Lightness first, then a filter
A black-and-white conversion has exactly one decision in it: how light should each colour become once it has no colour. The default answer here is the lightness the colour already had to the eye, measured in OKLab. A red jacket and a green hedge that looked equally bright stay equally grey.
A filter changes that answer on purpose. Each preset is a weighted mix of the picture's linear red, green and blue light, named after the glass filter it imitates, and the result is carried back onto the same lightness scale. The weights are on screen as numbers, because a preset you cannot read is a preset you cannot reproduce.
| Filter | Blue sky | Skin | Foliage |
|---|---|---|---|
| Neutral | as seen | as seen | as seen |
| Red | much darker | lighter | darker |
| Yellow | a little darker | close to neutral | close to neutral |
| Green | darker | darker, more texture | lighter |
| Blue | lighter | darker | darker |
Where a conversion goes wrong
- Two colours of equal lightness become one grey. If a red sign vanishes into a green wall, a filter is the fix: it separates them by hue before the colour goes.
- A strong curve clips. Lifting shadows past +15 hundredths flattens them into a haze; the numbers are there so the change can be undone exactly.
- Forms that ask for a colour photograph will refuse a black-and-white one; the rail says so when the picture is in the usual exam-photo size range.
Identity photographs such as passport photos must be unaltered. Do not use a recolour on a photo you will submit as ID.
Questions people bring to this page
- Why not just average the red, green and blue values?
- Because the three channels are not equally bright to the eye. An average makes a saturated blue as light as a saturated yellow, and a sky that looked deep turns into a pale grey slab. The default here is OKLab lightness, the same perceptual measure the rest of the site selects colour in, so each colour keeps the lightness it appeared to have.
- What does the red filter actually do?
- It is a channel mix that counts red light heavily and blue light hardly at all, exactly what a red glass filter in front of black-and-white film did. Blue sky and green foliage therefore come out darker, and skin and lips come out lighter. The weights are printed beside the preset and can be edited; they are normalised so only their proportions matter.
- Is the saved file really greyscale?
- It can be. Once the black-and-white operation leaves every pixel with equal red, green and blue, the Save as list offers a greyscale PNG written by this page as colour type 0 — one byte a pixel instead of four — or colour type 4 when the picture has transparency. JPG and WebP stay available and simply store the grey as colour.
- Can I add a tone after converting?
- Yes. The sepia page and the colourise mode both keep each pixel's lightness and only move its colour, so a black-and-white conversion followed by a tone is the same arithmetic as toning a monochrome scan. On the general remapper the two are separate operations in one stack.
Next door
- Colour splashKeep the colour you tap; everything else turns black and white along the same soft band.
- Sepia and toningSepia, selenium, cyanotype, platinum or split toning, laid along each pixel's own lightness.
- Colourise a monochrome imageMap every visible pixel to one hue while preserving the original luminance structure.
- Black and white channel filtersWhat a red, yellow, green or blue filter does to sky, skin and foliage, as weights.
Image data is never posted anywhere: decode, mask, remap and encode all happen inside this tab's memory, and that memory is released when you leave.