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Re: Log Math Function

Posted by Herbie on Apr 17, 2018; 8:20pm
URL: http://imagej.273.s1.nabble.com/Log-Math-Function-tp5020510p5020514.html

Good day Jacob,

I don't know what is insane for you but ImageJ uses the natural
logarithm per default, i.e. the log_e.

Other logarithms can easily be computed from log_e and the manual even
explains it for log_10.

BTW, the log_2 is used to compute entropy etc.

Still some headaches?

Hopefully not

Herbie

::::::::::::::::::::::::::::::::::::::::::
am 17.04.18 um 21:55 schrieb Jacob Keller:

>   By deductive reasoning from the documentation, the answer is that "log" in
> imagej means base 2. I just found out that in general:
>
>     -  x = log y  often means  x = loge y  in *mathematics* texts.
>
>     -  x = log y  often means  x = log10 y  in *science* and *engineering*
>     texts.
>
>     -  x = log y  often means  x = log2 y  in *computer science* texts.
>
> This is pretty insane, almost guaranteeing we will stay in our fields and
> not understand each other. I had been used to Log = base10 and ln = base e,
> but never ran across the base2 parlance before. Maybe adding clarification
> to the imagej documentation below would help some of us
> non-computer-scientists? Here is the online documentation for the log
> function, clearly-explained for computer scientists ;)
> 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).
>
>
>
>
>
>
>
> On Tue, Apr 17, 2018 at 3:17 PM, Gabriel Landini <[hidden email]>
> wrote:
>
>> On Tuesday, 17 April 2018 20:08:10 BST you wrote:
>>> Does anyone know what is going on with the Log function in
>>> process->math->Log? I have a post-Log'd image that goes up to 302, which
>>> seems rather high, esp considering that my pre-Log image maximum is 302.
>>> Does it just re-scale to the original min/max or something?
>>>
>>> Changing to 32-bit lowers the final numbers, but the numbers are still
>> off.
>>> Any way to get the actual values for the logs?
>>
>> Please read the online documentation. It is clearly explained.
>>
>> Cheers
>>
>> Gabriel
>>
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>> ImageJ mailing list: http://imagej.nih.gov/ij/list.html
>>
>
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