Using High Pass functionality of "Convolution shaman" filter

Using plugin for photo correction

Compare three photos above. The left photo is source; the middle photo shows the result of simple "Auto Levels" expanding tonal range. Many areas of the image remain dark, and cannot be recovered by changing gamma without introducing significant banding. The right image shows the result of highpass followed by "Auto Levels" - by discarding some "general" lighting difference between the left and right parts of the photo, it efficiently recovered and emphasized small details, including minor clothes ones and background texture.

Original photo above (c) Sergey Ryzhkov, 2001

Tip: if your image has a broad range already and you're only about removing some "long-distance" light gradation, switch "Keep ranges" option "On". In case your image has a short tonal range, it is recommended to switch it "Off" and do regular auto tonal adjustment after using highpass filter.

Tip: for best results, play with Apply modes, in most cases "Normal" and "Luminocity only" are most useful but check out the rest as well.

Using plugin for improving seamless patterns

Quite often, the patterns that are "formally" seamless (that is, their edges perfectly match) have a general lighting (or color) gradation across the whole image, which makes "repetitions" easily recognizable and annoying. It often happens with "artificial" patterns and even more often - with patterns prepared from real stones, marble etc. photos. With "Across edges" option switched "On", "Convolution shaman" filter's "Highpass" function may be effectively used for fixing it. Compare pattern examples below, source ones are at the left column and the patterns at the right one were passed through "Convolution shaman" with "Keep range" option turned "on":

As you can see now, annoying "long-distance" repetitions seen at the left are effectively eliminated without destroying the pattern details.

Original patterns (c) I-Tex; check out their free texture generator "GenTex", excellent texture explorer "I-Texture Discovery" useful for quickly previewing images in a "seamless" manner, and quite sophisticated shareware texture processor "Infinite Texture"!

How does the filter work

It seems to be often misunderstood by users how exactly "highpass" filtering clarifies the image. The main point of confusion seem to be the fact that filters like "Sharpen" or "Unsharp mask" somehow clarify the image too. The difference may be easily shown by a small sample below:

	Source image contains both smooth long-distance gradation (left to right) and small details (text) over it.
	Result of "sharpening" the test image - the filter improves the detail contrast between the text and smooth background by altering the text color. General gradation is not affected.
	Using "highpass" on a text image: while text details are not significantly affected, background gradation is completely removed, replaced with "medium" solid color.
	"Highpass" followed by sharpen - background gradient is removed, then small details improved. Both filters work side-by-side to make your image look better.