Lesson 37 · Choosing the mask - Build a QR Code Encoder

Choosing the mask

With eight masks and four penalty rules, the encoder tries every mask, scores the result, and keeps the lowest. Today you run the selection for HELLO WORLD and confirm which mask wins.

The goal

Score all eight masks and select the one with the lowest total penalty.

Start here - the target

TO DO

Scenario: The lowest-penalty mask is chosen

Giventhe HELLO WORLD data grid, where each candidate is the grid with one mask applied and that mask's format information placed

Whenall eight masks are scored by summing the four penalty rules

Thenthe totals for masks 0 through 7 are 347, 470, 506, 441, 539, 516, 314, 558

Andthe lowest is mask 6 at 314, so mask 6 is selected for the final symbol

Background

Now the four rules earn their keep. The encoder does not guess a mask - it tries all eight. For each mask it applies the flip to the data region, places that mask’s format information (the score is computed on the complete grid, format bits included), sums the four penalty rules into a single total, and remembers the lowest. Ties break toward the lower mask number.

For HELLO WORLD at level Q the eight totals come out 347, 470, 506, 441, 539, 516, 314, 558, and mask 6 wins clearly at 314. That is the mask a compliant encoder selects for this input, and the format information you place in the final symbol is the level-Q, mask-6 value 010111011011010 from the last lesson. Everything is now decided: the codewords, the layout, the mask, and the format bits. One lesson remains - assemble it all and prove the result is a real, scannable QR code.

Make it work

best, bestScore := 0, math.MaxInt
for m := 0; m < 8; m++ {
  g2 := applyMask(grid, m)
  placeFormat(g2, level, m)
  s := rule1(g2) + rule2(g2) + rule3(g2) + rule4(g2)
  if s < bestScore { best, bestScore = m, s }
}

CheckpointDONE

The best mask is chosen by penalty score. Commit and stop here.