A way of visualizing YUV color-representation.

In the past I have linked to This WiKiPedia article, to explain what YUV color-encoding is. But I find that even though the article does a good job of explaining, their ‘UV Chroma Map’ is flawed. This would be the square image which they used, together with an assumed Luminance value of 0.5, just to give a basic impression of how the colors get mapped.

The reason I see the above visual as flawed, has to do with the fact that at any one value for Y’ – i.e. for any one value of Luminance – the full range of UV- chroma values is not in fact available, in a way that leads to full color saturation.

I think that any visual should highlight, that many Y’UV combinations do not lead to possible RGB values, for which reason even a full gamut of RGB colors will experience losses, when encoded into practical, integer-based YUV.

I would propose that a modified method be used to exhibit what the YUV encoding does, specifically the ‘UV’ part, when the Y part is allowed to vary, in an arbitrary way that adapts to the full range of UV-chroma values. And the visual which I obtain would be as follows:

yuv_plot_1-001

http://dirkmittler.homeip.net/YUV_Plot_1.html

I realize that the Math is somewhat fudged, with which I created the visual above, but also know the more-precise Math which gets used, for YUV or Y’UV color-encoding. Here is my worksheet on that:

http://dirkmittler.homeip.net/YCbCr_1.html

The reader would need to allow JavaScript from ‘MathJax.org’ to run on his or her browser, for the Math to display correctly.

It would be correct to infer, that in this one Y’UV profile, the Android developers chose ‘Umax’ and ‘Vmax’ to exceed 0.5 deliberately, which effectively ‘over-modulates’ U and V, and that therefore, Android devices will not be able to encode fully-saturated primary colors Red and Blue, when using this profile. And one reason fw the developers may have done this, would be at least to improve the available resolution somewhat, for ‘natural colors’, that do not correspond to saturated Red or Blue.

(Updated 08/02/2018, 20h05 : )

It’s a basic fact in (U,V)-chroma encoding, that the (U) and (V) components can become negative by the same amount, by which they can become positive, which in today’s syntax is also referred to as ‘Umax’ and ‘Vmax’.

But the fact did occur to me, that when the Green primary color, which I’ve named (cGreen), has as amplitude (1.0), while my (cRed) and (cBlue) each have an amplitude of zero, the negativity of (U) and (V) should be less than those of (-Umax) and (-Vmax), even though my first visual above would show the ranges for both (U) and (V) as (±1.0) …

(Updated 08/05/2018, 14h10 … )

Continue reading A way of visualizing YUV color-representation.