Image Filters

Converting to grayscale collapses three colour channels into one brightness value, and how that collapse is weighted decides whether the result looks right.

The human eye is far more sensitive to green than to red, and least sensitive to blue. A naive average of the three channels therefore renders greens too dark and blues too light, which is why averaged conversions look muddy. Perceptual weighting — green counting for roughly twice red and far more than blue — matches how brightness is actually perceived.

The practical consequence shows up in images where colours carry meaning. A red chart line and a green one at similar saturation can converge to nearly the same grey, making a legible colour chart unreadable in grayscale. Check the result rather than assuming information survived.

One consequence is worth knowing before you use the result. Processing happens by drawing the image onto a canvas and encoding what was drawn, and a canvas holds pixels only. Every piece of metadata is therefore dropped: EXIF, the camera model, the timestamp, the colour profile, and any embedded GPS coordinates. That is excellent if you are stripping location data before posting a photo, and a real loss if you were relying on capture dates to keep an archive in order. Keep the original.