Your Curves

[Mark D Segal]

I have created this thread for any feedback and discussion of my essay "Do Your Curves Throw You a Curve?", published on Luminous-Landscape today. It would be ideal if all contributors to this discussion posted in this thread, for continuity and ease of reference.

[OnyimBob]

I'd love to read your article but, as has happened once or twice lately, there's nothing to read. The home page "what's new" indicates that it's there --- but it ain't!
I'll keep loking,
Cheers,
Bob.

[Schewe]

Scroll down...

[Mark D Segal]

There's now a link from "What's New" Your Curves of July 25th - this takes you to the Intro from which you can download the article.

[digitaldog]

Its a PDF which in this case, is really useful to print out (in color if possible) and read. While I think most articles on this site can be read on line, this one is very through and too long to be on line,  and really needs to be printed and studied carefully.

[FredT]

Very nice, Mark.  Thank you.  I'll have to do as Andrew suggests and print it out for a more thorough study.

[Eric Myrvaagnes]

Excellent essay, Mark.

Having gotten used to Capture One, RSP, and lately DxO, I've never really given CR a decent try (I didn't like whatever version I first tried, way back whenever.) But your well-written essay makes the new CR look very appealing and not as baffling as I expected.

Thanks for all your good work on this.

[Ray]

Mark,
This is an attempt at a very thorough treatment of aspects of curves and I've downloaded the pdf as have others. But my feeling is, the bottom line is, you must experiment. Using curves is an extremely heuristic process. The slightest movement of any point on a color specific curve can produce a significant change.

One feature I would very much like in PS is an ability to zoom on the image whilst the curves dialog box is open. That way, one could get a clear idea of how localised changes affect the over all image.

[digitaldog]

One feature I would very much like in PS is an ability to zoom on the image whilst the curves dialog box is open. That way, one could get a clear idea of how localised changes affect the over all image.
[a href=\"index.php?act=findpost&pid=129817\"][{POST_SNAPBACK}][/a]

You can do this (have been able for many versions). You can zoom/pan using the standard key commands while all dialog boxes are open. With Levels, curves etc open, just use the Command/Control and Option/Alt keys to invoke the zoom in or out cursor and click away.

[Ray]

You can do this (have been able for many versions). You can zoom/pan using the standard key commands while all dialog boxes are open. With Levels, curves etc open, just use the Command/Control and Option/Alt keys to invoke the zoom in or out cursor and click away.
[a href=\"index.php?act=findpost&pid=129840\"][{POST_SNAPBACK}][/a]

Eh! Can you be more specific. I have an image open at a specific degree of enlargement. I open a curves dialog box and my curser changes from the zoom tool to the eyedropper. I'm on a windows platform. Ctrl or alt, ctrl and alt, do nothing to change this whilst the curves box is open.

[digitaldog]

Eh! Can you be more specific. I have an image open at a specific degree of enlargement. I open a curves dialog box and my curser changes from the zoom tool to the eyedropper. I'm on a windows platform. Ctrl or alt, ctrl and alt, do nothing to change this whilst the curves box is open.
[a href=\"index.php?act=findpost&pid=129843\"][{POST_SNAPBACK}][/a]

You need the cursor to be a zoom or pan tool using the key commands:

Space bar + Command/Control is zoom in
Space bar + Option/Alt is zoom out
Space bar alone is pan

You have to use the above two key combo's and click on the image. Use just space bar to pan about.

In CS3, there's even more control over palettes and dialogs thanks to the UI design.

[Ray]

Ah! The space bar. I've got a lot to learn   . Thanks for that.

[Eric Myrvaagnes]

Eh! Can you be more specific. I have an image open at a specific degree of enlargement. I open a curves dialog box and my curser changes from the zoom tool to the eyedropper. I'm on a windows platform. Ctrl or alt, ctrl and alt, do nothing to change this whilst the curves box is open.
[a href=\"index.php?act=findpost&pid=129843\"][{POST_SNAPBACK}][/a]

Ray,

Try holding CTRL while pressing either + or - on the numeric keypad (to zoom in or out, respectively.) Works fine for me (CS2 on Win XP Pro.)

[Eric Myrvaagnes]

When I try CTRL plus SPACE and then click, the Levels dialog box disappears before the zoom happens, but with CTRL plus "+" (on numeric keypad), the zoom happens immediately and Levels stays open.

