One colour, three coordinate systems
A colour space is a choice of axes, not a different colour. This shows one colour written three ways at once: RGB, Rec. 601 luma, and HSV on OpenCV's own 8-bit scale, where hue runs 0–179 rather than 0–359.
Try this
Hold saturation and value still and sweep hue alone. Everything about the colour changes except how bright it is.
Where this lab comes from
Colour Spaces: Choosing Axes You Can ThresholdLesson 4 of The Image as Data. A colour space is a choice of axes for the same three numbers, and the choice is what makes a question easy or impossible. Worked on one measured chart patch: RGB (149.8, 58.3, 34.0) becomes luma 82.8, hue 12 degrees stored as 6, and a Lab triple — plus the five-line red detector that is brittle in RGB and robust in HSV.
More in the image itself
- One colour per photosite — Throw away two thirds of the colour, then guess it back.
- Hitting the colour a meter measured — Drag the gains and watch ΔE against 22 metered patches.
- Sixteen by twelve real pixels — Hover a pixel, read its index and its three numbers.