Lesson 26 · Timing patterns - Build a QR Code Encoder

Timing patterns

Two dotted lines - one across, one down - run between the finders so a scanner can count off module positions. Today you draw the timing patterns.

The goal

Draw the alternating timing patterns along row 6 and column 6.

Start here - the target

TO DO

Scenario: Alternating timing lines connect the finders

Givenrow 6 and column 6 between the finder regions (positions 8 through 12 in Version 1)

Whenthe timing modules are set, dark where the coordinate is even and light where it is odd

Thenrow 6 columns 8 through 12 are dark, light, dark, light, dark (1, 0, 1, 0, 1)

Andcolumn 6 rows 8 through 12 are the same pattern 1, 0, 1, 0, 1, so the lines start and end on a dark module

Background

Between the finder patterns run two timing patterns: a horizontal line along row 6 and a vertical line down column 6, each an alternating dark-light-dark sequence. They act like a ruler - a scanner counts the alternations to work out exactly where each module column and row falls, which keeps it aligned even if the printed symbol is slightly distorted.

The rule is simply dark on even coordinates, light on odd. In Version 1 the free stretch runs from position 8 to 12 (the finders and separators own 0-7, and the far finder owns 13 onward), giving 1, 0, 1, 0, 1 - starting and ending dark, connecting the dark borders of the finders they run between. These are the last of the always-fixed patterns whose values never change. One special single module and the reserved format strip remain before data can flow.

Make it work

// Row 6 and column 6, only the cells between the separators.
for i := 8; i <= 12; i++ {
  g.set(6, i, dark(i%2 == 0)) // dark on even coordinate
  g.set(i, 6, dark(i%2 == 0))
}

CheckpointDONE

The timing patterns connect the finders. Commit and stop here.