Fabric Value Sorter

Value is not the average of red, green and blue

Every quilting book says to sort by value, and everyone squints at the pile trying to do it by eye. The reason it is hard is that the eye is not equally sensitive to the three primaries: green carries about 72% of perceived brightness, red 21% and blue only 7%.

A pure yellow has 12.9 times the luminance of a pure blue. Averaging puts them a factor of two apart. It is not a rough approximation of value — it is a different quantity that happens to agree sometimes.

One pair in four comes out wrong

Swept over a grid of 216 colours — 23,220 pairs — the naive average puts 20% in the wrong order. Count the pairs it calls exactly equal when they are not, which is also a failure to rank them, and it gets 24.7% wrong. Roughly one pair in four.

And the worst case is not a near miss. Pure green and the blue-violet rgb(51, 0, 204) average to exactly the same number, so averaging calls them identical in value. Their real lightness is 87.7 against 26.9 — a gap of sixty L* units, most of the entire scale. In a black-and-white photograph one is nearly white and the other nearly black.

The two colours on the left average identically. The two on the right are what they actually look like in greyscale.

Which is exactly why the standard advice is to photograph the pile in black and white, or squint through a red value filter. Both do the weighting for you. This tool does the same arithmetic and shows you which two fabrics you are most likely to get backwards.

Why the bands are not at a third and two thirds

Luminance is how much light comes off the cloth; lightness is how light it looks, and the two are related by a cube root. Splitting light, medium and dark evenly on the perceptual scale puts the boundaries at 7.7% and 36.2% of the light — nowhere near a third and two thirds. It is the same reason a photographer's middle grey reflects about 18% of the light rather than 50%, and the same reason a fabric that seems "mid tone" is usually darker than you think.

Notes

• Sample the fabric, not the picture of it. Photograph your pile in flat even light and pick the colours off that. A studio shot on a retailer's site has been adjusted, and a fabric that reads medium in the shop can read dark at home.
• Value is relative. A fabric is only "dark" next to something lighter. The bands here are absolute so you can compare across projects, but what matters in a quilt is the spacing between the fabrics you actually chose.
• A quilt needs all three bands. The commonest reason a quilt reads flat despite lovely fabrics is that they are all mediums. The tool flags a missing band.
• Print and texture average out at a distance. A busy print reads as its average value from across the room, which is often lighter than the darkest thing in it. Squint at it — that is exactly what squinting does.