What is "frequency"

[IanSG]

Both frequency separation and high pass filtering involve separating the high and low frequency data in an image, but what are they measuring the frequency of?  I understand that low frequency equates to things like tonality, while high frequency catches the detail, but I don't know what those nice sine waves that are supposed to explain things are actually showing - it's spatial separation along one axis, but what's the other? 

[John Rostron]

Do not confuse

• the graph of sine wave frequency (=1/wavelenth) on the horizontal, against  frequency (=abundance) of that frequency/wavelength) on the vertical scale, with
• the two-dimensional representation of the image detail expressed as the sum of sine waves of different frequencies.

What frequency separation does is to separate those parts of the two-dimensional image that can be represented by a the high frequencies (of both x and y) from those represented by the lower frequencies in each of the x and y axes.

John

[IanSG]

Hi John

Thanks for that!  I was too focused on the idea that frequency = Hz, and I'd forgotten that the "abundance" is affected by the radius slider, so that didn't make sense either!