RE: [APML] A Curves Exercise [was: Histograms, curving and layers]

[Matt BenDaniel <matt@starmatt.com>]

Greg,

The levels tool is useful but not as versatile as curves.

Yes, my image is somewhat clipped on the low end. I did that to suppress grain noise. Perhaps now that I know how to use SGBNR, I should fix the clipping and deal with the grain by smoothing it away.

To load a curves file, open a curves dialog in PS (Image->Adjust Curves  or  Layer->New Adjustment Layer->Curves) and click on the Load button.

Matt

At 13:28 05-12-02 -0500, Greg Hartke wrote:

>This is an excellent exercise to PS neophytes like me and I really
>appreciate your effort in helping us, Matt.
>
>I downloaded both the uncurved and curved image and did some diddling. I
>confess that after spending some time playing with the curves adjustment to
>the individual color channels, I got very frustrated at my inability to get
>a good image. I proceeded to go in a direction more familiar to me, working
>with levels. This may be the wrong approach but I was curious what would
>happen. I suspect it's less versatile than curves but I don't enough to say
>that for sure.
>
>I looked at the information in the individual color channels and adjusted
>the sliders on the low and high ends so that the range was more compressed.
>I moved them on either end such that they were right at the end of the
>significant parts of the curves. I then adjusted the black point to 15, 15,
>22 and looked at the result. After comparison to Matt's curved version, I
>adjusted the center red slider for a little more red. The result was much
>more like Matt's image, though not identical.
>
>I'm sure this isn't the right way, it's just what I could do. An interesting
>consequence was that the resulting histograms were all clearly quantized. I
>would assume that's the result of a compressed range and working with only 8
>bits of info.
>
>The curved version had interesting features. Am I mistaken or are the blue
>and green histograms clipped at the low end? My efforts resulted in similar
>clipping but I thought we were trying to avoid this?
>
>I really wanted to look at Matt's curves file but I'm embarrassed to say I
>couldn't figure out how to import and look at it. This must be simple but I
>couldn't manage it. Matt would you please elucidate? Sigh. Somebody point me
>at a command line Unix system so I can grep 'em to death. ;)
>
>Thanks.
>
>Greg Hartke
>Sykesville, MD
>
>
>
>> -----Original Message-----
>> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
>> Behalf Of Matt BenDaniel
>> Sent: Wednesday, December 04, 2002 9:39 PM
>> To: astro-photo@seds.org
>> Subject: [APML] A Curves Exercise [was: Histograms, curving and layers]
>>
>>
>> Alan,
>>
>> The most important skill to master in image processing is curves.
>> By far, you'll get more bang out of curves than any other tool.
>> Both Photoshop and PictureWindow have good curves tools.
>>
>> I made an exercise for you (and anyone else who might be interested).
>>
>> I place an uncurved image of Central Auriga on my web site. Save
>> the uncurved image onto your disk. Using solely a curves layer,
>> try to make the color balance and contrast of the uncurved
>> picture look like my curved one. All you need concern yourself
>> with at this point is adjusting the R, G, and B curves in a
>> single adjustment layer. That's it. I already took care of
>> stacking, AV-ing and star shaping the uncurved image, so you
>> don't need to worry about any of that with this image.
>>
>> Now, I don't claim that this exercise is easy. But if you do it
>> right, you should be able to make the uncurved image look just
>> like the curved one on my web page.  The link to the uncurved
>> image is at the bottom of the page:
>>
>> http://starmatt.com/gallery/astro/central_auriga.html
>>
>> I hope this exercise is helpful to you and others. Please let me
>> know if you have questions, problems or have results to share.
>>
>> Matt
>>
>> At 17:45 04-12-02 -0800, Alan Voetsch wrote:
>> >Hi Matt,
>> >
>> >--- Matt BenDaniel <matt@starmatt.com> wrote:
>> >> Thanks. I'm always looking for suggestions for more
>> >> articles.
>> >
>> >How about: making a copy to work on, setting black
>> >points (whatever that is), making or using layers. All
>> >I feel partially proficient at is adjusting contrast,
>> >lightness and watching how much the image changes as I
>> >move the curves slider around.
>> >
>> >> The order matters a lot. If one layer clips the
>> >> image, nothing above it in the layer stack will
>> >> rescue the lost info. I find that an AV layer works
>> >> best if it is right above the source layer. When
>> >> doing AV, I'll alternate adjustments to the AV layer
>> >> and to the main curves layer. Their adjustment is
>> >> intertwined.
>> >
>> >I understand the concept, but this sounds like there
>> >are a lot of ways to learn a lesson the hard way.
>> >Maybe another article on the sequential steps to use
>> >throughout the whole process.
>> >
>> >Thanks Matt,
>> >Alan
>>
>> --
>> Matt BenDaniel
>> matt@starmatt.com
>> http://starmatt.com


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