Posted by
Jim Peterson on
URL: http://imagej.273.s1.nabble.com/Log-Math-Function-tp5020510p5020516.html
The key point for the original question is that the final value is scaled, but has nothing to do with base.
Just remember that to convert from base a to base b:
log_a(x) = log_b(x)/log_b(a).
Apply this to the log quotient in the equation below and you'll see that the conversion factors in the
numerator and denominator cancel, so the base is completely irrelevant.
- Jim
On 4/17/18, 4:52 PM, "Jacob Keller" <
[hidden email]> wrote:
I copied and pasted the documentation as found at
https://imagej.nih.gov/ij/docs/menus/process.html#math, and it does not
specify natural log??? Is there another place for documentation? Again,
here it is:
Log...For 8-bit images, applies the function *f(p) = log(p) * 255/log(255)* to
each pixel (*p*) in the image or selection. For RGB images, this function
is applied to all three color channels. For 16-bit images, the image min
and max are used for scaling instead of 255. For float images, no scaling
is done. To calculate log10 of the image, multiply the result of this
operation by 0.4343 (1/log(10).
After a bit of searching, I found this page:
http://imagej.net/docs/guide/146-29.html#toc-Subsection-29.9 where it seems
to have been improved:
29.9.12 Log
For 8-bit images, applies the function *f*(*p*) = ln(*p*) × 255 ⁄ ln(255) to
each pixel (*p*) in the image or selection. For RGB images, this function
is applied to all three color channels. For 16-bit images, the image min
and max are used for scaling instead of 255. For float images, no scaling
is done. To calculate log10 of the image, multiply the result of this
operation by 0.4343 (1 ⁄ ln(10)).
Anyway, perhaps, as you imply, the term "log" could be changed in future
process>math> menus to "ln?" It certainly would have helped avoid my
confusion. Who knows, there may be some scientific mistakes out there due
to this notation issue.
JPK
On Tue, Apr 17, 2018 at 4:16 PM, Gabriel Landini <
[hidden email]>
wrote:
> On Tuesday, 17 April 2018 20:55:48 BST
[hidden email] wrote:
> > By deductive reasoning from the documentation, the answer is that "log"
> in
> > imagej means base 2.
>
> No, the documentation specifies "ln", i.e. in base e.
> And it works fine and the conversion recipe to log10 is correct too.
>
> Perhaps you are getting confused because the command is listed in the menu
> as
> "Log".
>
>
> Cheers
>
> Gabriel
>
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http://imagej.nih.gov/ij/list.html >
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