Strange curves effect – bug or feature?

J
Posted By
jenelisepasceci
Aug 19, 2005
Views
406
Replies
4
Status
Closed
I was playing with curves a little bit (CS2) and got a strange effect when I applied an admittedly rather uncommon curves transformation to a monochrome gradient. You can see the result at:

http://tinyurl.com/8xh98

the image displayed links to the actual PS file with the gradient layer still in place, which you may download for inspection. The bottom left part shows how the image looked like at the beginning and the top right what I got after adding my contrived curves layer. You will find that the gradient is made up between two colors with exactly the same hue and saturation, and with brightness varying between zero and 100%. Since I did not modify individual color channels I’d have expected a result which is actually obtained when the curves layer blending mode is set to luminosity. This is what I do routinely to prevent unwanted changes in saturation.
What I got here is really surprising to me, since there is not only a change in saturation as expected but also in hue. It looks to me as if there were some serious rounding or overflow error in the curves routine.
BTW, I opened the file in CS and 7 as well and it looked always the same.
Please share your opinion

Peter
Related Tags
bug curve effect feature strange

Must-have mockup pack for every graphic designer 🔥🔥🔥

Easy-to-use drag-n-drop Photoshop scene creator with more than 2800 items.

MR
Mike Russell
Aug 19, 2005
"Peter Wollenberg" wrote in message
I was playing with curves a little bit (CS2) and got a strange effect when I applied an admittedly rather uncommon curves transformation to a monochrome gradient. You can see the result at:

http://tinyurl.com/8xh98

Although the gradient is monochrome, it does contain color – just the same color. Your image is a great example of how the RGB master channel alters hue. Try the same curve on the lightness curve of Lab mode and you’ll see no hue change at all.
—

Mike Russell
www.curvemeister.com
S
saswss
Aug 19, 2005
In article ,
(Peter Wollenberg) writes:
… Since I did not modify individual color channels

Ah, but you did. The RGB curve is applied to each channel individually, after applying the separate R, G, and B curves.

—

Warren S. Sarle SAS Institute Inc. The opinions expressed here SAS Campus Drive are mine and not necessarily
(919) 677-8000 Cary, NC 27513, USA those of SAS Institute.
J
jenelisepasceci
Aug 22, 2005
"Mike Russell" wrote:

"Peter Wollenberg" wrote in message
I was playing with curves a little bit (CS2) and got a strange effect when I applied an admittedly rather uncommon curves transformation to a monochrome gradient. You can see the result at:

http://tinyurl.com/8xh98

Although the gradient is monochrome, it does contain color – just the same color. Your image is a great example of how the RGB master channel alters hue. Try the same curve on the lightness curve of Lab mode and you’ll see no hue change at all.

BTDT 😉
Well, the lesson I learned is to use curves adjustment layers exclusively and set the blending mode to luminosity in order to retain hue and brightness. I still don’t understand the mathematics behind the whole thing completely. I’d have anticipated the complementary hue (H-180 deg) to appear but actually I got the whole spectrum. And I can’t see that this behaviour of the curves dialog is good for anything. The dialog might well offer the luminosity instead of RGB as the master curve, because most people don’t want or even expect changes in hue, but rather want to lighten or darken parts of the image without a color change when applying curves.
Thanks to you and Warren

Peter
MR
Mike Russell
Aug 22, 2005
"Peter Wollenberg" wrote in message
"Mike Russell" wrote:

"Peter Wollenberg" wrote in message
I was playing with curves a little bit (CS2) and got a strange effect when I applied an admittedly rather uncommon curves transformation to a monochrome gradient. You can see the result at:

http://tinyurl.com/8xh98

Although the gradient is monochrome, it does contain color – just the same color. Your image is a great example of how the RGB master channel alters hue. Try the same curve on the lightness curve of Lab mode and you’ll see no hue change at all.

BTDT 😉
Well, the lesson I learned is to use curves adjustment layers exclusively and set the blending mode to luminosity in order to retain hue and brightness. I still don’t understand the mathematics behind the whole thing completely. I’d have anticipated the complementary hue (H-180 deg) to appear but actually I got the whole spectrum. And I can’t see that this behaviour of the curves dialog is good for anything. The dialog might well offer the luminosity instead of RGB as the master curve, because most people don’t want or even expect changes in hue, but rather want to lighten or darken parts of the image without a color change when applying curves.
Thanks to you and Warren

Although I favor using the Lab color space, luminosity is an excellent way to achieve the effect. For minor adjustments I still find myself using the RGB curve with no bad effects. Certain underexposed images also fare well with the RGB master curve.

Interestingly, luminosity still screws up for CMYK images.

Must-have mockup pack for every graphic designer 🔥🔥🔥

Easy-to-use drag-n-drop Photoshop scene creator with more than 2800 items.

Related Discussion Topics

Nice and short text about related topics in discussion sections