There's one other kind of
The true moire takes place when there are two or more layers of a transparent material overlapping each other. One fine mesh over another fine mesh and you get moire. Pantyhose, tulle, any transparent lingerie can do that. Grab a pair of pantyhose if you have some, stretch one leg a bit and have a look at it – there will be a double layer of fine mesh material and it will ripple as you move it.
Or just have a look at this image – there's a moire pattern when there's a double mesh, and there's no
If you see true
Here we have a scarf or maybe something else, I don't know exactly as the thing is folded. And as there are quite a few layers of fine mesh, there are moire patterns all over. As I said, trying any method but the split frequencies would be totally useless. So I'll skip the boring part where I prove it, and just run the splitting action from the Moire_killing set. The ripples are quite huge, don't get shy with the radius. It can be as big as 100 if that's what it takes. The default 1.5 setting seems okay for the texture faking business, so I'll leave it as it is. Now it's time to wipe through the mask with the Eraser to see what we've managed to achieve. Don't expect any great results here, as it's just not possible.
But the ripples are no longer tainting the surface of this folded thing, which is better than nothing. If you try to resize the image straight away, it might not work. The ripples will appear again, this time because of the interpolation, so instead of resizing in one step, I'll run the stage_sizing action a couple of times, and only then will I be able to resize for good.
Not bad, but the texture is definitely too blurry, so it wouldn't hurt to add some noise. I'll select the insides of the object with the Marquee Tool, switch to the Quick Mask mode and blur the selection with the Gaussian Blur filter to make a smooth transition. We don't need much noise, 0.7 is quite enough. That's it. Let's compare the result with the original resized without any precautions. It looks absolutely fake, which is no surprise in this situation.
As true moire is something that exists in real life, it's completely up to you whether you want to retouch it or not. Leaving it as it is is also an option. And sometimes it's inevitable, like with the black transparent garment from the other image. There's no way to make it look nice no matter how hard you might try to. The frequencies splitting technique works only when you have a texture that can be transferred to the high pass layer. And in order to be transferred there, the texture has to be consistent. Not mentioning the fact that the splitting moire removal technique is the most destructive of them all and will wreak havoc on your images in a blink of an eye.
Dealing with moire: a scheme to follow
Now after we've discussed all the possible kinds of moire and methods how to fight it, let's work out a scheme that you should use when encountering any problems of this sort. So, imagine that you open an image and see moire. What do you do? The first thing you should try is Zooming to 100%. If moire disappears at this scale, it means there's no moire at all. If you can still see moire, check if the image was sharpened or reduced by the camera processor or someone else. If so, you should notify the photographer, because there's a high chance the problem can be resolved by altering the camera settings. If you oversharpened the image in Camera Raw or changed the size yourself and got moire, you should undo it and use the stage resizing action instead. If that's not the case and you got a raw image, or a hi-res original jpeg, or have to deal with the low-res sharpened jpeg, check if the moire is colored or not. If you see blue and yellow stains, that means yes – you have a case of colored moire. Use any method to colorize it, like the Brush in Color blending mode. If that helps, great job. But if there are still moire stains that are not affected by colorizing, or if it was not really colored in the first place, it means that you have the terrible monochrome moire, which is the worst of them all. So if you did any color alteration, undo it and check the RGB channels one by one by clicking on them in the Channels panel. If there's a channel that is clear, that doesn't have any moire pattern, use the “Channel_mixer” action. If there are no channels devoid of moire, try the Channel Mixer action and do twiddle with the three channels sliders until you get a decent image. If that doesn't work, try the “Lab” action. If that doesn't work either, all you can do is use the “Split_frequencies” action. It might work if your texture is right, or it might not, but in any case, you did all you could.