CcittFaxFilter

CCITTFaxDecode — ITU-T T.4 (Group 3) and T.6 (Group 4) facsimile encoding, used as PDF stream filter /CCITTFaxDecode (ISO 32000-1 §7.4.7). The dominant encoding for monochrome scanned PDFs.

Mode is selected by /K in DecodeParms:

• K < 0 → Pure 2D ("Group 4", T.6) — the modern default

• K = 0 → Pure 1D ("Group 3 1D", T.4) — common in older scans

• K > 0 → Mixed 1D/2D — not yet implemented; falls back to 1D

Output is 1 bit per pixel, packed MSB-first, padded to a byte boundary per row. The bit polarity matches the spec default: 0 = black/foreground, 1 = white/background — unless /BlackIs1 is true.