.

The field can be absent. In this case, often the JPEGQTables, JPEGDCTables, and JPEGACTables tags are included, and these tags should be consulted to find the table data.

The field can be present. If so, JPEGInterchangeFormatLength can be used to indicate the length of the embedded JPEG stream, but the Length tag can also be absent. The JPEGQTables, JPEGDCTables, and JPEGACTables tags can be present, but they can point to incorrect positions or even positions beyond EOF, so reading this data from the JPEG stream pointed to by JPEGInterchangeFormat, when present, seems safest.

The JPEG stream pointed to by the JPEGInterchangeFormat tag and possibly limited in length by the JPEGInterchangeFortmatLength tag, can be a complete and valid JPEG stream. But it can also be only the start of such a stream, up to but not including the SOS marker. If that is the case, most often an SOS marker is included right at the beginning of the first strip's JPEG compressed data.