Curves Are an Abomination

LH
Posted By
Lawrence_Hudetz
Feb 15, 2005
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1081
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I remember a discussion with a famous architect for whom I was being commissioned to do a series of photos of his churches. The curator and I had an argument about presenting the churches as he built them vs my interpretation of them. The architect resolved the dilemma neatly. He said "People are comming to the exhibit to see photographs. if they want to see the churches, they will go to the churches" (Absolutely lovely client he was!)

End of problem. A photograph is just that, and at the most, can be a sort of window to the world, but it isn’t the world as we see it. Thank god!

Curves are fundamental. Even a straight line is a "Curve" albeit a special case.
GA
George_Austin
Feb 15, 2005
Good Grief! The normally rational Jay Arraich must have had a bad-hair day! His Curves tirade is not only way off-base, it is grossly inconsistent as well. If Curves are "abominable" then it follows that Levels must also be abominable, since Levels is Curves in handcuffs. There is nothing Levels can do that Curves can’t do better. Both tools "distort" relative tonal valuesΒ—they’d be worthless if they didn’t. Distortion, which implies perversion, isn’t the right word. The Curves adjustment may counter existing distortion in the input. Or it may exaggerate existing contrast to reveal detail that would otherwise be lost. Or it may bring out detail that was weak in the scene itself where the reproduction of that weak scene was perfect but useless. Hey! This could go on and on.
SS
Susan_S.
Feb 15, 2005
An odd one. One might almost suspect that Jay Arraich was stirring the pot a bit just to get a reaction (the page is described as an editorial). A lot of the stuff on his site (especially for Elements) is very helpful.
TL
Tim_Lookingbill
Feb 15, 2005
I did my first photo restoration job only using curves. Curves are especially useful when minilab scanners have to squeeze the wide gamut and dynamic range of negatives into the comparatively narrower sRGB space.

My histograms of these scans are all wall to wall full of data without clipping. Unfortunately this makes for very high contrast images with blown highlites that can only be fixed using curves. Layer blending and all the other image editing tricks are a PITA to control.

Curves rule for me.
L
Lundberg02
Feb 15, 2005
This guy Jay must have tripped over a logarithm and broken his leg sometime. He’s nuts.
CC
Chris_Cox
Feb 15, 2005
Film (negative and chrome) has curves built into it.

Photographic paper has curves built into it.

Human vision has another set of curves built in.
DM
dave_milbut
Feb 15, 2005
i’m confused. πŸ™‚
LH
Lawrence_Hudetz
Feb 15, 2005
Curves=Transfer Functions.

Sad, but in digital, applying curves (or anything else), means throwing away pixels. In analog, it simply means a change in slope.

I have a curve, several really, built into the scanner so I can mimic the transfer functions of earlied emulsions. Ever wonder why a Steiglitz looked the way it did? Transfer function! Including paper, film lens flare, you name it. So, I added some curves that I can apply if I wish to images I am scanning. Not perfect of course because as Chris points out all have their own T.F. Sometimes the results are ghastly! Sometimes, well, sometimes the magic works. πŸ™‚
GA
George_Austin
Feb 15, 2005
Hi Larry,

"…Sad, but in digital, applying curves (or anything else), means throwing away pixels…"

Hardly sad—Certainly you ought not shed tears about lost pixels when 16 bits are used (even if only 12 of those bits represent sensed data). The extra bits provide enough editing headroom to make the lost data superfluous. Nor should you regret the demise of analog data, since the noise in analog systems is at least as limiting as digitization is in digital systems.
BB
Bert_Bigelow
Feb 15, 2005
I’ve been using ps for about 5 years and still don’t have a handle on curves, hence it’s under utilized in my toolbox

My experience with PS is not as long as Dave’s but I feel the same way. I generally use Levels to make color and contrast adjustments…I’m very comfortable with that tool, and for most images, it’s all I need.
Occasionally, if I have a special problem, I’ll tackle it with Curves, but rarely do I get a result I’m completely happy with. That’s not a criticism of the tool…it’s an admission of my lack of skill and experience with it. It is MUCH less intuitive, as Dave says.

I disagree with Arraich’s rant. Photography iw not constrained to simply record what the camera sees. It’s an art form, and anything…ANYTHING…that the artist wants to do to an image has the same license as a brush-and-oil artist has in creating his image.
At least that’s my opinion.
Bert
LH
Lawrence_Hudetz
Feb 15, 2005
Right George. I use 16 bit as much as possible. Sad maybe not quite the right word.

Somehow, I much prefer stretching to eliminating. But, I no longer have that choice.

You pays your money and you takes your choice.

