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Sorry for not answering before. I've just seen your problem. My technique to get the right values in my tries is this;
I get the Mean Value from the image. I take note of this value, for instance, it is: 0,455
I take this value and multiply it for 256 and I get other value: 116,48. I take note of 116. This value is my T0 = 116 (From this value I'll get the right value to get T2 to apply to B layer and T1 to apply to D layer).
To get T2; T0*170/128
To get T1; T0*85/128
So from my sample, it will be:
T2 = 116*30/128 = 154 (to apply on the left side in threshold) on B layer
T1 = 116*85/128 = 77 (to apply on the left side in threshold) on D layer
I hope it is clearer.

Issabella

Don’t be afraid to start over. It’s a new chance to rebuild what you want.

Sorry for not answering before. I've just seen your problem. My technique to get the right values in my tries is this;
I get the Mean Value from the image. I take note of this value, for instance, it is: 0,455
I take this value and multiply it for 256 and I get other value: 116,48. I take note of 116. This value is my T0 = 116 (From this value I'll get the right value to get T2 to apply to B layer and T1 to apply to D layer).
To get T2; T0*170/128
To get T1; T0*85/128
So from my sample, it will be:
T2 = 116*30/128 = 154 (to apply on the left side in threshold) on B layer
T1 = 116*85/128 = 77 (to apply on the left side in threshold) on D layer
I hope it is clearer.

Issabella, the "quick and dirty" formula is not applicable with mean values very distant from 127.5 (2.8) or 0.5 (2.10). Sorry for having suggested this quick but...too dirty formula.
So I suggest you now to update the tutorial.
One way is to add a sentence similar to the one I posted in my reply, letting the user to estimate approx reasonable values.
Another way could be this new "quick and (less) dirty" formula: let's speak only in absolute values as in 2.8 to simplify
considering that the theoretical values 85 and 170 were intended for a mean of 127.5 , we could clarify:
- 170 represents an increase of 1/3 (0.333) towards the right extreme (255)
- 85 represents a decrease of 1/3 (0.333) towards the left extreme (0)
Hence:
given a detected mean value of M, calculate:
- T2 as M + ((255-M)*0.333)
- T1 as M - ((M-0)*0.333)
Take a big value for M: 200 (very rare, but possible in very light images).
- T2 should be = 200 + ((255-200)*0.333) = 218 (rounded)
- T1 should be = 200 - (255*0.333) = 115 (rounded)

Sorry for not answering before. I've just seen your problem. My technique to get the right values in my tries is this;
I get the Mean Value from the image. I take note of this value, for instance, it is: 0,455
I take this value and multiply it for 256 and I get other value: 116,48. I take note of 116. This value is my T0 = 116 (From this value I'll get the right value to get T2 to apply to B layer and T1 to apply to D layer).
To get T2; T0*170/128
To get T1; T0*85/128
So from my sample, it will be: T2 = 116*30/128 = 154 (to apply on the left side in threshold) on B layer T1 = 116*85/128 = 77 (to apply on the left side in threshold) on D layer
I hope it is clearer.

Issabella, no doubt about the technique itself, your tutorial was excellently enlightening.

We are saying that it is not in all images that one can follow this formula to the letter. See the example I gave for an image where the Mean value is 0.840.#48

As for the calculations you use, I suggest simplifying them, just enough: T2 = M * 340
T1 = M * 170 See using the example you gave with M being 0.455: T2 = 0.455 * 340 = 154.7 T1 = 0.455 * 170 = 77.35

"I feel that in both art and music, it's not the success that matters but the pleasure it gives you. Focus on the pleasure and the learning will come naturally." - Brian Weston

Issabella and Diego - I think Krikor has pinpointed/resolved my particular problem with Mean values in the Histogram window. It seems from what Krikor has posted (0.840), in 2.10 they are expressed in a different manner from 2.8 which I am still using.

I will probably need to divide my Mean values by a hundred first and then apply Issa's tutorial calculations.

OMG, you can do a simple conversion using a direct relation:

127.5 = 0.5
Your Mean Valur = X

Then
(Your Mean Value * 0.5) / 127.5 = X

Example, your Mean value is 85, then
(85 * 0.50) / 127.5 = 0.3333

Now just find T2 and T1: M=0,3333 T2 = 0.3333 * 340 = 113.322 T1 = 0.3333 * 170 = 56.7

"I feel that in both art and music, it's not the success that matters but the pleasure it gives you. Focus on the pleasure and the learning will come naturally." - Brian Weston

Just popped on before retiring for the night ... and WOW ... Diego thank you very much for both the calculation instructions clarification and the 2.8 steps.

Krikor- thank you for your maths lesson too

A very rainy day forecast for tomorrow so plenty of time for me to play with Issa's tut.

Thanks you Krikor and OMG. Just Dinasset has recalculated the formula so that the results are alright even on very dark or very clear photos.
I hope this updated tutorial is a bit better.
Dinasset, thanks for your effort and patience.