Same with Curves. CTRL together with numeric + and - seems much nicer to me. To pan just use the horizontal and vertical scroll bars.

[Ray]

When I try CTRL plus SPACE and then click, the Levels dialog box disappears before the zoom happens, but with CTRL plus "+" (on numeric keypad), the zoom happens immediately and Levels stays open.

Same with Curves. CTRL together with numeric + and - seems much nicer to me. To pan just use the horizontal and vertical scroll bars.
[a href=\"index.php?act=findpost&pid=129851\"][{POST_SNAPBACK}][/a]

Eric,
Yes, you're right. The curves dialog box does have a tendency to disappear when using the space bar. Ctrl with + or - works just fine.

[mitra]

I'm fascinated by this discussion, but still in the Dark Ages (CS2). Is there an article I can read on parametric curves and how one determines, given image's characteristics, where to move them along the X axis?

[Mark D Segal]

I'm fascinated by this discussion, but still in the Dark Ages (CS2). Is there an article I can read on parametric curves and how one determines, given image's characteristics, where to move them along the X axis?
[a href=\"index.php?act=findpost&pid=129875\"][{POST_SNAPBACK}][/a]

There is excellent resource material on Lightroom (for example Michael and Jeff's DVD download on this website, plus recent books by Martin Evening and Scott Kelby on Lightroom. Also, Ben Willmore's CS 3 Up to Speed Chapter 2. I point you to Lightroom because the Develop Module is essentially the same as what you find in Camera Raw 4.x.

But frankly, the very best thing to is experiment. Make copies of a few raw files, put them in another folder, don't worry about what happens to them as you play - all you have to lose is some duplicate pixels if the worst happens (but needn't).

Go to the parametric curve and before adjusting anything on the X axis under the box, make an S Curve by decreasing the Shadow slider a lot, decrease the Darks slider some but less, increase the Lights slider and decrease the Highlights slider. Now you have clearly demarcated brighter and darker zones in the image - by design. Make sure you've created a steep enough S so the contrast between these zones is very obvious.

Now start moving each zone slider under the Curves box back and forth, and you will see relative to where each slider demarcates on the histogram how the tonality of the two adjacent zones changes while you adjust.

The next thing to do is to go to the Point Curve tab, select the Eyedropper tool, press Control (Command on a Mac I think) and as you left-click the mouse, mouse-over what appear to be the darker and lighter parts of the image and watch on the tone curve where the distinctions between light and dark lie. Remember the placements, or do a rough diagram with a pencil and paper and mark Xs on the curve about where you see the breaks between highlight/lights, lights/darks and darks/shadows. Now go back to the Parametric curve, and adjust your sliders so that if you were to drop verticals upward from the sliders, they would intersect the Curve about where you placed thoses Xs. It's a bit awkward (but it works) why I suggested in my conclusions that this capability we have in the Point Curve to demarcate the zones from analyzing the image should also be conveyed to the Parametric Curve where it is actually useful.

But you need not stick with that demarcation. It depends on what effects you wish to get. You essentially have a matrix of seven controls in that parametric dialog which gives you huge flexibility to combine any set of placements that best meets your taste and your requirements. After some experimenting, you'll get the hang of it, and it becomes somewhat second-nature.

Enjoy!

[laughfta]

I inadvertently posted to a new topic, though my questions belong here. Meanwhile, Mark answered one of them so I will cross that off my list.  Thanks, Mark  

From Mark's essay:

QUOTE
CR  Curves have a hue lock. They map the minimum and maximum RGB values (in linear ProPhoto RGB) through the tone curve, and the middle value is interpolated to exactly preserve hue. The PS-RGBc does not have a hue lock. But setting the PS-RGBc to Luminosity Blend Mode preserves the colour of the underlying layer. Using the Lab L*Curve for this purpose has only fair colour consistency as L* is changed holding a* andb* constant.


1. Does the CR curve, then, exactly preserve hue even with the mild saturation changes?
2. If so, must one continue to work in ProPhotoRGB when in PS to realize this advantage? Can Adobe RGB also preserve exact hue?
3. I think I understand that (in LAB) a* and b* do not remain constant as to hue as the L* is changed; they drift slightly. But in the PS-RGB curve is the hue exactly preserved in the Luminosity Blend Mode? Would this be this considered "locking" the hue, at this point?