I paid mine to Adobe, so I guess that’s my choice! πŸ˜€

I guess being an analog person so long gives me a bit of discomfort around digital.
CC
Chris_Cox
Feb 15, 2005
Man Ray also might have a few things to say about using extreme curves….
FN
Fred_Nirque
Feb 15, 2005
Sheesh, what an absolute load of codswallop Jay Airraich has come up with there. It really demonstrates just how the Internet is able to host some incredible garbage written by totally clueless idiots.

So much for his "Photoshop Tips" – if he has as little handle on PS as he has on photography it’s a wonder that he’s been able to get past the splashscreen thus far.

One thing’s for sure – he’s obviously never processed a roll of B&W film, and he certainly has not a clue as to the intent and workings of AA’s Zone System.
MD
Michael_D_Sullivan
Feb 15, 2005
As Chris pointed out, every element of the silver-based photographic paradigm had a curve as its transfer function. Ansel Adams’ Zone System was a brilliant attempt to modify those curves to maximize the amount of information that could be portrayed by a picture. He modified what the camera’s lens "saw" through exposure (to capture the darkest tones), development of the film (to retain those dark tones and avoid blowing out the highlights), printing (selection of paper contrast, exposing for the highlights, dodging and burning), and developing (developing to ensure that all zones come out, generally requiring developing for sufficient contrast among the highlights).

Every stage was lossy. If he had exposed for the highlights, the shadows would have been underexposed and lost at the negative stage. He manipulated the negative to get the most out of it. All of the shadows were exposed alike, whether they deserved it or not. Same with the development of the film. He had no way to reverse any highlight loss caused by overdevelopment or shadow loss caused by underdevelopment. He manipulated the development to get the most out of the picture, not to capture truth. He didn’t have any interest in publishing "true" snapshots. Instead, he chose to expose, develop, and print for dramatic effects that would only have been dimly perceived in a literal translation of light to paper.

Take a look at "Clearing Winter Storm," possibly the finest photograph I have ever seen (I have had a poster of it on my wall for many years). This is not an accurate, natural representation of the light levels in Yosemite. It is the highest photographic art, where the light has been bent to the photographer’s will through complex, multistage techniques, in accordance with his Zone System. In a digital workflow, the Curves tool would be a key element in Ansel Adams’ repertoire for reaching a result such as this.
JJ
John Joslin
Feb 15, 2005
Like Dave and Bert, a lot of people are daunted by Curves. I don’t claim to have mastered them – the deeper you dig the more you find behind that beautiful simple dialogue box – but I was lucky enough to hit on a good basic tutorial which got me off to a good start in the early days.

That’s the trouble with learning Curves, there are a lot of lessons and explanations out there but they are too complicated for most users, who then tend to stick with Levels.

Learning Curves has a steep learning curve.
DM
dave_milbut
Feb 15, 2005
but rarely do I get a result I’m completely happy with. That’s not a criticism of the tool…it’s an admission of my lack of skill and experience with it. It is MUCH less intuitive, as Dave says.

right. and most of the tutorials, instead of spelling out in english how to use the tool correctly, do so assuming the user has a background in photography or whatever secret language you guys often speak! πŸ™‚ If i wanted to be a photographer, i woulda bought a camera and developer kit. I want to use software! It should be as easy (or hard <shrug>) to learn as the rest of the program and not make a left turn in to photograph land!

variable contrast paper

Curves are especially useful when minilab scanners have to squeeze the wide gamut and dynamic range of negatives into the comparatively narrower sRGB space.

My histograms of these scans are all wall to wall full of data without clipping

Transfer Functions

change in slope

mimic the transfer functions of earlied emulsions

not picking on those who used these terms, just pointing out a few examples just from this thread. professionals should talk to like professionals in their own languages, but the uniformity of the conversation makes it tough for an ‘outsider’ to get a feel for what sounds like a great part of the program. sure i could look up all those terms and many others with google or webopedia, but that gets annoying fast after about the dozenth time on page one of a 2 page tutorial.

I WANT to use curves. who wouldn’t the way those who can use it rave about it? but every time i try i get wildly distored images. or i get close to what i want, then tweak a bit, THEN get a wildly distorted image and can’t get back to where i was close. where does all this control come from that people talk about. to me, the curves filter seems like a sloppy mess!

I think adobe needs to make this tool easier for the non-photographer to use and understand. imho, of course. πŸ™‚
DM
Don_McCahill
Feb 15, 2005
Well Dave, to be fair the first half of the product name is Photo.

I suspect that the graphic terms like CMYK, bleed and layers are just as confusing to the photography folks until they learn about them.

