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 after the marker (0xFFDA) a two byte number follows which defines how long this segment is -- for example here -- as with most variable size markers. But I seem not to understand it correctly in this case. It works for every other marker type.

I checked multiple files but just can't get this right. Is this number not defining how long the complete SOS field is? So for a baseline JPEG there should be exaclty one SOS segment and after this should the End Of Image marker follow. If it is progressive there can be multiple SOS segments but still all should have a length field.

I have one picture with a SOF0 Marker so it should be baseline. I believe that this is the correct SOFn marker because the image resolution can be found after that marker. With a hex editor I have found 3 0xFFDA marker and all of them have 0x000C in the following 2 bytes. So that segment, as I understand it, should always be 12 Byte long. But in all 3 cases no new marker is following after 12 byte of data. I guess the very last one is the scan I am looking for because if the value 0xFF comes up it is followed by 0x00 -- except the reset markers.

Are those two byte following 0xFFDA not the length fields?

EDIT: So thanks to the comments and answer there seems to be no length field for the actual compressed data and only way to know where it ends seems to be decoding it.

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?

But there is one more thing. According to DRI Marker (Define Restart Interval) it contains the value after which a Scan should have a restart marker 0xFFD0 - 0xFFD7. But I seem to misunderstand that too or I'm not doing it right. For example a marker contained the value 0x0140 as restart interval. In the following Scan I started from the beginning and searched for the first 0xFFD0 but it came after 862 bytes instead of 320.

Answer 1

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.