Lesson 33 · Penalty rule 2 - blocks - Build a QR Code Encoder

Penalty rule 2 - blocks

The second penalty rule punishes solid 2x2 blocks of one color, which make dense clumps a scanner can misread. Today you write it.

The goal

Score a grid by penalizing every 2x2 block of a single color.

Start here - the target

TO DO

Scenario: Solid 2x2 blocks are penalized

Givenpenalty rule 2: every 2x2 area whose four modules are the same color scores 3, counting overlapping placements

Whena single 2x2 all-dark block is scored

Thenit contributes 3

Anda 3x3 all-dark area contributes 12 - it contains four overlapping 2x2 blocks, each worth 3

Background

Rule 2 discourages solid rectangular clumps. Every 2x2 area in which all four modules share a color scores 3, and the placements overlap - you slide the 2x2 window one module at a time, so a larger solid region contributes many overlapping blocks. A 3x3 solid area, for instance, contains four 2x2 sub-blocks and scores 4 * 3 = 12.

The implementation is a double loop over every module that could be a 2x2 top-left corner (all but the last row and column), checking whether it and its three neighbours match. This rule, together with rule 1, is why a good mask spreads dark and light into a fine texture rather than large fields. Two rules down, two to go: next is the one that protects the finder pattern’s signature.

Make it work

// Every top-left corner of a same-colored 2x2 adds 3.
func rule2(g [][]int8) int {
  s := 0
  for r := 0; r < len(g)-1; r++ {
    for c := 0; c < len(g[0])-1; c++ {
      v := g[r][c]
      if g[r][c+1] == v && g[r+1][c] == v && g[r+1][c+1] == v {
        s += 3
      }
    }
  }
  return s
}

CheckpointDONE

You can score a grid on 2x2 blocks. Commit and stop here.