Don

PS. If it helps … I did spend a lot of time with my hands in fixer as a teen, and I have trouble with curves as well.
GS
Gustavo Sanchez
Feb 15, 2005
To make good sashimi you’ve got to use Ginsu knives.

And they cut.

A lot.

πŸ˜‰
FN
Fred_Nirque
Feb 15, 2005
Dave, you seriously want to have a look at any good basic photography book (Michael Langford’s "Basic Photography" is a good starting point – pp 217, *223*, 293 [1971 edition], or Google "film characteristic curve") for a concise description of exactly where the curve comes from and what it relates to.

Once you get a handle on that, the whole thing becomes a snap to understand.

Good thing is that silver-based photography is a well refined process that has changed little in the last half-century or more, so reference books such as these are easy to come by cheap as second-hand items, and are entirely relevant in this regard even though they may date back well over 30 years.

Fred.
LH
Lawrence_Hudetz
Feb 15, 2005
Dave, perhaps your curve settings are confusing. I don’t know why default settings on Curves puts the highlights at the lower left and shadows upper right, but I reverse this function by clicking on the arrows on the bottom of the chart. Now, shadows are left, highlights right. Make something darker, pull the curve down, Lighter, pull up. Perhaps it’s because i have worked the film based curves so long, but reversing the PS default seems to be the intuitive thing to do.

OK, now, click on the line somewhere in the center. Pull it up slightly while watching the image. It will lighten up. Pull it down. It will darken.

Compare it to a similar move with the gamma slider in Levels. You obtain the same overall brightness changes, but the local contrast is different. Sometimes levels is best, sometimes Curves are better.

OK, dave, next step. Open Curves. Click on the center, say, 126,126. now, move off center and pick a spot. Move it up and down. You will produce an "S" shaped curve. With a normal looking "S", the contrast increases; and inverted "S" it decreases. Pull the center around a bit also and you will see some brightness changes as well.

Do this with small moves. For instance, grab the center (126) and move it 10 points higher, or lower at first. You can do this with the numbers pad or mouse.

Systematically go to different portions of the curves and try again. Note how the image behaves different.

It’s a systematically approach, and one that, once you master it, you will wonder how you ever got along without it.

Now if you really want to have fun, pick the pencil and make the wildest looking curve you ever saw!

There are a multitude of possibilities (did I mention solarization? A cinch!).

Have fun!
DM
Don_McCahill
Feb 15, 2005
Lawrence

I don’t think Dave is misunderstanding curves to that degree. (I know I don’t). The point I think we are making is that curves are not a simple tool to master.

I can solarize with curves, and improve an image by playing with the curves in the manner you explain. The problem is that I don’t know exactly what is happening, or why. And, as Dave mentions, almost all tutorials that try to explain such get deeply into photographic terminology.

Don
LH
Lawrence_Hudetz
Feb 15, 2005
Point taken, Don.

But then, PS itself is a big Tool, and boy, will I ever master it?

The only way I know to master it away from photo terminology is to do it mathematically, that is plot the input data (horizontal) against the output (vertical) using a complex wave form. That wave form can represent anything; light, sound, or an abstract mathematical function. For our purposes, an evenly spaced step tablet in numerical form will do nicely.

Another way to think about it: See a transfer function as a mirror. A flat mirror projects a faithful rendition. Go to a house of mirrors and see the distortions. S shaped mirrors are the most interesting, IMO.

Visualize them as bits being pulled apart or quashed together.
DM
dave_milbut
Feb 15, 2005
I don’t think Dave is misunderstanding curves to that degree. (I know I don’t). The point I think we are making is that curves are not a simple tool to master.

No i think i am. πŸ™‚

I can solarize with curves, and improve an image by playing with the curves in the manner you explain. The problem is that I don’t know exactly what is happening, or why. And, as Dave mentions, almost all tutorials that try to explain such get deeply into photographic terminology.

‘zactly

I’ll give this a try tonight larry. thanks. i’ve never heard the left is shadows/right is bright thing before! (or that ps uses the reverse!) at least not in terminology i can understand. πŸ™‚

I’ve seen basic tonal adjustments like this, but where do adding multiple points and adjusting them come in? How does that map (reliably) to the image you’re working on. Does curves work on colors like hue/sat? or is it strictly for brightness/contrast/lightness/darkness?
C
ChristopherD
Feb 15, 2005
Dave,

By default the Photoshop curves adjust all three RGB (red/green/blue) channels, so basically it is adjusting the brightness. However, there is a drop-down menu at the top of the curves box that lets you choose if you want to adjust all three RGB channels, or just the red, green, or blue channel of the image.
DM
dave_milbut
Feb 15, 2005
However, there is a drop-down menu at the top of the curves box that lets you choose if you want to adjust all three RGB channels, or just the red, green, or blue channel of the image.

ah yes, i recall that now. thanks.
MV
Mathias_Vejerslev
Feb 15, 2005
Also, a Curves Adjustment Layer can be put into any mode.
LH
Lawrence_Hudetz
Feb 15, 2005
Play with RGB together first, dave.

