How determine the crossover frequency

I'm having a hard time to understand how the high-pass filter determines vs high frequency. What value it measures. ? A change of RGB values or contrast? I see the sharpness of words or blur, but what values determine the amount of blur or sharpening? Is there and the actual frequency being high or is it just a metaphor to compare it to the audio crossover?

No details on the workings of this filter is appreciated.

A few additional explanations:

Audio signals:
frequency = cycles per second
It is the frequency in the time domain.
An arbitrary signal can be represented by a set of harmonic signals (sine, cosine)
In addition, some DC (direct current, non-periodic component).
A typical high-pass filter for audio signals:
The cut-off frequency fc is at the transition between the increase in gain
(low frequencies) and fixed gain 1.0 (high frequency).
frequency reduction (mitigation)
CF 0.01 0.01
CF 0.10 0.1
CF 1,00 0.707
10.0 fc 1.0
100 CF 1.0

A typical high-pass filter for digital images:
frequency = cycles per unit of length or (better) cycles per pixel
It is the frequency in the space field.
The highest frequency is 0.5 cycles per pixel (alternation of black and white pixels)
A line of arbitray in a digital image can be represented by a set of harmonic signals
In addition, DC-part.
Each channel that r, G, B is filtered individually, one after the other.
Frequency and gain are as above.

A high pass filter applies to mitigation<1 to="" low="" frequency="" signal="">
and removes the DC component entirely. This would result in a black background.
Therefore, the background is extended to R = G = B = 128 (8 bits per channel) or
R = G = B = 0.5 (standard).

Edges contain stronger high-frequency components. These are preserved.

http://en.Wikipedia.org/wiki/high-pass_filter

In this doc, we find a simple implementation:
y [0]: = x [0]
for i from 1 to n
y [i]: = a * y [i-1] + a * (x [i] - x [i-1])
return y

x [i] are values of R (or G or B) row in column i in the original image
y [i] are values of R (or G or B) on the same line in column I after filtering.
Factor (a) indirectly contains the cutoff frequency.
This example does not yet apply the SHIFT R = G = B = 128.

The implementation can be different (filter not recursive instead of recursive, as indicated above,
higher order instead of first order, using a filter kernel 3 * 3 5 * 5 pixels instead of work
in each row independently).

I'm not trying to explain how Photoshop!  There are so many alternatives.

Cordially - Gernot Hoffmann

Tags: Photoshop

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