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The Power of the High Pass Filter

May 23, 2001 Article Start Previous Page 4 of 4
 

Pattern Overlays and Detail Textures

Whether you were working on a normal texture, or a so-called detail texture, High Pass can greatly help you in creating patterns that can be overlaid on top of textures.

Sometimes it's a nice idea to mix several layers of gritty textures together with the Overlay blend mode. High Pass simply ensures that the small details overlay the underlying texture without larger details interfering, and that the overlay is centered around 50% gray, minimizing the overall luminosity-changing effect of the blend.

Or, let's imagine you're working on a photo-based leather jacket texture. To seam the different photos of the jacket taken from different angles together, you've had to smudge and fade the edges of the layers a lot. Smudging blends the colors nicely, but results in a lack of small detail.

FIGURE 15. Original texture + High Passed overlay = A cool new texture.

To restore the leather pattern onto the texture, select a section of leather that contains an intact bump map, copy it to a new image, and desaturate, then High Pass it. This keeps the small detail you need, but removes the large ones that would make tiling the pattern difficult. You may also want to Offset the pattern with Wrap Around to bring the edges to the center, then use the Clone tool to make it tile seamlessly - however, you may notice that the smaller High Pass radius you've used, the less seam-sealing you have to do.

FIGURE 16. Smudged leather jacket texture.

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FIGURE 17. (left) Crop a section of intact leather pattern; (right) High Pass, Desaturate, and Define as Pattern.

Nevertheless, when you're satisfied with the results, turn this image into a Pattern, and use the Pattern Stamp tool in "Overlay", "Soft Light" or "Hard Light" mode to paint the leather pattern right back on. If the pattern doesn't seem prominent enough, simply increase its contrast before turning it into a pattern.

FIGURE 18. The leather pattern can then be painted back on with the Pattern Stamp tool.

Detail Textures

The same rules apply for so called detail textures (a fine example of them can be found in games that use the Unreal engine.) Since the very purpose of detail textures is to add small, sharp details to textures up close, they are obviously ideal targets for the High Pass filter. Because the details needed are indeed very small, feel free to use High Pass quite harshly with small radius values.

Multiplicative detail textures are a pretty straightforward case. Just make sure that they are mostly white, and only contain small crisp black details - otherwise they will darken the underlying textures too much when they get MIP-mapped. That, or you need to increase the brightness of all the underlying textures to compensate.

Some newer 3D accelerators support the Indexed Additive blend mode. This blend mode works the same way as the Hard Light blend mode in Photoshop: everything in the bitmap brighter than 50% gray, brightens the underlying bitmap, and everything darker than 50% gray, darkens it. This blend mode is ideal for detail textures and decals, but because not all the older hardware out there supports it, most games still use alpha blended detail textures. Since High Pass automatically shifts the image luminosity to around 50% gray, its results are immediately applicable.

Alpha blended detail textures are probably the most common application today. When working with alpha blended detail textures, it is best that the color map only contains black and white (no grays), and that the total average brightness of the bitmap when summed together is 50% gray. This ensures that the detail texture doesn't lighten or darken the underlying texture when it gets MIP-mapped.

How do you create an alpha blended detail texture that looks and behaves like an indexed additive one? First, start working on the texture as if it was indexed additive, creating a bump map on a 50% gray background. You can actually keep it in Hard Light mode in Photoshop while working, to see what the result will look like. Use the High Pass filter to remove any larger details. Make sure it tiles perfectly and everything.

When you're satisified with the results, turn the texture into a layer, and switch its mode to Difference. Create a new 50% grey Background behind it. As you can now see, all the pixels in the texture, that were either darker than gray, or lighter than gray, show bright against a black background this is the alpha map for the texture. Copy Merged the result into a new layer. As you can see, the more contrast the original detail texture has, the more opaque the alpha map will be. Make sure the contrast isn't too high, or the detail texture will be too prominent and may overrun the underlying textures.

Finally, take the original gray detail texture, switch its blend mode to Normal, and increase its contrast to maximum, so that all colors brighter than gray turn white, and all darker than gray turn black. This eliminates the grayness from the final result. Use this bitmap as the color map, and the "Difference bitmap" as the alpha map for it. The resulting texture behaves very much like an indexed additive detail texture.

FIGURE 19. Creating an alpha blended detail texture from a greyscale bump map.

And this concludes our feature. These were the primary uses I've come up with for the High Pass filter. Get creative, and you may invent something I never figured out. I sincerely hope these tips will help the texture artists in the industry. Now go impress your project lead.


Article Start Previous Page 4 of 4

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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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