| Hi @jjcm and @seanwilson First of all thank you for these comments, I do proactively seek out comments like these as there are too few at the official APCA discussion forum https://github.com/Myndex/SAPC-APCA/discussions
That said: A px is not a device pixel, it is referenced to the canvas abstraction layer as the canonical CSS reference px. It represents 1.278 arc minutes of visual angle as subtended onto the retina. Or 0.0213° 𝑽𝜽 This is the case with a 96ppi monitor at 28" away. Visual Angle in arc minutes is what is used in research, and in fact what the Snellen eye chart is based around, 20/20 is based on a capital E that is 5 arc minutes high, where each line is 1 arcmin, and each space between the three horizontal lines is 1 arcmin, where we then find a spatial frequency of 30 cycles per degree. But that is for minimum acuity . Minimum acuity is NOT best readability, which needs to be ~2.5 or times larger than the acuity size (critical size). For a standard display a reference distance away, that means an x-height of 9.4px, which is 12 minutes of arc 𝑽𝜽. Critical size and critical contrast are recited and empirically tested for decades by eminent readability researchers Whittacker, Lovi-Kitchin, Ian Bailey, Legge, et alia. The font sizes for APCA are based on this, and assuming a reference font like Helvetica with a 0.52 x-height ratio. ALSO, if you want a "catch all" contrast value,Lc 75 is more appropriate IMO. NOT SET IN STONE YET This kind of discussion is good at the forum, so that it can be tracked and considered in the conversations about guidelines. ALSO If you want an easy way to use APCA and be fully backwards compatible with the old WCAG 2.x, then try BridgePCA at https://bridgepca.com |