Mettre une image en noir et blanc

The grayscale preset sets saturation to zero. Each pixel’s new brightness is a weighted mix of its red, green and blue values: about 21% red, 72% green and 7% blue, the Rec. 709 weights used for HD video and by the CSS saturate() filter. Green carries most of the weight because human eyes are most sensitive to it. A simple average of the three channels would make blue skies too bright and green foliage too dark; the weighted mix keeps them close to how bright they look in colour.

The result is grayscale, with 256 shades between black and white. It is not a two-tone image where every pixel is either pure black or pure white. If you need something close to that, for a signature or a stamp on a form, push the contrast slider towards 200% so the light greys go white and the dark ones go black.

Colour photos often look flat once the colour is gone, because a red jumper and a green hedge of the same brightness turn into the same grey. Raising contrast to 110–130% separates them again. Brightness helps with backlit portraits, where the face sits in shadow. Both sliders run from 0 to 200% with 100% as neutral, and the preview updates as you drag, side by side with the original.

The same panel has rotate, flip, straighten and crop, so a slightly tilted scan can be fixed in the same pass. If you want some colour back after all, the Original preset restores it, and Undo steps back through your changes.

A black and white JPG is still stored with three colour channels that happen to be equal, so do not expect it to be a third of the size. It does compress better, because the colour detail is gone: our 1200 × 800 test JPG re-saved at quality 92 came to 222,800 bytes in colour and 205,659 bytes in grayscale, about 8% smaller. PNG output is lossless and suits scans with text; JPG and WebP follow the quality slider. BMP input is saved as PNG.

EXIF data is not carried over to the new file. For printing that does not matter, and for sharing it removes the camera details and any GPS location.

Printing on a mono laser printer is the everyday case: converting first lets you check contrast on screen instead of discovering muddy greys on paper. Team pages and event programmes look more consistent when headshots taken in different light are all grayscale. Application portals sometimes ask for a black and white scan of an ID or certificate, and old family photos with faded colour casts often look better without the colour. Each of these is done in the browser tab, with no copy of the photo stored anywhere.