We're talking about a micro adjustment here and Curves just don't do the same job.
Ain’t that the truth (expect your idea is hardly a macro adjustment).
"false profiles" (or whatever you want, or don't want, to call them) are a powerful tool. I use them all the time, mainly for unblocking shadows in gamma 2.2 files (Adobe RGB etc).
Lets look at making a "false profiles". In Photoshop’s Custom RGB dialog (Color Settings), you have the “precision” of making a “false profile“ from say 1.80 to 1.79 or 1.81 or for sRGB, 2.19 etc. Maybe that is what gives Gromit the idea he has some precision here. The tiny numeric values make tiny, insignificant differences in our testing. Lets do that. Make a “false profile” for Gromit‘s workflow. Build a profile at gamma 2.1 and save it to disk after naming it “False sRGB tweak“.
Make a new document in Photoshop: sRGB, 100x100 pixels. Fill with Lstar 50 and place an eyedropper sampling point in the center. Set the info to read Lab, you’ll see of course its Lstar 50.
Go into the Assign Profile
command. Make sure its set for sRGB then toggle to Adobe RGB (1998). Same Lstar 50. For fun, toggle to Apple RGB, or ColorMatch RGB and notice the predicted values using this assignment are L57. The so called micro adjustment of moving from a 2.2 TRC in sRGB to a 1.8 gamma curve. Lighter yes, but was that too much? Try assigning the 2.1 profile. Lstar 50 is now Lstar 52, Now try making such a micro adjustment in curves. If you are careful as anyone who uses curves can be, you can set a single point dead center in the curve and using your keyboard, move that point one movement up or down to produce (Input 50/output 51), and examine the Lab values. Before and after Lstar now reads 50/49 (or depending on the direction of said curve, 50/51 with input 50/output 49). A pretty precise macro adjustment in terms of the results on this 8-bit file. More precise than the assignment using a gamma curve in a profile. But wait, you’ll see that viewing this at Lstar 50 only tells us one part of the story (that we can
produce a finer adjustment on this solid color using curves, dismissing part of Gromit’s theory that a “false profile” is a marco adjustment and curves are not).
Now lets try using this idea of a macro “false profile“ as defined by Gromit on more tones. We’ll start with an image with dark tones, an Lstar 2 to Lstar 50 gradient, (keep the a&B star zero). Its in sRGB as Gromit stated that he uses a "false profiles" in so called gamma 2.2 files to unblock shadows. This will represent some tonal areas of our blocked up shadow area extending into midtones. Again, with the info palette set, use Assign Profile
, pick a “false profile“ of say 2.0. You can try using the tiny tweaks when building a “false profile“, like 2.19, 2.15 etc but the results are totally insignificant in the lower area of the images that we are supposed to be "fixing." Try 2.0 and here you see the big issue with Gromit's idea. Yes, as you assign the 2.0 profile, you see the image lighten up, but where?
Around Lstar 20. Everything below isn't touched. And since this IS a curve, as you move up in Lstar, the effect is greater! Like a Photoshop curve, this curve is one where the point defining the curve has more of an effect and it gradually decreases from that point in either direction (duh, that’s why its called a curve). If you wanted to open up the blocked up shadows, seems you'd want to do this far lower in the tonal areas (say Lstar 4-6). But this is a gamma curve! Gromit is actually altering and affecting the data far more in the midtowns than the shadows! By the time we get to Lstar 50, the “false profile“ curve has moved those tones to Lstar 55. Any savvy Photoshop users could lock down curves where they don't want the curve to affect an area of the tone curve. They could decide with absolute precision where they want a curve to be in its max position. A “false profile“ has no such control. Its like using a axe when you really need a scalpel. And as Mark pointed out, you can adjust these dark tones using a myriad of other Photoshop techniques where you could control exactly where you start, stop and how far, you adjust the values, in an increment as little as 1 or two values.
Are using curves to open up blocked shadows the best method? Well the best method is not to block up shadows in the first place, something Gromit has never answered my query about. Its pretty clear that using curves has more control, percision, allows the use of adjustment layers, layer masks, blend-if control etc that far, far exceeds anything anyone can accomplish with a "False Profile" and for good reason. Profiles were never intended to solve this particular problem. Unfortunately, dear Gromit doesn't see that.
Another fun tidbit that illustrates how the sRGB TRC isn't the same as a 2.0 gamma "false profiles" and how this could be an issue. Take the gradient in sRGB and place a sample point in the center. Now duplicate it and Assign Adobe RGB (the numbers don't change, the Lab values do indicate the new translation). Since you duplicated the image, the sample point in the dupes stay exactly in the same position. In my case, I see that the sRGB profile has an Lstar of 23, the Adobe RGB doc has an Lstar of 21. Now make a "false profiles" using 2.0, in fact just open sRGB as supplied by Photoshop and select Save RGB...
to build a new "false profiles". Duplicate the Adobe RGB doc, Assign this "false profiles", you get the same Lstar as the Adobe RGB doc (Lstar 21 in this case) but not the sRGB doc due to the fact that sRGB is defined using a more complex curve (and it was specified for a reason). It is not a simple gamma curve like all these "false profiles". Gromit could actually be reducing the so called blocked up shadows in his sRGB document, certainly any area where sRGB calls for its shadow curve, with a "false profiles", not the other way. And not until the Lstar gets far above the value anyone I believe would consider a dark area of an image would the Lstar values now get lighter. Talk about counter productive!
Interesting update, playing around with a much higher rez file, Lstar 2 to 50 gradient, then posterized into 10 steps. With the 2.0 “false profile” look what happens in the deeper shadow tones (the opposite of what Gromit expects):
0/0 (0 change we did map Lstar 2 to zero with the Posterize command which may explain the next to results but I doubt it), 3/2 (-1), 9/7 (-2!), 15/15 (0), 21/22 (1), 27/28 (1), 32/34 (2).
With a “false profile” using 1.8.1, a much larger “curve”:
0/0 (0), 3/4 (1), 9/13 (4), 15/21 (6), 21/28 (7), 27/34 (7), 32/40 (8, curve is maxing out here, far from our shadows), 38/46 (
, 43/51 (
, 48/55 (7).
Clearly this idea of a “false profile” affecting shadows is bogus when its producing far larger numeric changes in late quarter/midtones. In the case of the 2.2 to 2.0 “false profile”, I’m actually seeing more deep shadow tones blocking up, not the opposite but this could be from the Posterize command, used to make it easier to see the effect of the profile although I suspect that’s what happened to the Lstar 0, it doesn’t explain why the “false profile” makes a negative value here. What a complete mess of a tonal correction.