Redness (a* proxy)
Mean CIE a* (red–green axis) over the cheeks and nose relative to the forehead, from uncalibrated sRGB.
How to read it
Uncalibrated sRGB from a phone is not colorimetry. Melanin and haemoglobin absorb in overlapping bands, so at deeper skin tones a* is melanin-dominated and this value is not an erythema measure — the app says so when it applies.
Why every value has an error bar
Every value Canthion reports carries a 1σ uncertainty propagated from MediaPipe Face Mesh's published accuracy (3.96% of inter-ocular distance mean absolute error per landmark, Kartynnik 2019). For a typical 300 px inter-pupil photo that is ≈12 px per landmark, which is why small ratios and angles get wide intervals. A number without an interval is a number pretending to be more precise than it is.
Related: how bone-level measurements are taken from a photo · methodology · all insights