Parsing JPEG SOS Marker

Question

There is a question with the same title but unfortunatly it doesn\'t help me.

I am trying to parse the data of a SOS marker. All documentations I can find say that afte

Answer 1

Summary of how to find next marker after SOS marker (0xFFDA):

1. Skip first 3 bytes after SOS marker (2 bytes header size + 1 byte number of image components in scan).
2. Search for next FFxx marker (skip every FF00 and range from FFD0 to FFD7 because they are part of scan).

*This is summary of comments below post of user3344003 + my knowledge + Table B.1 from https://www.w3.org/Graphics/JPEG/itu-t81.pdf.

*Basing on Table B.1 I can also suspect that values FF01 and FF02 through FFBF should also be skipped in point 2 but I am not sure if they cannot appear as part of encoded SOS data.

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Additional question above:

Why does a Baseline DCT Image have multiple scans? I would understand why it has two; the main image and a thumbnail, but what is the third scan?

If image stream contains APP2 marker (0xFFE2), that tells us it can be (but not have to be) Multi Picture JPEG (tag named MPF), we can have more than 3 images. In general APP markers can store anything, there are a lot of standards related to APP segments in JPEG files.

First table tells us about "things" that can be stored in each APP segment: https://sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html

Answer 2

The SOS marker contains the compressed data; the most complex part of the JPEG stream. The SOFn marker indicates the format of the data. SOF0 and SOF1 are processed identically. SOF2 (progressive) is quite a bit different. (The read of the SOFn markers are not in common use or commonly supported).

The length is that of the SOS header, not the compressed data. Most of the header is only applicable to progressive scans (SOF2).

The compressed data follows the header. The compressed data has no length. You have to scan through the data to find the next marker.