PhotoDithering

README.md

@@ -184,6 +184,26 @@ refuses a monochrome job on two-colour tape, saying *Black/Red on White paper is
 Change it to Monochrome media.* The roll setting controls how the job is built, not just its
 colour.
 
+### Photos on the badge
+
+A thermal printer has one bit per dot: every pixel is either burnt or not. A photo therefore has
+to be reduced to pure black and white, and how that is done makes the difference between a
+recognisable face and a few solid blobs.
+
+Under **Sites → Edit → Badge printing**:
+
+| Setting | What it does |
+|---|---|
+| Error diffusion | Scatters the rounding error into neighbouring dots, so mid tones survive as a pattern. The default, and much the best for faces. |
+| Hard threshold | Every pixel darker than the cut becomes solid black. Crisp for line art, ruinous for photographs. |
+| Leave it to the driver | Sends greyscale and lets `brother_ql` decide. |
+| Threshold | Where the cut falls. Higher is darker. |
+| Contrast | Applied before the reduction. Webcam photos are flat and flatten further at one bit, so a lift of 20 to 30 usually helps. |
+
+**Preview the photo settings** renders the most recent real visitor photo at several settings side
+by side, so the choice is made by eye. The halftoning happens in the server's renderer, not in the
+printer driver, so the preview and the printed label are the same image.
+
 **Tell it which roll is loaded.** *Roll loaded in the printer* under **Sites → Edit → Printer**
 must match what is physically in the machine. A two-colour job sent to a plain roll is refused
 outright: the printer shows **Wrong Roll Type** and nothing comes out. The setting is separate