Black and White Image Converter

Grayscale is a choice of weights

A colour pixel has three values — red, green and blue — and a grey pixel has one. Turning one into the other means deciding how much each colour contributes to brightness, and there is no single right answer. Human vision is far more sensitive to green than to red, and least sensitive to blue, so a fair conversion weights green most heavily. How heavily is where the standards differ.

Rec. 709 luma uses 0.2126 R + 0.7152 G + 0.0722 B. These weights come from the ITU-R BT.709 standard for HD television, whose colour primaries are the same as sRGB's — so for an ordinary photo on an ordinary screen this is the most defensible default, and it is the one selected here.

Rec. 601 luma uses 0.299 R + 0.587 G + 0.114 B, from the older BT.601 standard for standard-definition television. It is still what JPEG uses internally to separate brightness from colour, and what many image libraries call "grayscale". Compared with Rec. 709 it renders reds noticeably lighter and greens darker.

Average simply takes (R + G + B) ÷ 3, giving every colour the same weight. Blues come out much lighter than they look and greens much darker. Desaturate takes the average of the largest and smallest channel — the L of HSL — which renders every fully saturated colour, red, green or blue alike, as the same mid-grey.

Linear-light luminance applies the Rec. 709 weights after undoing the sRGB gamma curve, then re-encodes the result. It is the physically correct relative luminance, and it renders saturated colours lighter than the luma methods do.

The same red, five different greys

Pure red (#FF0000) converts to these grey levels out of 255: Rec. 709, 54; Rec. 601, 76; average, 85; linear luminance, 127; desaturate, 128. Pure green gives 182, 150, 85, 220 and 128. Pure blue gives 18, 29, 85, 76 and 128.

Those differences are visible in real photos. A red flower against green leaves, converted by Rec. 709, becomes a dark flower against light leaves; by average, the two come out almost the same grey and the flower disappears. A blue sky is nearly black under Rec. 709 and pale under average. None of these is a mistake — they are different interpretations — but it is worth trying two or three, because the one that keeps the subject distinct from its background is the one that works.

Black-and-white film photographers did the same job with coloured filters: a red filter darkens blue skies dramatically, a green filter lightens foliage. Choosing a conversion method is the digital equivalent, applied after the fact.

Pure black and white, for documents and stencils

Grayscale keeps 256 shades. Sometimes you want two: a scanned page for a fax-style PDF, a signature to place on a document, line art to colour in, a stencil to cut, or an image for a laser engraver or a vinyl cutter. Tick Pure black and white and every pixel becomes either black or white depending on whether its grey level falls below or above the threshold.

The threshold matters. Too low and faint pencil marks and light text vanish; too high and the paper's texture and shadows turn into black speckle. Auto (Otsu) chooses it for you using Otsu's method, published in 1979: it looks at the histogram of grey levels and picks the cut that best separates it into two groups, which on a document is ink and paper. It usually lands in the right place for a clean scan; adjust by a few levels from there if needed.

For a photographed page rather than a scanned one, uneven lighting is the enemy — a single threshold cannot suit a page that is bright at one end and shadowed at the other. Photograph in even light, or raise the brightness first. And for a scan you intend to send as a PDF, save as PNG: two-tone images compress extremely well losslessly, while JPEG adds grey fringes around every letter.

Sepia, contrast and finishing

Sepia toning was a darkroom process that converted the silver in a print to silver sulphide, giving the familiar warm brown and making the print far more resistant to fading — which is partly why so many surviving old photographs are brown. The name comes from the ink of the cuttlefish, Sepia, an earlier brown pigment. The sepia here uses the standard colour matrix behind the CSS sepia() filter; blend it at 20–40% for a subtle antique warmth or 100% for the full effect.

Most colour photos look flat when converted, because colour contrast — red against green — disappears when both become the same grey. A contrast boost of 10 to 30 usually restores the punch, and a light vignette helps hold the eye. The Noir look combines high contrast with a vignette for a dramatic result; open More adjustments for every other control in the editor.

Choosing JPEG keeps a black-and-white photo small. The image is still stored with three channels, so the file is not three times smaller than the colour version, but the colour information compresses to almost nothing.