In fact, take an image and convert to grayscale first, then play. You will get the concept easier, IMO, as sometimes playing with curves on an RGB image also shows subtle color shifts as well, possibly confusing the issue.

Curves are more than just for brightness!
LH
Lawrence_Hudetz
Feb 15, 2005
Just saw Mathias’ post.

Do your playing in Adjustment Layers, as then you can go back and re tweak.
G
Gener
Feb 15, 2005
Well, he does have a nice quick tutorial on quick mask, something I haven’t used that much, but will be working on in the future thanks to it.
MV
Mathias_Vejerslev
Feb 15, 2005
Lots of goodies on Jays site. With this article.. Well, he made you look!
G
Gener
Feb 15, 2005
That he did, Mathias, that he did πŸ™‚
TL
Tim_Lookingbill
Feb 15, 2005
This is what I find curves most useful for as shown in this sample edit of my 35mm Fuji minilab scans from Walgreen’s one hour photo:

<http://www.uploadyourimages.com/view/567577curvdemo.jpg>
FN
Fred_Nirque
Feb 15, 2005
Larry,

It would appear that PS chooses to default curves as plotted for positive images as in transparencies and as used in the printing industry, hence the "reversed" curve. Makes sense, I suppose, as we’re looking at a positive image on the screen.

I reckon us older B&W negative hacks automatically do the transposition in our heads, mainly from a familiarity of that situation brought on by visually "reading" a negative before printing it, unlike the digital age where the negative image stage is a thing of the past and everything is presented as a positive.

It’s always the first thing I reset after I delete prefs, so the shadows are on the bottom left where they belong as if you were plotting the curve of a B&W neg with a densitometer. It just makes more sense to me that way. The software writers also obviously saw the need to pander to the old guard by including that setting.

Fred.
SS
Susan_S.
Feb 15, 2005
I do the same (reverse the curves so lifting them makes them lighter, shadows are on the left hand side – makes more sense to me). I use a fairly ad hoc way of using curves, working on the combined channel. With it set up so the darkest areas are on the left, then if you make the curve steeper over a given range then you are increasing the contrast over that range; if you flatten the curve you are reducing the contrast.
So I look at the image and decide for example what area I want to lighten; then with the curve dialogue open and holding the (control? – I’m slumming here, I use a Mac!) key down and clicking with the eyedropper you can add handles onto the curve which correspond to the brightness level of that point in the image – then just slightly shift up the control handle (you can use the arrow keys) Similarly for an area you wish to darken, shift the handle down. And if you want to increase contrast over a range use the eydroppers and the (control) key to add handles at each end of the range and then increase the slope of the curve over that range by moving the bottom handle down and the top one up (all ups and downs rely on having the shadows at the bottom end of the curve!)
The key is very small shifts of the curve; and it does affect colours if you do larger shifts – if the apparent increase in saturation (don’t ask me to explain, I don’t know why it happens!) is offensive then I put the curves adjustment layer to luminosity mode (or fade a curves adjustment to luminosity)
LH
Lawrence_Hudetz
Feb 16, 2005
Good point about Luminosity. I use it also, but it’s almost mechanical so I forgot to mention it.

I frequently do a corner burn and I usually do it using Luminosity, as it doesn’t then increase the color saturation.
DM
dave_milbut
Feb 16, 2005
you can add handles onto the curve which correspond to the brightness level of that point in the image

what point in your image? how does the curves dialog relate to the image on your screen?
SS
Susan_S.
Feb 16, 2005
Dave -The point on your image that you clicked with the eyedropper. The curve shows a graph of each current possible level of luminosity for the composite channel (horizontal axis) plotted against the luminosity you will end up with after the curves adjustment occurs (vertical axis) running from 0 to 255. (again I’m assuming that you have curves set up with the shadows (level 0) on the lhs and the brightest areas (255) on the rhs of the graph). So if the curve does nothing to the image it’s a 45 degree line passing through (0,0) and (255, 255). If you want to brighten the image you may want a pixel that started with luminosity of say 170 to end up at say 200 – plotting this gives a curve which still goes through (0,0) and (255, 255), but will be bent upwards in the center going through (170,255). Any pixel with brightness other than 0 or 255 will end up brighter once the curve is applied.

