Lesson 03 · A segment's length - Build a JPEG Encoder and Decoder

A segment's length

Most markers introduce a segment whose size is given by a two-byte length field. Today you read that length, learning the one quirk that trips everyone up - the count includes the two length bytes themselves.

The goal

Read a segment's two-byte big-endian length and report how many payload bytes follow it.

Start here - the target

TO DO

Scenario: Reading a segment length field

Giventhe two length bytes 0x00, 0x11 right after a marker code

Whenthe segment length is read

Thenthe length field is 17 and the payload is 15 bytes (the length minus the two length bytes)

Andthe two length bytes are read most-significant byte first (big-endian), and the minimum length 0x00, 0x02 describes an empty payload of 0 bytes

Background

Right after most marker codes comes a two-byte length field, stored big-endian (most significant byte first), so 00 11 is 0x0011 = 17. The quirk that catches everyone: this length includes the two length bytes themselves. A field of 17 therefore describes a segment of 15 payload bytes plus the 2 bytes of the length field. Get this off by two and every later segment lands at the wrong offset.

This framing is what lets a decoder skip a segment it does not care about: read the length, jump length bytes past the start of the length field, and you are positioned at the next marker. Not every marker has a length - SOI and EOI do not, and neither do a few others you will handle next lesson - but every table and header segment you actually parse begins with this field.

Make it work

// length is a 2-byte big-endian value that COUNTS ITSELF:
// payload size = length - 2
func readLength(b []byte, pos int) (length int, payload int) {
  length = int(b[pos])<<8 | int(b[pos+1])
  payload = length - 2
}

CheckpointDONE

You can read a segment's length and know how many payload bytes it carries. Commit and stop here.