Map a photograph onto two colours

A duotone starts by measuring brightness — the standard weighted mix of red, green and blue that matches what the eye counts as light — and then forgets everything else about the pixel. What it does with that single number is the whole effect: zero becomes your shadow ink, 255 becomes your highlight ink, and every value between lands on the straight line joining the two.

Because there are only 256 possible answers, the lab builds the answer for all of them once, as a table, and then does nothing per pixel but look up an index. That is why this is the one effect on the site that stays live under a dragging colour picker even on a large photograph. It is also why it is the safest thing to reach for when a picture has to work at a small size: nothing here depends on having pixels to spare.

Shadow ink

Where black goes. It sets the darkest thing the finished picture is capable of, so a shadow ink of mid-navy produces a duotone with no true black in it — which is often exactly the intended effect, and occasionally an unwelcome surprise.

Highlight ink

Where white goes. Choosing something well short of white is what gives a duotone its printed, slightly dirty quality; choosing white keeps the picture open and clean.

Midpoint, 5 to 95 percent

Which input brightness arrives halfway along the ramp. Applied as a curve, so it lifts or sinks the picture as a whole rather than putting a hinge in the middle of it.

The six pairs in the control panel are starting points rather than presets to be used as they are. Each one loads two colours into the pickers and leaves you to move them; there is nothing behind them that you could not have typed yourself.

Two rules get you most of the way. Keep a real distance in brightness between the two inks — a duotone made from two colours of similar weight has nowhere to put its midtones and reads as a flat wash whatever you do to the midpoint. And keep the shadow ink the darker of the two, unless you deliberately want the picture inverted, in which case swapping them is the whole trick and it works exactly as you would expect.

Distance in hue matters far less than distance in brightness. A deep green paired with a pale green is a perfectly good duotone; a mid-blue paired with a mid-orange is usually a mess, because the two ends are equally bright and the picture loses the structure that told you what was in front of what.

A duotone throws away hue and keeps structure. That makes it the gentlest effect in this lab, and gentleness is sometimes the problem: it is a colour decision applied to an otherwise intact photograph, and the photograph still looks like a photograph afterwards. If what you wanted was for the picture to visibly become something else, the dither and the halftone next door start from the same brightness measurement and then go much further with it.

One genuine combination worth knowing: a duotone followed by nothing at all is flat, but the dither on this site builds its palette from whatever colours are actually in the picture — so a picture that has been reduced to two inks first dithers into a pattern made from those two inks, which is how a screened two-colour print is put together.

Is this the same as tinting a black-and-white photograph?

It is one step past it. A tint pushes the whole picture towards a colour and keeps its own light and dark ends; a duotone replaces both ends. The blacks become your shadow ink and the whites become your highlight ink, and every tone between them lands somewhere on the straight line joining the two. That is why a duotone with a pale blue shadow and a cream highlight has no true black in it anywhere — there is nowhere for one to come from.

What does the midpoint actually move?

Which brightness in the original photograph arrives exactly halfway along the ramp. It is applied as a curve rather than as a break, so moving it lifts or sinks the whole picture smoothly instead of kinking it at one value. Drop it towards 20 percent and the image opens up, because more of the original tonal range is spent on the highlight end. Push it towards 80 and everything but the brightest areas sinks into the shadow ink.

Does a duotone keep a transparent background?

Yes. This is the one effect here that does. Duotone reads brightness and writes colour without ever needing to decide what an empty pixel means, so the alpha channel is carried across untouched and a cut-out stays a cut-out. Save it as PNG and the transparency survives; save it as JPG and the format itself drops the channel, because JPG has no alpha at all.

Why is this the only effect that stays smooth while I drag a colour?

Because it is cheap in a way the others cannot be. The whole operation is a 256-entry lookup table, rebuilt whenever a colour or the midpoint moves, plus one pass that reads a brightness and writes three bytes. There is no neighbourhood to consider, no error to carry, no grid to rotate. A halftone at the same resolution is a box blur plus a rotation plus a shape test per pixel, and that is the difference you feel under your hand.

• DitherError diffusion and ordered matrices, at palettes from one bit upwards.
• HalftoneDots on a rotated screen, one plate or four, with the angles a press would use.
• The effects labAll six effects, chosen from your own photograph rather than from a list of names.
• Effects, and why they are not filtersThe one test that sorts a colour grade from a rebuild, applied to fifteen awkward cases.

Rendered on your computer, not on a server.