12-11-2025, 08:57 PM
(12-11-2025, 07:32 PM)oshfdk Wrote: You are not allowed to view links. Register or Login to view.But is it possible to determine if they became transparent or just nearly the same shade as the parchment?
I am not sure I understand the question, or what difference it would make. Here is the histogram of pixel values in the MB940IR_012_F image of f17r, clipped to the page proper minus a narrow margin all around:
[attachment=12247]
Note that the histogram count scale is lograithmic, and that of the cumulative histograms is "bilogarithmic", with log-like behavior near 0 and near 1.
Here is the histogram of pixel value over the six patches of the gray scale that was placed just above the page, near the upper right corner of the image:
[attachment=12249]
And here is the histogram of the Spectralon patch, that was placed just above the page, near top center of the image:
[attachment=12248]
Spectralon is a white material used as a standard in technical imaging. It is advertised as scattering 99% or something of the light (that is, being "white") at all wavelengths except UV, where its reflectance drops. So you can compare brightness levels in images taken at (say) 700 and 840 nm by scaling all pixels in each image so that the Spectralon patch has the same gray shade in both images.
The Spectralon histogram is not a sharp peak because it has a mottled appearance:
[attachment=12250][attachment=12251]
The first image is just contrast-stretched (linear map), the second one is both contrast- and brightness-stretched. I don't know whether that mottling is normal, or the result of dirt having stuck to the surface of the patch. Anyway, as long as the dirt is the same on all images, the peak of the histogram for each image can be used as the "white" value. I myself need a table of those values and other parameters, but I haven't made it yet.
For this image, specifically, a perfectly white matte surface should have pixel values a little above 2160. (Assuming the dirt is matte too. The light direction is about 45 degrees, so those brightest spots should not be shiny-finish highlights...)
The last hump in the full-page histogram is the vellum. It is not a sharp peak because of warps and stains of all kinds. It is obviously well below the Spectralon white.
The gray patches are not as accurate as the Spectralon patch and their reflectance varies a lot with the wavelength outside the visible range. So they should not be used to calibrate the pixel scale of the IR images.
Here are the coordinates of the various parts of the TIFF images (in the ImageMagick's "-crop" notation). The "whref" is the Spectralon patch.
Code:
# To be sourced by shell scripts. The {*_crop} parameters are relative
# to the original image, before rotation.
page_crop='6500x4650+0934+1352' # Full page with a bit extra margin ("full.png").
txref_crop='0196x4650+7405+1352' # Transmission ref patch ("txref.png"); same Y range as page.
hist_crop='6050x4000+1100+1584' # Region to consider in "full" histogram.
C0_crop='0400x0300+1488+2008' # "C0" image clip (test).
C1_crop='0320x3700+0945+1952' # "C1" image clip (top marginalia).
C2_crop='0850x1800+2712+2500' # "C2" image clip (lines 7-11, end).
C3_crop='1320x0900+1400+4660' # "C3" image clip (lines 1-6, start).
scale_crop='2200x0200+5822+0688' # Coords of gray scale patcthes ("scale.png").
whref_crop='0300x0300+3422+0418' # Coords of white ref patch ("whref.png").
grref_crop='0300x0300+4162+0406' # Coords of gray ref patch ("grref.png").
ruler_crop='2400x0170+5710+0916' # Coords of ruler ("ruler.png").
rot=90 # CW rotation to apply (degrees)Hope it helps, --stolfi
