Reducing moire/aliasing with a filter.

[Dave2ic]

I've posted this thread on another website, but thought I'd have better luck getting a response here on a specialist video website.

I have a Sony NEX-6 and I think it does great video, but it is marred by moire/aliasing artifacts and I'm interested in looking at ways this can be reduced.

As I understand it, moire/aliasing is caused because each high resolution frame of video is not properly downsampled to 1080p as the processing required to do this would be too much at 25/50 fps (PAL) or 30/60 fps (NTSC). Instead, lines are just skipped, so in areas where there are hard lines or fine patterns these artifacts can appear.

Solutions to help reduce moire/aliasing (other than purchasing a dedicated camcorder!) seem to revolve around softening/blurring the image that reaches the sensor - enough to reduce the moirse/aliasing effect but not so much that it effects the sharpness of the final 1080p video significantly.

The first suggestion I saw was to turn sharpness down to -3 in whatever Creative Style you are shooting in. I found though that this didn't seem to be enough to make a noticeable difference for me.

The other thing that a lot of people seem to be trying to reduce moire/aliasing is various filters on the lens of the camera. I am interested in trying this, but I was wondering if anyone had tried anything similar and what results they had.

I was thinking of trying a simple diffusion filter, but here is an interesting video of what can be achieved with a simple eyeglass lens:

www.youtube.com/watch?v=xIVMvCDOt54

Caprock do a set of filters:

http://caprockdev.com/antimoire.htm

This video demonstrates a solution (available for certain DSLR's) that involves a filter going behind the lens. It's not available for NEX, but does show the possibilities:

http://www.youtube.com/watch?v=pUXEm6sD8oM

Make sure you view the videos at their highest resolution to properly see the effect.

Any thoughts/advice on what might be worth trying?

[racer-x]

As I understand it, moire/aliasing is caused because each high resolution frame of video is not properly downsampled to 1080p as the processing required to do this would be too much at 25/50 fps (PAL) or 30/60 fps (NTSC). Instead, lines are just skipped, so in areas where there are hard lines or fine patterns these artifacts can appear.

Don't believe that B.S. The cold reality as I see it, is they purposely reduced the quality so it won't take away sales from their high end cameras. Think about it.......the micro processors have plenty of power to implement CPU intensive noise reduction, but can't even use a simple bicubic resize algorithm? Pure BS in my opinion........

My t2i also suffers from this. You can try those crazy filters, or do what I do and shoot only with a shallow dept of field.

Good Luck.....

[smrpix]

Moire has appeared for years on the highest end cameras, so put the conspiracy theories away. Moire is an artifact of detail in the image closely matching the resolution of the sensor and causing interference.

In the old days a little softening of the image helped somewhat. All the techniques described here do exactly that.

[jagabo]

Moire is an artifact of detail in the image closely matching the resolution of the sensor and causing interference.

Yes, but the sensor has much more resolution than the 1920x1080 output. The problem is indeed poor resizing on the part of the cameras. If you take a full resolution photograph and downsize to 1080p in software with a decent resizing filter you'll get much better results.

The cameras' processors may simply not have enough computing power to downscale 15+ megapixel images to 1920x1080 at 24 or 30 frames per second. As I understand it, most of them sample a line at a time. To downscale the vertical dimension they just skip lines while sampling.

[Dave2ic]

In the old days a little softening of the image helped somewhat. All the techniques described here do exactly that.

It all seems rather hit and miss though with the filters. I've been looking at a thread on another website discussing the Caprock filters and it seems to be filled with posts from people returning them to try a different strength filter. They have 8 different filter strengths in all, and living in New Zealand it could be a very long exercise trying to find one that will do the job.

Even then, as I understand it any solution will probably only work best for a specific focal length within a certain aperture, so it won't be an ideal solution for me working with a zoom lens. But I don't mind having a solution for a certain focal length, as zooming during video doesn't look very professional anyway.