What do you mean, degrees are not linear? I'm a mathematician, and I think they are inarguably linear, so I'm assuming we're using different definitions of "degrees" or of "linear".
Edit: By any chance, do you mean "the projection of an angle measured in degrees on a straight line measured in {cm/m/whatever} is not linear"?
That is exactly what he means. I guess it might have been explained better in the comment, but that's the whole point the article is making: that the "reference pixel" for a given display is calculated by taking a certain visual angle (based on a specified DPI/distance) and projecting it onto a perpendicular surface at the expected viewing distance.
If the reference pixel is the viewing angle then multi-pixel measurements don't map linearly to on-screen sizes unless the screen they're designed for is the concave face of a sphere centred on the user's eye
As I understand this article (and my long-past reading of the spec), the only thing in the spec that talks about visual angle is the part about defining the size of one reference pixel. That's a very small angle, and at those sizes angular size is essentially linear even when projected onto a flat screen.
Nothing that I've seen in the spec ever remotely suggests using angular measurements for more than one pixel at a time, let alone enough of them for 90 or 180 degrees of the visual field.
>and at those sizes angular size is essentially linear even when projected onto a flat screen.
But not quite, and summed over many pixels, it will result in a noticeable discrepancy (which is still of no practical consequence). I wonder how it would feel to use a computer that used head tracking to dynamically transform the screen's contents so that pixels were mapped to equal angular size (from the point between the user's eyes).
That's all completely true, but the article that this linked article is a response to didn't draw that same conclusion from the spec, and that's what started this whole mess
Edit: By any chance, do you mean "the projection of an angle measured in degrees on a straight line measured in {cm/m/whatever} is not linear"?