Portrait blemishes and low pass

[tomczak]

I don't do portraits, but I have a belated Valentines project on my hands. Some time ago, I figured out how to smooth the skin blemishes by removing the mid frequency coarseness with low-pass blending. I can't find lowpass compositing option anymore - is there another way of implementing this technique or perhaps some better way to do what I want?

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[pierrelabreche]

Have you considered Advanced Sharpen's Noise réduction with masking ? I'm not sure it would perform as you intend, but worth a try.

[tomczak]

Cheers! I tried it, but in the AS Noise Reduction tab, unlike the Sharpen tab, there is only the upper roughness limit - I can protect the major edges, but not the high frequency noise from blurring.

p.s. But I'm trying to do something similar with Multipass Sharpen with Sharpen Factor of zero, using the mask as a paintbrush, and treating Blur Threshold as the upper roughness limit and Sharpen Threshold as the lower one, trying to remove the mid range roughness only. With judicious choice of parameters the retouching seems to look mostly natural.

[jsachs]

Try the Detail tool.

[tomczak]

I'm having some luck with the Brightness Curve in Advanced mode, keeping the Detail Slider to less then 1 and using the Mask paintbrush to choose where to apply it. This is probably similar if more convoluted to the Detail Tool, but instead of grafting high pass details from the source to target areas, it supresses or emphasises the existing details in situ.

The Detail Tool looks promising too. I had a little trouble choosing a suitable source areas for grafting details and then with the source moving along with the target - as the suitable source areas are sometimes small and scarce and it's easy to mis-clone from the adjacent areas.

[jsachs]

That's exactly what the Detail Tool does.

[tomczak]

One more comment on the technique above (i.e. the one with the Brightness Curve Advanced Mode and a painted mask): it seems to work fine, but I wish I could threshold-protect the finest details from smoothing just like the coarsest - I think it would add to the realism of such blemish retouching.

One can argue that this is an advantage of the Detail Tool which doesn't smooth or sharpen any detail frequency but replaces one natural high-pass with another instead.

[tomczak]

I just want to report that my Valentine portraits project has been received with a relative enthusiasm. Here is my secret weapon. It's more blunt but faster than the Detail Tool. The key is not to overdo it.

What this technique is lacking is protecting finest details from blurring, like grain and skin pores. My output were 800x600 pixels images for the Kodak picture frame, so most of those details got smeared anyway during down sampling, but for bigger output images being able to blur only mid-frequency blemishes could add to the realism.

[jsachs]

There is a mask texture tool that lets you select areas with different amounts of texture. You could possibly use it to limit the blurring to a range of detail sizes.

Also, instead of using Brightness Curve Advanced Mode would Multipass Sharpen give you the sharpen threshold option you are looking for?

[tomczak]

I tried Multipass Sharpen, hoping that I could restrict blurring from both ends on the roughness scale, but the results looked artificial. Maybe it's just a biased impression, but I thought it was because, unlike the AS, the definition of 'roughness' differ for blur and sharpen thresholds there and the high/med/low frequency parts of the image are not on the same continuum and can't be neatly separated into for smoothing or not smoothing without overlaps.