APCA & SAPC

Frequently Asked Questions for the Accessible Perceptual Contrast Algorithm.

–THIS IS A WORK IN PROGRESS–

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APCA FAQ post #001

Q: What is APCA?

A: It’s the Accessible Perceptual Contrast Algorithm, a mini-appearance model, and a way to predict the perceived contrast for best readability. It is tuned specifically for readability of text on self-illuminated displays.

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APCA FAQ post #002

Q: How do you use it?

A: Simply send it the text color and the background color, and APCA will calculate a contrast value measured as Lc (perceptual lightness contrast). For more complete design guidance, there is a lookup table for font size and weight.

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APCA FAQ post #003

Q: What is the range of values?

A: For beta version 0.98G-4g, the range is -108 to +106, with positive numbers for dark text on a light background, and negative numbers for light text on a dark background.

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APCA FAQ post #004

Q: Why is there a lookup table?

A: Visual contrast for readability is affected not only by a pair of colors, but also spatial frequency, in other words the size and weight of the fonts used for the text. The APCA font lookup is part of WCAG 3 conformance for web content. The lookup table can be revised for different languages and character sets.

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WIP