Using High Pass Filter To Sharpen Images In Gimp

Some people are familiar with using Unsharp Mask to sharpen images in Gimp. But there is another method, which I think does a better job. It’s the High Pass Filter. Gimp doesn’t come with a High Pass Filter by default (as of version 2.6), but there is a plugin available on Rob A’s (Im)personal Blog.

Most of the time, sharpening is the last step you want to do when working on an image. If you apply other filters to a photo after you’ve sharpened it, those filters tend to affect the sharpened areas and make it look wrong.

The plugin comes with five modes you can use:

• Colour
• Preserve DC
• Greyscale
• Grescale Apply Chroma
• Redrobes

Let’s compare the different modes, and Unsharp Mask. Click on any photo to see a larger version. I have to warn you, though, they are larger-than-normal files, but I wanted you to be able to see the difference between photos.

No sharpening

The photo has slightly fuzzy edges throughout.

This photo is slightly out of focus. The trees in the background are kind of foggy. The outline of the elk is a little fuzzy. The grass tends to run together. A haze hangs over the whole thing.

Unsharp Mask

Unsharp Mask applied

With Unsharp Mask, the photo has definitely improved. The trees look more defined, as well as the outline of the elk. There is a slight improvement in the sharpness of the grass and it seems like the haze has been lifted. But there are now blown-out highlights on things now. Particularly the elk’s antlers and branches and leaves that were in areas without much contrast previously. Unsharp Mask increases the contrast to sometimes unbearable levels, leaving halos around edges. Now let’s look at some high pass filter techniques.

High Pass Filter — Colour

High Pass Colour applied

This is a much gentler method of sharpening the photo. All the blown-out highlights and halos are gone, but the sharpness of the trees has increased, the outline of the elk is much clearer and the grass has more texture to it. Here is how I used it:

1. Click on Filters → Generic → High Pass Filter. A dialogue box appears with some options. This is where you choose which mode you want. The default is Colour. I didn’t adjust Filter Radius or Contrast Adjust from their default settings of 10 and 0.
2. Click OK and the filter runs its course. It creates a new layer that appears mostly grey with outlines of the image.
3. In the Layers palette, change the mode from Normal to either Overlay, Hard Light or Soft Light. I chose Overlay. The sharpened image appears.

I think the Colour mode will be suitable for most situations, but if you want to experiment and have a little fun, there are four other modes you can choose. Let’s look at those now.

High Pass Filter — Preserve DC

Preserve DC mode

The Preserve DC mode functions similarly to the Colour mode, but also adds the average image color back in, so in this instance the photo becomes much greener.

High Pass Filter — Greyscale

Greyscale Mode

The Greyscale mode desaturates a copy of the image before applying the High Pass Filter. With this image, I cannot see any difference between this mode and Colour mode.

High Pass Filter — Greyscale, Apply Chroma

Greyscale, Apply Chroma mode

This mode also uses a desaturated copy of the image, but blends it with the source layer colors. The result is an image with very saturated colors. It borders on looking unrealistic.

High Pass Filter — Redrobes

Redrobes mode

This mode is similar to the Preserve DC mode. However, while flipping back and forth between the two images, I can see that Redrobes produces a slightly sharper image than Preserve DC.

So those are the different methods of High Pass Filter in Gimp. Colour is probably the mode I’ll use most when simply touching up photos, but Preserve DC, Greyscale Apply Chroma and Redrobes would be fun to play around with.

I’d like to hear from you about other methods of sharpening photos.

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