Those color patterns you get on fine details

jpl

I can't remember the name... moraine effect maybe? Google says that is wrong. I'm talking about the color pattern that shows up in fine lines like the water below (just below center on the right). You'll especially see it in ties of people on TV when the ties have very fine stripes. The effect makes it look like the tie is moving.



Anyway, what is it called and how/can you fix it in photoshop?

Thanks

jpl

Is this just a extreme case of chromatic aberration? I'm familiar with chromatic aberration and I thought this was a different problem... but maybe it isn't. When I think about this problem is pretty much the same except it happens on many more and much smaller edges.

davidweaver

The term you are looking for is the moiré pattern.
Wikipedia: http://en.wikipedia.org/wiki/Moir%C3%A9_pattern
I don't see it much in your image but it is very apparent here:
http://upload.wikimedia.org/wikipedia/commons/1/16/Moire_on_parrot_feathers.jpg

A minor moiré issue can sometime be address by copying the afflicted area and then creating a layer that is moved a pixel or two depending upon the image. You can play around with lighten, darken, multiply, screen, overlay for that layer as well as playing with the opacity of the layer. i don't know how to correct a dramatic problem as demonstrated in the wikipedia photo (linked above)
A polarizer might help when shooting in some situations when reflected light is a concern. When the image is overlayed screens you would probably see it anyway w/o the aid of a camera.

The Wikipedia article does a good job here.

You may also be suffering from some chromatic aberration, or a mix of both!

Cheers,
David

jpl wrote:

I can't remember the name... moraine effect maybe? Google says that is wrong. I'm talking about the color pattern that shows up in fine lines like the water below (just below center on the right). You'll especially see it in ties of people on TV when the ties have very fine stripes. The effect makes it look like the tie is moving.

Anyway, what is it called and how/can you fix it in photoshop?

Thanks

LiquidAir

Your camera sensor is made up of a grid of dots in red, green and blue. If you have a very fine black and white pattern close to the resoution of the camera it can selectively illuminate some colors while skipping others which can fool the camera into thinking it is seeing colors when it isn't (the issue in composite television is slightly different in the details, the net result is similar).

The process of interpreting the pattern of red, green and blue is called demosaicing and is part of the RAW conversion process. If you still have the RAW image you can try different RAW converters to see if you can find one which does a better job. However, if you shot JPEG then the RAW conversion was done in camera and your moire pattern is now baked in.

To fix this in Photoshop, you want to blur the color information without affecting the luminance. One way of getting there is to take the image into Lab color space, blur the a and b channels, and the convert back to RGB. I'd do that on a copy of the original file, layer the blurred in version over the orignal in color blend mode and use a layer mask to paint in the new version only where the moire is an issue.