Find the right gamma

gamma is the per-printer darkness-calibration knob. Different thermal heads, different tape stocks, different ambient temperatures — they all shift how dark a given bitmap actually prints. gamma lets you compensate without re-rendering source artwork.

This recipe is the empirical procedure for picking a value.

What gamma does

gamma is a pow-curve applied to luminance after autoLevels and before threshold/dither:

• gamma < 1 brightens midtones — use when the printer comes out too dark.
• gamma > 1 darkens midtones — use when the printer comes out too light.
• gamma === 1 is a no-op (the default).

Pure black input pixels stay black at every gamma; pure white stays white. Only the midtones shift.

Procedure

1. Generate a gamma sweep test card. Print the same source image at five gamma values — 0.7, 0.85, 1.0, 1.15, 1.3 — using a representative dither method ('floyd-steinberg' is fine).

   import { padBitmap, renderImage, stackBitmaps } from '@mbtech-nl/bitmap';
   
   const sweep = [0.7, 0.85, 1.0, 1.15, 1.3].map((g) =>
     padBitmap(
       renderImage(testRgba, { dither: 'floyd-steinberg', gamma: g }),
       { bottom: 8 },
     ),
   );
   const card = stackBitmaps(sweep, 'vertical');
   await printer.send(card);

2. Print on the actual stock. Don't calibrate against your screen, against a different printer, or against a different tape — gamma is per-printer-per-tape.

3. Pick by eye. Look for the panel where:

   • Solid blacks are dense but not bleeding.
   • Light greys (skies, faces, gradient highlights) are visible but not muddy.
   • You can still read fine text and thin lines.

4. Bake the chosen value into your driver's defaults. Don't make end-users tune this — they shouldn't have to.

   const PRINTER_GAMMA = 0.85;
   renderImage(rgba, { dither: 'floyd-steinberg', gamma: PRINTER_GAMMA });

Per-tape adjustments

Some printers (Brother QL with DK-22251, two-colour DYMO) print red/blue foils with a different effective darkness from black. For those, calibrate gamma per plane via renderMultiPlaneImage's planes[name].gamma override:

renderMultiPlaneImage(image, {
  palette: [
    { name: 'black', rgb: [0, 0, 0] },
    { name: 'red', rgb: [204, 0, 0] },
  ],
  defaults: { dither: 'floyd-steinberg' },
  planes: {
    red: { gamma: 1.15 },     // red foil burns lighter on this printer
  },
});

When to use autoLevels instead

If you're calibrating to compensate for low-contrast input (faded scans, washed-out photos), you want autoLevels, not gamma. autoLevels rescales the input range; gamma reshapes midtones. They compose — and autoLevels runs first in the pipeline, so a typical "auto-enhance" config sets both:

renderImage(rgba, {
  dither: 'floyd-steinberg',
  autoLevels: true,
  gamma: PRINTER_GAMMA,
});