You can similarly have more complicated curves which drop the brightness of pixels with lower luminosity and increase the brightness of lighter pixels, thus increasing contrast in the middle regions of tone. (I’m still hazy about exactly when I should use brightness or luminosity – if I’ve used the terms incorrectly , I apologize!)
DM
dave_milbut
Feb 16, 2005
hmm… processing… pondering… πŸ™‚
MD
Michael_D_Sullivan
Feb 16, 2005
In my copy of CS, the curves have the shadows at the lower left and the highlights at the upper right. Either the default changed from 7 to CS or I clicked on a setting to do this somewhere and don’t remember doing so.
SS
Susan_S.
Feb 16, 2005
You just click on the little double arrows on the horizontal axis of the graph to switch between the two methods of display. I can’t remember which is the default!
LH
Lawrence_Hudetz
Feb 16, 2005
Default is usually the shadows upper right.

Dave, in order for you to gain an understanding of the relationship between the image and Curves, you have to practice. It’s not at all conclusive simply by looking at the dialog.
JJ
John Joslin
Feb 16, 2005
Dave, just for a bit of fun, click on the little pencil symbol at bottom right of the x-axis and draw your own crazy curves with Preview on.

Andy Warhol look out!

Seriously I used to get my students to play around with this for a bit, just to get a better feel than mathematical/technical terminology would give.

Edit: Of course, if you get an effect that you like, you can save it for use on other pictures.
PC
Pierre_Courtejoie
Feb 16, 2005
I remember that someone here gave a nice tip: switch to pencil mode, click a single point right next to the curve, and switch back to curve mode… instant control points…

Dave, you need to click and hold while you move on the image with the eyedropper to see the point on the curve(of course, you need to be in curve mode). Depress the mouse button and CTRL+click on the image itself to add on the curve.

See this page on Trevor Morris’ site for more curves shortcuts: <http://user.fundy.net/morris/photoshop144.shtml#adjustments> (the number 3 update ! )
DM
dave_milbut
Feb 16, 2005
dammit! i was gonna bring in my copy of 7 and play around here at work today b/c the boss is out! <shoot!>

again thanks pierre and larry (everyone) i really need to sort this out. wish i had brought the sw in as i didn’t have time to mess with it last night! πŸ™
RK
Rob_Keijzer
Feb 16, 2005
Dave,

Make sure your boss isn’t a regular on this forum without you knowing that! πŸ˜‰

Rob
DM
dave_milbut
Feb 16, 2005
well i still work, but i surf while compiling. yea, that’s the ticket! πŸ™‚
D
deebs
Feb 16, 2005
See that’s the trouble in 21st Century

Everyone assumes that the worker is shirking on a forum

And os course in Dave’s case it is evidently not so.

But what presumptions have been made?

What assumptions have been made>

OK i’ll phrase them here: what’s the Boss doing on the forum?

LOL

8/
BB
Bert_Bigelow
Feb 16, 2005
what’s the Boss doing on the forum?

Checking up on Dave! πŸ™‚
DA
Dante Aligheri
Feb 18, 2005
On Wed, 16 Feb 2005 04:29:10 -0800,
wrote:

I remember that someone here gave a nice tip: switch to pencil mode, click a single point right next to the curve, and switch back to curve mode… instant control points…

Dave, you need to click and hold while you move on the image with the eyedropper to see the point on the curve(of course, you need to be in curve mode). Depress the mouse button and CTRL+click on the image itself to add on the curve.

See this page on Trevor Morris’ site for more curves shortcuts: <http://user.fundy.net/morris/photoshop144.shtml#adjustments> (the number 3 update ! )

If you want to understand and use curves, you need to get the book Professional Photoshop 7. It’s about professional color correction, by Dan Margulis. Look on Amazon. I don’t pretend to understand curves. This book is beautiful, I reread it just for fun. I think I understand everything he says, then my mind gets muddy 15 minutes later. I’m sure you will do better. He says curves is the only tool you should be using, makes me feel guilty when I do a quick Levels adjustment. Try the book, not cheap but you will like it.
– ron
D
d23
Feb 18, 2005
I really have read the documentation but I can’t find or am to stupid to understand any directions about using other paper. I’m using Photosho7 and an Epson R800 printer. I have some non-Epson paper I want to use and there is supposed to be a paper profile I can load–somewhere–to make the printer work well with it. I have such a profile which I created with Monaco. There is a file ith the right name in Windows\System\Color. So now what? I can’t find anywhere in the printer settings or in Photoshop to load that file.
Thanks
Charles

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