Computer Graphics with Photoshop - Shadow/Highlight tool question

Author

Shadow/Highlight tool question

toomuch@att.net

2004-09-18, 7:14 pm

In PS CS' Shadow/Highlight tool, how are the Black and White Clip %
values defined? The tiniest change in these values can cause major
changes in the shadow and highlight.

For example, to bring out shadow details and to keep highlight
unchanged, I find that I have to reduce the White Clip value to .001%
(smallest value). Any value above it would blow out the highlight to 255
and raise too much shadow, even if the Highlight amount is set to zero
and the Shadow amount is minimized to 1.

Would appreciate some help. Thanks.

jjs

2004-09-18, 7:14 pm

<toomuch@att.net> wrote in message news:414C59F8.E40C4BAE@att.net...
> In PS CS' Shadow/Highlight tool, how are the Black and White Clip %
> values defined? The tiniest change in these values can cause major
> changes in the shadow and highlight.

I can't give you the metrics, but 'curves' remains the best way to adjust
tonality. IMHO.

Bart van der Wolf

2004-09-18, 7:14 pm

<toomuch@att.net> wrote in message news:414C59F8.E40C4BAE@att.net...
> In PS CS' Shadow/Highlight tool, how are the Black and White
> Clip % values defined? The tiniest change in these values can
> cause major changes in the shadow and highlight.

Depends on image content. Of all pixels in the image, you can define
the percentage that will fall in (or be added to) the minimum/maximum
histogram bins before other tonescaling is applied to the image data.

> For example, to bring out shadow details and to keep highlight
> unchanged, I find that I have to reduce the White Clip value to
> .001% (smallest value).

Yes, that's fine. Whatever helps the image, you can even set it to 0%.

> Any value above it would blow out the highlight to 255 and
> raise too much shadow, even if the Highlight amount is set to
> zero and the Shadow amount is minimized to 1.

The defaults are just that, defaults. Change as required, it's the
result that counts.

Bart

Mike Russell

2004-09-19, 4:14 am

toomuch@att.net wrote:
> In PS CS' Shadow/Highlight tool, how are the Black and White Clip %
> values defined? The tiniest change in these values can cause major
> changes in the shadow and highlight.
>
> For example, to bring out shadow details and to keep highlight
> unchanged, I find that I have to reduce the White Clip value to .001%
> (smallest value). Any value above it would blow out the highlight to
> 255 and raise too much shadow, even if the Highlight amount is set to
> zero and the Shadow amount is minimized to 1.
>
> Would appreciate some help. Thanks.

The new shadow/highlight tool does offer some exciting new possibilities for
manipulating brightness information that was difficult to do previously,
even with curves. The key seems to be the dynamic mask that is generated
using the radius settings, which are unique to this tool.

As for the thresholds - my advice is not to use them. They may be of
interest if you wish to quickly clip shadow and highlight detail, but this
job is best done manually. As jjs suggests, it is better done using curves.
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net

toomuch@att.net

2004-09-19, 12:14 pm

Mike Russell wrote:
>
> toomuch@att.net wrote:
>
> The new shadow/highlight tool does offer some exciting new possibilities for
> manipulating brightness information that was difficult to do previously,
> even with curves. The key seems to be the dynamic mask that is generated
> using the radius settings, which are unique to this tool.

What is a dynamic mask, and how do you generate one from this tool, and
how do you use it?

> As for the thresholds - my advice is not to use them. They may be of
> interest if you wish to quickly clip shadow and highlight detail, but this
> job is best done manually. As jjs suggests, it is better done using curves.

I always lower the thresholds to reduce clipping, as in the Auto tools.
I lower them from the default .01% to .001% in this case. My question is
why would changing from .001% to .002% make such a drastic difference in
the Shadow/Highlight tool.

As with any new tools in a new PS rev, none of the CS books go into any
details about Shadow/Highlight. The authors are all too eager to rush
their books to the market.

Mike Russell

2004-09-20, 4:14 am

toomuch@att.net wrote:
> Mike Russell wrote:
>
> What is a dynamic mask, and how do you generate one from this tool,
> and
> how do you use it?

The mask is my own guess about how the internals of S/H operate. It is an
integral part of the mathmatics, and AFAIK there is no way to use it outside
of the command. You may be able to approximage the mask using a combination
of gaussian blur and levels.

>
> I always lower the thresholds to reduce clipping, as in the Auto
> tools. I lower them from the default .01% to .001% in this case. My
question
> is why would changing from .001% to .002% make such a drastic difference
> in the Shadow/Highlight tool.

I don't recommend clipping your highlights and shadows this way. It's too
blunt a weapon to use on your images. Unless your time per image is very
limited, it's much more effective to use curves to control your shadow and
highlight explicitly, based on your chosen shadow and highlight areas The
threshold numbers may well have a different significance for the
Shadow/Highlight command.

> As with any new tools in a new PS rev, none of the CS books go into
> any details about Shadow/Highlight. The authors are all too eager to
> rush their books to the market.

The publishers certainly are looking at the bottom line, and perhaps we can
forgive them this. It's also true that Adobe has not, AFAIK, provided
details. I would not be surprised if a patent is in the works.
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net