CS2 Noise Reduction Issue

[Ken Fournelle]

I just finished a retouch and noticed something odd in using the CS2 noise reduction filter. I was reducing noise in the advanced mode for each channel. I used a strength of 5 and 6.

When I applied the filter, it left a slight magenta cast to the whites. The tint was in the filter dialog window too. The tint was there in the Basic mode also. I then used Neat Image for my noise reduction. No magenta cast.

Anyone have any thoughts on this?

K

[delic]

Well without testing anything.. WHen you modify each channel independently there is a very good chance you will add a color cast. Try turning the denoised layer blend mode to Luminosity. Only the color noise won't be denoised this way.

[Ken Fournelle]

delic,

Yes, that does take the cast out of the whites in the image, but there must be an explaination.

byRo?

k

[byRo]

Not so much of an explaination, more like some observations....

I set up a test image and sprinkled some eye-droppers. Did Noise Reduction with various settings and discovered that the principal culprit seems to be in the "Reduce Color Noise". A higher setting will cause more colour "drift" - although even at zero there are still some shifts.

My hunch would be that the answer lies in the colour mode conversions between RGB and internal PS representation. (I'm not too good at this stuff - would have to call Stroker in to draw the pictures)

I tested my hunch by converting to LAB mode (where you should have been anyway - nudge, nudge) and then applying a 100% Reduce Color Noise - of three values (L/A/B) for the four eye-droppers only one moved, and that only by one count.

So, until somebody comes up with something better:
- Yes, there is a marked colour shift (although I saw less Reds and more Green / Blue);
- Lowering "Reduce Colour Noise" should lessen the effect;
- Using LAB mode should (just about) eliminate it.

Hope this helped,

Rô

[Ken Fournelle]

Ro,

Thanks. Yes, I did have a high setting on Reduce Color Noise. That must be the culprit. It's something to watch out for.

regards,

k