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The Power of the High Pass Filter

May 23, 2001 Article Start Page 1 of 4 Next
 

Introduction: Common Problems

If you're a graphic artist working in the game industry, it is almost certain that at least at some point of your career you have had to deal with tiling textures in Photoshop. Textures, that must not only tile seamlessly, but also look good even when tiled tens of times over. Ever too often, your brick wall may tile seamlessly at the edges, but something, just something there doesn't work: when the texture repeats many times over, you see annoying repetition, covering the distant hills with an unnatural raster-like pattern. You may realize that there's a cloudy, darker area that needs to be removed, try to Dodge it a little, and try again. Okay, the dark blotch is now gone-but wait, now there's a bright area in the other corner! And so it goes, driving you nuts.

More recently, using digital photo material for textures has also been an issue. Taking the pictures is an art in itself, but working textures out of them is often where the most hair-pulling takes place. Ever had to take pictures of a concrete wall in a dark underground garage, using a flash? The picture will end up with a prominent highlight in the middle, which may look good in a completely non-lit environment, but not when rendered with lightmaps. How do you deal with it? Creating gradual selections by using gradient tools in the Quick Mask mode may do the trick, but not perfectly, and not without a lot of trouble.

FIGURE 1. A brick wall with an annoyingly repeating texture.

The third area that has traditionally been somewhat tricky, is making detail textures for today's 3D accelerators. These, if any, must tile perfectly unrepeated, or any surface using them may look even worse with one than without.

For all these problems, there is an incredibly handy little filter that most people seem to ignore. It is called the High Pass filter.

FIGURE 2: A photograph of a wall texture, taken in dark with a flash.

Underrated, Yet Powerful

If you've ever explored the secluded Other.. section in Photoshop's Filters menu, you may have seen the High Pass filter. You might have tried it, and seen how it pales and flattens the image. What possible use could there be for such a filter, except for some odd postmodern weathering effects? You may have looked at Photoshop's online help, or even some tutorial books on the subject, but none of them do much about explaining what the filter does beyond its conventional uses - that is, "extracting detail out of continuous-tone images prior to posterizing". This isn't something a graphic artist working in game industry often needs to do.

I once decided to experiment a bit further with High Pass, however, and realized its true potential - and nowadays, I couldn't live without it.

First, I am going to tell you something about the world of signals and waveforms. Strange it may sound, but pictures and sound have much more in common than many people think.


Article Start Page 1 of 4 Next

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Comments


Scott McDaniel
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Great article - this helped me with research for a blog post on a Salvador Dali painting in which he hides a portrait of Lincoln in a portrait of his wife. He masked the portrait of Lincoln in the low spatial frequencies, and in the high frequencies he did the portrait of his wife. I link to this article from there. http://www.scottmcd.net/artanalysis/?p=1131



I'll also definitely be able to use the high-pass filter in PS, which I'd never really understood before. Thanks!

Ferdinand Joseph Fernandez
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Takes me back. I remember reading this when I was still a student.

Barry Scharf
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I always use a copy layer for High Pass and use it as an overlay layer on the original image. One of the adjustments that I apply after conversion to high pass is to add a curve adjustment to bring more sharpening to edges. Then it is modified in the layer modes and opacity for the layer. It adds a fully controllable sharpening to the image. Finely add an inverted layer mask and paint in the effect where needed.


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