Re: hue shift with Photoshop curves
Re: hue shift with Photoshop curves
- Subject: Re: hue shift with Photoshop curves
- From: Eric Chan <email@hidden>
- Date: Thu, 19 May 2011 11:53:55 -0400
You can get perceived hue & saturation shifts with RGB curves in PS because the curves are applied per-channel. The exact influence of a given curve will also depend on your choice of working space, because of the gamma encoding of the image values (e.g., Adobe RGB is 2.2, ProPhoto RGB is 1.8). This behavior has been there since the beginning ... it's not new. It is possible you just happened to notice it now, on a particular image, due to a particular curve you're using.
To make a long story short, there are many reasonable ways to apply a given 1D curve to a color image. Applying the curve 3 times to a RGB image (once per channel, individually) is just one way. It has certain defining visual characteristics that make it good for some images, less good for others. Nothing surprising here.
Cheers,
Eric
On May 17, 2011, at 1:42 PM, Andrew Rodney wrote:
> This is a long and unnecessary controversy. When in Photoshop, using the RGB curves, there is an alteration in saturation by design (by Thomas Knoll decades ago). In a nutshell, it looks better, its what most users expect. It took more work to code and was done by Thomas for this reason. There’s been this urban legend that this is somehow a problem or a bad idea. That curves should be manipulated individually or in Lab (assuming you don’t mind the time hit and data loss, certainly in 8-bit per color docs). The solution is quite easy as Mark explained. Use the Luminosity Fade command or an adjustment layer with the same blending mode. You now have control over each option. So it doesn’t matter if the working space is well behaved or not, the alteration of saturation is a feature, not a bug you can use if you wish or not use if you wish.
>
> Its all very well explained by Mark here:
> http://www.luminous-landscape.com/essays/Curves.shtml
>
>
> Andrew Rodney
> http://www.digitaldog.net/
>
> On May 17, 2011, at 11:32 AM, edmund ronald wrote:
>
>> aRGB
>>
>> On Tue, May 17, 2011 at 6:58 PM, Scott Martin <email@hidden> wrote:
>>
>>>
>>> On May 17, 2011, at 11:28 AM, Dick Busher wrote:
>>>
>>>>> Every time I use the PS curves tool in RGB mode I see thing shift in
>>> hue. I
>>>>> don't want a hue shift, I expect a perceived brightness shift at the
>>> same
>>>>> hue. This is CS5. WTF?
>>>
>>> Just to check - your document is in a gray balanced working space right
>>> (sRGB, AdobeRGB, etc)? Not a device space?
>>>
>>> Scott Martin
>>> www.on-sight.com
>
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