And thanks for the great article!

[Mark D Segal]

IFrom Mark's essay:

QUOTE
CR  Curves have a hue lock. They map the minimum and maximum RGB values (in linear ProPhoto RGB) through the tone curve, and the middle value is interpolated to exactly preserve hue. The PS-RGBc does not have a hue lock. But setting the PS-RGBc to Luminosity Blend Mode preserves the colour of the underlying layer. Using the Lab L*Curve for this purpose has only fair colour consistency as L* is changed holding a* andb* constant.
1. Does the CR curve, then, exactly preserve hue even with the mild saturation changes?
2. If so, must one continue to work in ProPhotoRGB when in PS to realize this advantage? Can Adobe RGB also preserve exact hue?
3. I think I understand that (in LAB) a* and b* do not remain constant as to hue as the L* is changed; they drift slightly. But in the PS-RGB curve is the hue exactly preserved in the Luminosity Blend Mode? Would this be this considered "locking" the hue, at this point?
And thanks for the great article!
[a href=\"index.php?act=findpost&pid=129888\"][{POST_SNAPBACK}][/a]

Hi Gloria, thanks - glad you're enjoying the article. Let me try answering these in order:

(1) My understanding of this based on the information I reported in the Principles section of the article, and based on the test I did (page 17) seems very much to be YES.

(2) ProPhoto and Adobe RGB(98) are both RGB colour working spaces, the former being much wider than the latter. It stands to reason that if by converting your working space from the former to the latter you clip a channel or two in ARGB(98) that were not previously clipped in Pro Photo, the affected pixels should have a hue shift because the colour composition has changed. But to see this in a print, the affected hues would have to have been in printer gamut.

(3) Based on all I've read about it and the experiments I've done, using an RGB composite curve in Luminosity Blend Mode is supposed to preserve the hues of the underlying layer. I don't know whether it does so EXACTLY for every conceivable pixel value.

[mitra]

There is excellent resource material on Lightroom (for example Michael and Jeff's DVD download on this website, plus recent books by Martin Evening and Scott Kelby on Lightroom. Also, Ben Willmore's CS 3 Up to Speed Chapter 2. I point you to Lightroom because the Develop Module is essentially the same as what you find in Camera Raw 4.x.

But frankly, the very best thing to is experiment. Make copies of a few raw files, put them in another folder, don't worry about what happens to them as you play - all you have to lose is some duplicate pixels if the worst happens (but needn't).

Go to the parametric curve and before adjusting anything on the X axis under the box, make an S Curve by decreasing the Shadow slider a lot, decrease the Darks slider some but less, increase the Lights slider and decrease the Highlights slider. Now you have clearly demarcated brighter and darker zones in the image - by design. Make sure you've created a steep enough S so the contrast between these zones is very obvious.

Now start moving each zone slider under the Curves box back and forth, and you will see relative to where each slider demarcates on the histogram how the tonality of the two adjacent zones changes while you adjust.

The next thing to do is to go to the Point Curve tab, select the Eyedropper tool, press Control (Command on a Mac I think) and as you left-click the mouse, mouse-over what appear to be the darker and lighter parts of the image and watch on the tone curve where the distinctions between light and dark lie. Remember the placements, or do a rough diagram with a pencil and paper and mark Xs on the curve about where you see the breaks between highlight/lights, lights/darks and darks/shadows. Now go back to the Parametric curve, and adjust your sliders so that if you were to drop verticals upward from the sliders, they would intersect the Curve about where you placed thoses Xs. It's a bit awkward (but it works) why I suggested in my conclusions that this capability we have in the Point Curve to demarcate the zones from analyzing the image should also be conveyed to the Parametric Curve where it is actually useful.

But you need not stick with that demarcation. It depends on what effects you wish to get. You essentially have a matrix of seven controls in that parametric dialog which gives you huge flexibility to combine any set of placements that best meets your taste and your requirements. After some experimenting, you'll get the hang of it, and it becomes somewhat second-nature.

Enjoy!
[a href=\"index.php?act=findpost&pid=129882\"][{POST_SNAPBACK}][/a]

Thanks for your extensive reply.  I agree with the experimentation route but as of now only have CS2.  I'm creating a CS3 folder with tutes and other information and will save your comments there.