Chroma noise reduction

[tomczak]

Here is Den's nifty technique on chroma noise reduction in PWP, buried in a thread - so far I couldn't come up with anything better. One downside of it is that it can't be incorporated into a workflow.

http://www.dl-c.com/board/viewtopic.php ... sd=a#p2747

In short: extract Luminance from the original, also blur the original + apply a constant HSL-L curve y=50% (effectively replacing HSL-L channel with constant 50% gray but leaving H and S channels intact); then Composite the latter image with the Luminance from original as an overlay in Hard Light mode.

One possible improvement to Den's technique, which should limit blurred chroma leaking at bright edges, is to use Bilateral Sharpen instead of Gaussian Blur in the Blur step in the recipe above, with Sharpen Factor/Threshold=0 and adjusting Blur Threshold/Radius so that the noise is blurred but contrasty edges are not.

[tomczak]

This technique (especially, and somewhat counter-intuitively, when Gaussian blur is used) also seems to reduce chroma edge bleeding and reduces incidence of pixels clipped in one colour channel.

cf. http://www.dl-c.com/board/viewtopic.php?f=5&t=895#p4273

[tomczak]

Another variation of this effective technique is using the HSL-L channel instead of Luminance as an Composite Overlay in the last step.

The downside of either variant of this method is that it can not be automated.

There is also a possibility of reordering the steps (i.e. first flatten the HSL-L channel to 50%, then blur the image), but this seems to cause greater colour edge bleeding than the Den's original procedure, i.e.:

1) Extract HSL-L (or Luminance) channel from the original image
2) Blur the original image (Gaussian blur works fine - beware that larger radii may cause unpleasant desaturation of steep colour transitions)
3) Flatten the HSL-L channel of the blurred image to solid 50% gray (Brightness or Colour Curve in HSL-L: y=50%; you could replace HSL-L channel with 50% solid gray by Extract then Combine Channels, but that's more complicated)
4) Composite the latter with the HSL-L (or Lumianance) channel of the original image (step 1) in Hard Light mode.

[tomczak]

I just discovered yet another method of competent chroma NR that affords some edge protection (AS) or can mask the blurring without leaks across mask edges.

Both AS and Precise Gaussian can already blur chroma only. But while they do blur chroma relatively well, they both tend to leave un-naturally looking brightness 'poke marks' in exchange and tend to over-desaturate small chroma patterns.

The proposed method to effectively reduce these defects is simple: replace HSL-L channel of the chroma-blurred image with the HSL-L channel of the original image (Extract/Combine channels in HSL mode).

In addition to removing the brightness 'lumps', the procedure seems to restore saturation of small details that became desaturated or obliterated by the AS Chroma blurring.

Both AS and Precision Gaussian may take a while while blurring chroma with larger radii.

The AS and Precise Gaussian can be automated, but the necessary restoration of the original HSL-L channel that follows, can not.