This Experiment with Curves is actually the first of the exercises that are presented in the Luminosity manual. The complete manual can be downloaded here.
When Command is mentioned, Windows users should substitute the Control key.
Even if we are only considering tonal control in Photoshop, Curves is the most important dialogue box.
The question we are going to answer is: Does “Lobster” enable Curves RGB interface to be used in a more meaningful, precise and controlled way than its use with a regular RGB Photoshop file? How you value that answer is up to you.
This experiment begins with an exercise file that should be very easy for you to make yourself.
The file can be approximately 600 pixels square. The file should be in RGB mode and any edited RGB numbers given in later exercise steps are based on Adobe RGB 1998 as the RGB Working Space.
Use Selections and fill (Edit > Fill) the files with the RGB values shown. Double clicking on the foreground colour in the toolbar allows you to open the Color Picker and then Edit> Fill > Foreground Color with the RGB numbers as shown.
Save the file with the name Ex1 1. They should be saved in Photoshop format. The file should have only one layer. The displayed RGB numbers are for your guidance and should not be typed in.
Before running this file in “Lobster”, we will work on it as an ordinary Photoshop file.
1. Make sure that there is only one layer. Check the RGB numbers in the Info palette to make sure that the file matches the illustration.
In the Tool Bar, choose the Color Sampler tool and place a Color Sampler point in both the red and pale grey areas.
Set the Info palette read-out for these sample points to HSB. The read-out mode is changed from the small triangle under the eyedropper icon in each Color Sample window.
2. Open Curves and Command / Click in the red area. This creates a Control Point on the Curve. Try moving the red from this point. Note that the light grey changes if you move the Curve strongly so something is happening. Do not click OK.
3. Try adjusting the red from the Control Points by using Curves as an adjustment layer. Also try the Control Points with an adjustment layer Curve at Luminosity layer mode. Cancel all changes and close Curves. Note your results. Obviously there are some colours that cannot be changed at all by Curve RGB Control Points, no matter what the mode.
4. Changing to Luminosity mode will only correct any Hue changes that occur with RGB Curves or any other adjustment layer. It will not correct the actual accuracy of the edit otherwise the red would have changed.
5. Duplicate the file. Bring Lobster to the front, and File > Lobsterfy the file.
6. The Color Sampler Points should still be available.
7. When adding Adjustment layers you can choose them from the drop down list that appears when you click on this icon at the foot of the Layer palette. It is also possible to choose them from the Layers menu.
RULE FOR LUMINOSITY LAYERS
When using the foot of the Layers palette to add an adjustment layer above the Luminosity layer or above individual layers inside the Chromaticity Set, please:
Hold down the Option key when choosing the adjustment layer.
or Choose them from the Layers menu Layers > Adjustment Layers.
Whichever method you use, when the New Layer dialogue box appears, be sure to check Use Previous Layer to Create Clipping Mask.
THIS IS THE RULE FOR EACH PART OF EXERCISE ONE, AND FOR GENERAL “LOBSTER” USE.
THIS MEANS THAT THE ADUSTMENT LAYER ONLY AFFECTS THE LAYER IT IS DIRECTLY ABOVE. THIS IS IMPORTANT
IT IS PRESUMED THAT YOU WILL DO THIS THROUGHOUT THE REMAINDER OF THESE EXERCISES.
IF YOU WILL HAVE DIFFICULTY REMEMBERING THIS PRINCIPLE, PLEASE MAKE A NOTE.
8. Choose a Curves adjustment layer. Always use the previous Rule for Luminosity Layers.
9. Command / Click on the red and move the Curve from that point.
10. Make a note of your results and compare them to the previous results.
This simple example reveals one of the problems with Curves:
Curves RGB interface is the best tool for adjusting tonality in an RGB image. But the difficulty revealed with this example accurately connecting Curves to the values you have Command / Clicked on - is true to some extent with every single RGB pixel with any Saturation value above several percent, and becomes worse as Saturation increases.
Whilst this first exercise uses maximum Saturation and Brightness, later exercises do not.
Please particularly note that using Luminosity mode did not make Curves work in a regular RGB file. This is a real danger for people deciding that they can “already do” what “Lobster” offers.
THE BASIC PRINCIPLE IS : AS SATURATION OF AN RGB PIXEL INCREASES, THE CONTROL POINT ON THE RGB CURVE BECOMES A LESS AND LESS ACCURATE WAY TO MAKE A MOVE FOR THE CHOSEN PIXEL VALUE.
With the extreme Saturation of the red in this exercise, we can see that without “Lobster”, the RGB values cannot be adjusted AT ALL using the Curves Control Point. But this problem even exists to some degree in pixels of low Saturation.
The lack of control decreases from 100% Saturation, but can still be measured with pixels of only several percentage points of Saturation.
“Lobster” ALWAYS corrects this difficulty, and with “Lobster” ANY value that you Command / Click on, is efficiently and accurately adjustable by the number one editing tool Curves. No pixel is left behind.
As a by-product, note that when the values changed in the red area through “Lobster” the Hue did not change. Adjustments by ANY tool through the Luminosity layer do not change Hue.
QUESTIONS AND ANSWERS
Q. Do I really need “Lobster” to stop Hue changes? I can do this by setting my adjustment layers to Luminosity mode.
A. Correct but you still cannot change the red in this Example, and this points to an important principle the Curve Control Point is not accurately connected to the RGB values. No matter what layer mode that Curve is working in it cannot correct for this failure “after the case”. Whenever Saturation rises above several percent, the Control Points are inappropriately placed.
A. You are reduced to push it and see values that you don’t want to change, change, and values that you do want to change don’t! You just have to play around looking for an acceptable result.
Q. Ah then, I will duplicate the image layer I am editing, place this @ Luminosity mode and do all my tonal editing on that layer.
A. This produces exactly the same result in all RGB files as working on your image layer through adjustment layers @ Luminosity mode. You can’t get to accurate Luminosity editing by taking that road. It cannot be conscientiously recommended.
Q. What about converting my file into Lab mode? Doesn’t the Lightness Curves interface provide me with control ?
A. Yes it does and if you only want to bother with tonal changes, Lab is fine.
Please be sure thought to keep up to date with the issues surrounding Lab.
As well, colour editing in Lab does cause headaches for some people.
Q That’s OK! I will make my tonal corrections in Lab and then convert to RGB for colour.
A. Good thinking but see what happens to your adjustment Curves layer on the Lab Lightness channel - it is merged on conversion to RGB. If you want to re-edit tone after your colour changes you have to start all over again.
There is data now that argues that the change between RGB and Lab is not lossless.
This is part of the debate about Lab.
This is essentially an extract from the Luminosity manuals there are further examples there that do not use such extreme examples of Brightness and Saturation.
You are encouraged to download the entire manual to follow the argument about Curves through to its conclusion,