Not Your Everyday Banana

by Bart Arondson

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At a Synchronized Skating competition this weekend, I was forced to shoot through a mesh netting surrounding the ice (usually there to keep hockey pucks in the rink). Of course, the netting created havoc on auto-focus, but I still managed to get many shots that were reasonably sharp. However, on some shots that were otherwise clear, there are obvious artifacts from the netting visible in the pictures. I was wondering if there was any way to "save" these marginal photographs?

Here's an example of the netting (and the AF getting fooled by it, nothing can save this, but have a look at the other shots):

alt text

Another picture where it focused (nearly) correctly, but the netting is still subtly visible:

alt text

A picture where the focus is good, but the netting is clearly visible:

alt text

And finally, a picture where the netting is not seen at all (well, almost not at all, there are still some artifacts in the blue of the dresses):

alt text

Obviously nothing can save the out-of-focus picture at the top, but is there anything that can be done to save pictures where the focus is good, but where there are subtle (or not so subtle) artifacts from the netting visible in the photo?

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Is the Raw version available for some of these? It's hard to tell from your samples because the resize effectively took out any artifacts from the netting. Also, don't give up on the first, some desaturation with some further bluring may create an interesting shot, perhaps some form of abstract. –  John Cavan Dec 13 '10 at 3:47
    
Yes, I have Raw versions of all the shots. –  seanmc Dec 13 '10 at 13:17
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What's more interesting to me: How can you avoid the netting (achieve a shot similar to the last?). I guess by shooting wide open, though I couldn't check since the images don't have exif anymore. The AF problem can of course be solved by manual focus. –  ziggystar Dec 13 '10 at 20:27
    
Even the last picture has the grid pattern in it, it's just harder to see. I found that it worked best at full zoom (200mm F2.8) and focused far away. –  seanmc Dec 14 '10 at 0:22
    
@ziggystar, if you focus beyond the netting, it will reduce the impact, but not eliminate it. You can do the same with your eyes, though it's more subconscious when it happens. –  John Cavan Dec 14 '10 at 2:36
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4 Answers 4

up vote 16 down vote accepted

I've never attempted this sort of fix myself, but a Fast Fourier Transform plugin for Photoshop or GIMP might do the trick here.

Wikibooks has a tutorial on removing periodic interference with GIMP.

It might help to split the image into several sections, as the netting is not perfectly even.

I expect that this might help a bit, but without a lot of manual retouching, your image is not going to be "perfect". This might help make the netting even less noticeable, however.

(If you try this, let us know how it went.)

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This is a good suggestion. Although the netting spacing is uneven, it should still show up clearly in a power spectrum: it will just be a spread-out peak rather than a sharp one. FFT solutions can do amazing things with such interference. –  whuber Dec 13 '10 at 23:26
    
I have done this kind of fix myself, and it's the right call. It's the standard go-to solution I learned in grad school to fix repeated pattern interference in images (either from degraded satellite signals or a some other regular grid that appears in the image). You should see spikes in the power spectrum that, once removed, should remove the grid. –  mmr Dec 14 '10 at 2:12
    
That is a very useful link –  labnut Dec 15 '10 at 13:42
    
I have not tried this yet, but I intend to give it a shot. Thanks! –  seanmc Feb 11 '11 at 2:43
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While Evan's suggestion is quite good, there is a rather different possibility that can work well under the right circumstances. Specifically, if (for example) you were shooting from a specific seat number, and can get back into the same place when nothing is going on, and shoot basically the same shots, but without the skaters present. Put the two shots onto two layers, align them, and from there it's a matter of picking the right blending options to remove the second from the first. It's not necessarily easy to make it work, but can be quite effective when it does.

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You might even consider overpowering the ambient light with flash onto the netting, so that it shows up better against the background. This might make it easier to make a mask. –  Evan Krall Jan 8 '11 at 8:23
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This is a shooting suggestion how to max-eliminate netting on scene. Evan made a great suggestion how to remove it in post processing.

The first thing to know is to get as close to netting fence as possible and shoot from there (because it will make netting very out of focus - far from focused DOF). but on the contrary you may get smudges from the glass on your shots.

One more about periodic interference algorithms. You should try different shots with that post process. I'm a developer myself and I guess that semi-focused net would be easier to remove than the completely blurred one or the sharp focused.

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You can also try with Project Vega. The result seem impressive..

http://img-service.com/overview/periodic_quasi-periodic_noise_removal.html

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