11-27-2019, 03:26 PM
(11-26-2019, 10:21 PM)Ofnuts Wrote:(11-23-2019, 01:14 AM)marco-gimp Wrote: Thanks Ofnuts; I'm now investigating this tool.
Okay ... I think that I have understood it.
The scale is 256.
Therefore a fully black image would have a value of 0
A slightly less black image might have a value of 2.56
This would indicate that the slightly less black image was emitting 1% of light
... and absorbing 99% of light.
This is the required tool.
Thanks again Ofnut
No...You can tell that there is a difference, and increasing values mean increasing difference but it's likely not linear by default. The 0..255 scale is "gamma-corrected" so the true value (in a 0.00-1.00 scale) is more or less pow(x/255,2.2) (or is it pow(x/255,1/2.2)?). In Gimp 2.10 you can work in "Linear light" but even then the values are whatever is needed to make the image look right, and not an accurate measurement of incident light.
Thanks Ofnut for continued interest in this question
The problem for me is that I lack the fundamental knowledge regarding 'gamma' and 'gamma corrected'.
However, your post helped to guide my thoughts on the difficulties relating to linear progression.
First off ... I don't believe that there is need for comparing two images in 'difference mode'.
The image is either black, or less than black.
Black = 0
Therefore there is no difference to be gained.
I tested this with a black and less than black image, and found it to be the case.
So perhaps we can simply concentrate directly on a given image, and its histogram value (unless gamma correction has an effect).
Here are my thoughts (correct me if I'm wrong) :
Quote: When shining white light onto a page containing 256 shades from black to white ...
The power of the light should be such, to allow all the shades to be seen.
Too powerful, there will be numerous whites.
Too weak, there will be numerous blacks.
If all shades can be seen, and photographed, the histogram values should closely approximate to '0 to 255' for each shade.
The camera auto will need to be locked.
We will then have a standardised light-source power.
The power could then be increased by percentages, imaged at each instance, and the histogram values noted.
Ie. a 1% increase in power ... does the histogram value increase by 1% (or by how much).
If using a 12v halogen, through an adjustable AC transformer, the supply lead can be metered.
The question then, is whether the light output increases in a linear reaction to the increase in power?
Therefore I need to know how halogen light output increases against power increase (probably from a given starting point).
I can likely gain that information - ideally gaining an equation to enable 'steps in light output' to be achieved.
This would enable an equation based upon real life scenarios of changing light.
Otherwise:
It may be the case, that for Black 3.0 the power could be increased to the point where reflection begins to register as a histogram value
... and then powered back, to regain zero.
At this point, the light source could be increased by a known amount, and the histogram value noted.
... so providing a percentage of light being absorbed by the material.
This would avoid the need for determining 256 shades of grey (though that might be interesting anyway).
However:
There remains the question mark over gamma and gamma corrected.
What does that mean, in relationship to the experiment?