When he describes setting up his scanner and "Optimizing lamp lightness", he wrote: "The exact lamp setting for my unit is: +8 units overall, plus an additional +6 and +4 units respectively for green and blue." This approach is contrary to the usual recommendation and contrary to my practice. Is it wise to make color corrections before profiling? I'll need to read his article more carefully, but a lot of what I've glanced at raises questions.
That's what I'm looking for – articles that raise lots of questions. It's only by questioning that you learn. I haven't gone through the entire bragit.com
article yet, as I got sidetracked by all the links.
My initial answer to your query: "Is it wise to make color corrections before profiling?" is that it shouldn't matter. The idea of profiling is to correct for errors, so if someone wants to introduce gamma or colour corrections while scanning, it shouldn't make much difference, as long as those same settings are used for all scans. The reason why it may be desirable to introduce corrections while scanning, is to bring the scanned image of the target as close as possible to the real IT8 target. That way, the profiling software has less correction to do. One of the links that bragit.com
provides is to computerdarkroom.co
m, where it states:Those of you reading this who already own a scanner profiling application… will be wondering if the above procedure for determining the ideal gamma is either necessary or desirable. The simple answer is NO and Yes in that order! The reason it's not necessary is the IT8 calibration will always reproduce grey-scale patch GS11, etc, as per the associated reference data file irrespective of what gamma value you choose to make the original scan with. In effect the IT8 calibration process will produce the optimum Tone Curve.
Sounds reasonable to me. But his explanation of the "Yes" part – is gamma correction desirable, is not at all clear. Something to do with loss of shadow detail in some cases for some scanners.
Getting back to my original post about gamma: I assumed that a setting of Gamma 1 would give similar scans for all scanners. Of course I imagined different colours, different brightnesses, but basically I assumed a similar general look, no matter who made the scanner. It looks like that is a wrong assumption. When I set gamma = 1, the scan is extremely dark – the mean value for the GS11 patch is 48. GS11 should scan in the range 100-115. That's something I didn't realise, and which is not explained anywhere that I could find in the Coca or Argyll documentation. Here's a very clear explanation from computerdarkroom.com
(Optimising the Scanner Tone Response Curve):The purpose of scanning the grey calibration strip [of the] IT8 calibration target… is to find a the ideal Tone Curve or gamma-gradation value for your particular scanner. The simplest approach is to adjust the gamma value so that the grey patch GS11 on the IT8 calibration slide has an average RGB value in the range 100-115. This means scanning the IT8 at different gamma values then selecting the one that is closest to the optimum. You should also find that the average RGB values for patches GS0 (white patch) is in the range 235-250 and GS23 (black patch) is in the range 10-25. In each case it is better to avoid values outside these ranges.
Dean, it looks like your scanner has a different Gamma 1.0 than mine. All this talk from me about needing to vary gamma might come down to the fact that your Gamma 1 may be equivalent to my Gamma 1.8. In your case you see good results at Gamma 1 (GS11 ~100), whereas I need Gamma 1.8 to achieve the same result. If I use Gamma 1, the scan is so far away from what it should be, the profiling introduces small errors at the extremes.
How can you tell which of our scanners is actually scanning at a true Gamma 1.0? We both have IT8 targets that would be pretty much identical, yet my Gamma 1.0 scan (I'm assuming) is seriously darker than yours. Why? Is my scanner faulty, or is your scanner applying corrections before you are given the ability to adjust?
The only way I can think of to work out which scanner if actually giving true Gamma 1.0 is to go back to the maths:
Y/Yn = ((L* + 16)/116)3
Where Y/Yn is the relative luminance (Yn is max white), and L* is the brightness value in Lab coordinates – which are available in the data file for each IT8 target. I have just scanned my Velvia target at Gamma 1.0, 2.0 and 2.2 with these results for GS0, GS11 and GS23 (in order for each gamma):
Gamma 1.0 … 235, 40, 1.2
Gamma 2.0 … 244, 100, 15.8
Gamma 2.2 … 245, 109, 20.0
Taking the computerdarkroom.com
figures as a guide, that means I should be using Gamma 2.2. It would be interesting to see your readouts for Gamma 1.0.
Now the maths to work out if my Gamma 1.0 is actually gamma = 1.0. Assume that it is. If so, the 0-255 scale is linear would be linear with respect to luminosity and be equivalent to the "Y" values in the formula, with Yn being the maximum white for the IT8 target (GS0). From the IT8 data file, the L* values are:
GS0 = 92.18
GS11 = 41.95
Maximum white in the Velvia IT8 target has an L* value of 92.18. Plugging this into the formula gives: Y/Yn = 0.811. The maximum luminance of the Velvia brightest white is actually only 81.1% of reference white (I'm not sure what that is defined as). But as this is all relative, it doesn't matter anyway.
What is the relative luminance of GS11? It is defined as having an L* value of 41.95, which gives Y/Yn = 0.125.
The luminance ratio of GS11/GS0 is therefore 0.125/0.811 = 0.154. If my scanner is truly scanning at Gamma 1.0, then the RGB value of GS11 (measured in Photoshop as 40 – see above) should be 0.154 times the RGB value of GS0 (measured as 235):
Measured GS11: 40
Theoretical GS11: 0.154 x 235 = 36.
Indicating that my scanner when set to Gamma 1.0 is giving a close match to a linear series of luminance values (I consider within 10% close, given all the inaccuracies involved). A graph of the results for several greyscale patches gave a straight line (see attachment). Dean, it would informative if you could run through the same calculations for your Gamma 1.0 scan.
My understanding of colour theory is only superficial, so there may be errors in the above analysis. Anyone care to comment? What I'd like explained is: why my Gamma 1.0 scan appears much too dark on my monitor. My theory is:
1. It is a true Gamma 1.0 scan, meaning that linear values of luminance are being saved to the image file.
2. In Photoshop, my default colour space is sRGB, and that is the space in which the image is being shown. sRGB has a gamma of 2.2, meaning the input-output relationship is a power-law of the form Out = In2.2 (see attached graph).
3. If the above are true, then my image is going to have a lot of dark areas because each RGB value will be mapped downwards. Only if the profile has a linear characteristic matching the RGB data will the image display properly.
To confirm the above, I generated a linear sRGB profile following these instructions (Making a Linear IC Profile
. I then applied the profile to my supposed linear scan and … it appeared similar to my IT8 scans with gammas of 1.8 and 2.2. This has suggested to me a test for whether or not an image has linear gamma: Open the image in PS and apply a linear sRGB profile. If the image is way too bright, then you do not have a Gamma 1.0 image.
I have uploaded a zip file called Gamma 1 Test Documents
in case anyone wants to play around and offer comments. The file contains:
• Gamma 1.0 Plot
(image as attached here and Grapher version for opening on Mac);
• Gamma 1.0.tif
(a linear IT8 Kodachrome scan);
• Linear sRGB IEC61966.icc
( a linear sRGB profile)