Black and White Image Converter

About the Black and White Image Converter

Convert colour photos to black and white with the formula you choose — BT.709 or BT.601 luma, the channel average, HSL lightness, or true luminance computed in linear light — or with a channel mixer whose presets imitate the yellow, orange, red, green and blue filters photographers used with black-and-white film. Adjust brightness and contrast, invert, and tone the result like a sepia, cyanotype or selenium print.

For thermal receipt printers, e-ink displays, laser engraving, stencils and fax-style scans, switch to 1-bit output: a fixed or automatic (Otsu) threshold, Floyd–Steinberg or Atkinson error diffusion, or Bayer ordered dithering. Resize to your printer’s width first, compare at 100% or 200%, and save as PNG or a 1-bit BMP. Several photos can be converted with the same settings into one ZIP. Everything runs in your browser.

The greyscale formulas

• BT.709 luma (ITU-R BT.709-6): Y′ = 0.2126 R′ + 0.7152 G′ + 0.0722 B′ — the weights of HD video and the sRGB colour space.
• BT.601 luma (ITU-R BT.601-7): Y′ = 0.299 R′ + 0.587 G′ + 0.114 B′ — standard-definition video and JPEG files.
• Luminance (linear): the BT.709 weights applied to linear light (sRGB decoded with the IEC 61966-2-1 curve), then re-encoded — the physically accurate brightness.
• Average: (R + G + B) / 3. Lightness: (max + min) / 2 (the L of HSL).
• Channel mixer: your own weights in percent. Totals above 100% brighten, below 100% darken, and negative weights are allowed for strong effects.

The film-filter presets are approximations: a real glass filter’s effect depends on the film and the filter. A coloured filter lightens its own colour and darkens its opposite — red darkens blue skies, green lightens foliage.

Threshold, error diffusion and ordered dithering

• Threshold: every pixel at least as bright as the level becomes white. Auto uses Otsu’s method (Otsu 1979), which picks the level that best separates the dark and light pixels. Best for text, logos, signatures and stencils.
• Floyd–Steinberg (Floyd & Steinberg, Proc. SID 17(2), 1976): each pixel’s rounding error is passed on to its neighbours with weights 7/16, 3/16, 5/16 and 1/16, so the dot density follows the brightness. Best for photos.
• Atkinson: passes 6 × 1/8 of the error to six neighbours (¾ in all), which gives crisper, higher-contrast results with clean highlights.
• Bayer (Bayer 1973): compares each pixel with a repeating 2 × 2, 4 × 4 or 8 × 8 threshold matrix — a regular cross-hatch pattern that engraves and prints predictably.

Serpentine scanning alternates the direction on each row, which reduces the diagonal "worms" of error diffusion. Linear light dithering keeps the average brightness of the dots equal to the photo’s on screens and e-ink; the classic method (off) makes midtones look lighter on screens but often prints well, because printed dots spread.

Tips for printers, e-ink and engraving

• Resize before dithering, to the exact width the device prints or shows — scaling a dithered image afterwards ruins the dot pattern.
• Save 1-bit images as PNG or BMP (1-bit): JPEG and WebP add grey pixels around every dot.
• For engraving on dark materials (slate, anodised aluminium), use Invert so the laser marks the light parts.
• Raise the contrast a little before dithering photos for thermal paper; small receipt printers lose midtones easily.

Limitations

• Toning gives the look of sepia, cyanotype and selenium prints with fixed colour gradients; it does not simulate a chemical process.
• Film-filter presets are channel-mixer approximations, not measurements of particular filters or films.
• BMP output is 1-bit only (for 1-bit images). Greyscale images are saved as JPEG, PNG or WebP.
• Photos up to 50 megapixels. Enlarging with the width setting stops at 16,384 px per side and 50 megapixels, and adds no detail. EXIF metadata is not copied to the saved file.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.