Grayscale & Black-and-White Converter
Four ways to remove colour, and they produce visibly different pictures.
Only used by the black & white method.
Below 100% leaves some of the original colour, for a desaturated look.
Original
Converted
What the grayscale & Black-and-White Converter does
Removing colour from an image means collapsing three numbers into one, and there is more than one defensible way to do it. The choice changes the picture noticeably, which is why this tool offers several rather than one “greyscale” button.
Luminosity weights the channels by how sensitive human vision is to each: roughly 21% red, 72% green, 7% blue. Our eyes are far more sensitive to green than to blue, so this produces the greyscale that looks like the original in brightness. It is the right default and the formula behind the WCAG contrast standard.
Average treats the channels equally. It is simpler and wrong perceptually: blues come out too light and greens too dark, so a blue sky and green grass end up closer in tone than your eye says they should be. Lightness takes the midpoint of the brightest and darkest channel, which flattens saturated colours and gives a softer, lower-contrast result.
The single-channel options discard two channels entirely, which is what film photographers approximate with coloured filters. The red channel darkens skies and lightens skin; the blue channel does the opposite and is usually noisy, because most camera sensors capture less blue information.
Pure black and white is a threshold rather than a conversion — every pixel becomes one or the other. It is the right choice for line art, signatures and scanned documents destined for OCR, and the wrong choice for photographs.
How to use it
- Load your image and leave the method on Luminosity to start.
- Compare the other methods — the difference is most obvious on skies and skin.
- For a scanned document or signature, switch to pure black & white and adjust the threshold until the text is solid without filling in.
- Pull the strength slider below 100% if you want a partly desaturated look rather than full greyscale.
A worked example
The same photo of a blue sky over green grass, converted four ways:
| Method | Sky tone | Grass tone | Verdict |
|---|---|---|---|
| Luminosity | Mid grey | Light grey | Matches how the scene looked |
| Average | Light grey | Mid grey | Sky and grass swap places — wrong |
| Red channel | Very dark | Mid grey | Dramatic sky, the classic red-filter look |
| Blue channel | Very light | Very dark | High contrast and noisy |
The red-channel row is why photographers carried filters. A blue sky contains almost no red, so reading only the red channel renders it nearly black and makes white clouds leap out. You get the same effect here for free, and you can compare it against the others in a second rather than developing a roll of film.
Frequently asked questions
Which greyscale method should I use?
Luminosity, unless you have a specific reason not to. It weights the channels the way human vision does, so the result reads as the same brightness as the original. The others are creative choices rather than better conversions.
Why does my greyscale image look flat compared to the colour version?
Because colour was doing some of the work of separating objects. Two areas of different hue but similar brightness become the same grey. Raise the contrast slider, or try a single-channel method that pushes those areas apart.
What threshold should I use for scanning a document?
Start at 128 and move it until the text is solid black with clean white between the strokes. Too low and letters break up; too high and they fill in and merge. Uneven lighting across the page is the usual reason no single threshold works.
Does converting to greyscale make the file smaller?
A little, but less than you would expect. The output here is still an RGB PNG with equal channel values, which compresses well but is not a true single-channel image. For genuinely smaller files, run the result through the Image Compressor.