SOS marker.
The line in the file where the bookkeeping ends and the photograph begins. Everything after it is copied through untouched, which is the whole basis of the pixel claim.
SOS (Start of Scan, marker 0xFFDA) introduces the entropy-coded image data in a JPEG. From the end of the SOS header to the end of the file is the compressed picture itself.
What it is
A JPEG's segments describe how to decode what follows: quantisation tables, Huffman tables, frame dimensions, and then SOS, which names the components in the scan. After that header the file stops being a sequence of segments and becomes a stream of entropy-coded bits.
Why it matters
The SOS offset is the boundary that makes "removes metadata without touching the picture" a checkable statement rather than a slogan. Everything before it is structure and metadata; everything from it onward is the image.
A scrubber that decodes and re-encodes must produce different bytes, and usually loses a little quality doing it. A scrubber that copies from SOS to end of file cannot alter a pixel, because it never looks at one.
tests/verify_scrub.py checks byte-identity of that region — and finds the SOS offset with its own independent segment scanner, because using the tool under test to locate the bytes that vindicate it would prove nothing.
Related terms
Common questions.
Can I check this myself without running any scripts?
Yes. Find FF DA in each file with a hex viewer you trust, take everything from that offset to the end, and compare the two hashes. That is the check that matters most and it needs nothing but your own tools.
Is there ever more than one SOS?
In progressive JPEGs, yes — a progressive file has several scans. Everything from the first SOS onward is still image data, and it is still copied verbatim.
Get ScrubPony
Strips identifying metadata out of JPEGs without touching a pixel. C99, no dependencies beyond libc, macOS and Linux, MIT-licensed.