A Question about Highlights and Shadows sliders in basic panel

[carolz25348614]

I have read about a introduction about Highlights and Shadows sliders in basic panel, the article published in 'Adobe Press'. That is what it says in the article.

'The Highlights and Shadows sliders are symmetrical in the way they behave, allowing you to lighten or darken the highlight or shadow areas. These sliders only affect the tone region on either side of the midtone point. To be precise, the range of these sliders does extend beyond the midtone value, but the greatest effect is concentrated in the highlight tones for the Highlight slider and the shadow tones for the Shadows slider. Adjustments in the +/-50% range will have a normal type effect when lightening or darkening, but as you apply adjustment that are greater than this, the lightening or darkening adjustments are applied via a halo mask(this is similar to method used in HDR tone mapping). The thing to watch out for here is that as you apply a strong effect, the underlying halos can become noticeable. The goal has been to make the halo mask as unobtrusive as possible. For the most part, it is quite well disguised, but when pushed to extreme, it can be possible to detect halos.'

Here, I have questions about this statement.

1. What is the 'normal type effect' (which is highlight with blue in the paragraph above)? How do these two sliders affect pixels in different brightness?

2. What is the halo mask and what does it do?

Thanks.

[99jon]

1. Adobe made great improvements to the old recovery slider and in most cases I find the highlights/shadows sliders produce pleasing results. I would suggest turning on the clipping warnings if you need a visual guide to the normal effect. Click the triangle icon (top-left or right on histogram) when using either slider. Clipping is shown in the image with a red or blue warning.
2. The halo is often seen in hdr tone mapping, as the tonal range is compressed. Some people like the effect for a grungy look. The masking aims to reduce this. When pushed to extremes, halos will be apparent on close inspection but generally it is well disguised, subject to sensor exposure, even when sliders are pushed fully to the right/left.

[carolz25348614]

Thank you for your time and valuable information.

Further, do you know how the mask come about and how it works,  or the principle and the algorithm behind these two sliders(Highlights & Shadows)?

[richardplondon]

As I understand it, in that explanation "normal" means to say that a particular amount of lightening or darkening is applied to a given pixel, solely based on its starting tonal value.

And "halo mask" means to say that LR further considers whether a pixel of that same starting value, happens to be sitting within a generally light or a generally dark part of the photo. So different things will then happen either way.

IOW this has to do with "local contrast": when a consideration of what something is next to, comes into how it gets shown.