Once you have it installed open your photo and choose your line type. Drag it across the area of your photo where you want to measure noise. Cntrl+K creates the graph.
To change the Y scale of the graph go to edit, options, plot profile options
For my measurement in the black area of this poster I picked min 0, max 40
Once you have a flat noise profile, go to analyse, measure or hit Cntrl+M
For an accurate noise measurement signal average several. This is a square law thing so 4 measurements doubles your accuracy and 9 measurements triples it. Summarize will do the math where your Signal to Noise (S/N) is the Mean divided by the StdDev (Standard Deviation)
Is this conventional statistical definition of S/N to best way to characterize photographic signal to noise?Probably not. Noise peaks, especially colored noise peaks, that jump out of the average noise are far more distracting than a mild increase in the average noise. So is pattern noise. Both these noise problems will become lost in these numbers. But if you do these comparisons carefully ImageJ is a very useful tool for working out how well the various combinations of sliders and methods work for you.
Final note. You can not save a picture of your graph directly in ImageJ. 'Save as' creates a spread sheet file, 'Clear\' clears out you mistakes and 'Rename' allows you to start a new results list without losing your previous numbers.
Again if you want to know more about noise read Emil's article, http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p2.html
great use for this tool.
ReplyDeletemust one first convert the raw image to a jpg before using this method?
From the documentation
DeleteIt can display, edit, analyze, process, save and print 8-bit, 16-bit and 32-bit images. It can read many image formats including TIFF, GIF, JPEG, BMP, DICOM, FITS and "raw"
By "raw" it means the most basic type, a list of numbers without a header.
I guessed that may be the case.
ReplyDeleteWhen I open a NEF (Nikon raw) file the display shows a very pixelated image.
And I get very bizarre results when attempting to get a noise profile.
Bizarre in what way? Spiky or clipped profiles for example are all indications of happenings/problems in the camera or during postprocessing.
ReplyDeletescribble-
ReplyDeleteBack after a long summer. ImageJ will not work properly with a NEF raw file.
The documentation says:
"Image files must be in TIFF, GIF, JPEG,
DICOM, BMP, PGM or FITS format, or in a format supported by a reader plugin."
I have not been able to find a plugin for raw files for any camera. Tiff files and jpegs work fine.
mikesan
A suggestion for setting the Profile Plot Options (min and max Y scale values):
ReplyDeleteYour example of 0 - 40 works ok for darker areas in a jpeg (8 bit) file but you need much larger values when working with a 16 bit (tiff) file.
First run Analyze > Measure and note the Mean value. Set the Y scale values to include this value + or - 10%.
mikesan
Agree with both comments. ImageJ works with the the pure RAW data that you get out of scientific cameras but not with the compressed NEFs from my Nikons
ReplyDelete