by
MichelB » Tue Jun 10, 2008 10:44 am
The term 'high pass' stems from electronic devices used to filter high frequency signals to protect the signal from lower frequency signals.
http://en.wikipedia.org/wiki/High-pass_filterI have mentionned it in my text about tools for sketching in one of the first newsletters.
What is a high frequency in a digital image? It is a sharp change in value between close pixels. Where there is a sharp edge, (ie change of luminosity) the frequency is high. If the transition is less sharp (blurred for example), the frequency is lower.
What you see with the filter with a low radius in Photoshop or Elements is a gray image with the edges enhanced, the lighter part of the edge is lighter, the dark one is darker. That is a kind of edge detector.
What use can this filter have? It can be used for edge detection and sketching, by using a threshold or gradient map in addition, for example. However, its most common use is to sharpen when used in a layer in overlay or softlight mode. Both blend modes increase the edge contrast by making the lighter side of the edge lighter and the darker one darker, hence a more visible edge.
What is interesting is what you see when using a large radius (30 to 50 for example). You get an increase which is much wider than the pixels near the edges or contours. This gives more depth, more three-dimensional look, just as USM with a large radius.
Edit:
Help for Elements:
http://help.adobe.com/en_US/PhotoshopEl ... 14D89.htmlIt is important to note that this is the exact opposite effect to gaussian blur.