Logarithmic Histogram?

[DrJimSok]

Hey Folks,

Is there a way to get a logarithmic histogram of pixel values, like in Histogram / Curves Transform? This woud be very useful for visualizing the status of the high end / low number of the distribution. When stretching an image, then one could see the impact on this high end to see how saturated items have beocme.

Jim

 

[fredvanner]

The easiest way is to use PixelMath to generate a temporary logarithmic stretch (roughly log($T)), and look at the histogram of that. You are probably really interested in the luminance, so (log(CIEL($T))). The image will typically be normalised, so the log values will be negative (and so clipped to zero), hence you need to subtract log(min(CIEL($T))) (but this won't quite work as it stands since you want the min over the whole CIEL($T) image). You will also need to avoid zero values (log(0)=-infinity).
So something like: given image Im, extract ImL = CIEL(Im); on ImL execute (iif($T,log($T)-log(min($T),0))) giving ImlogL; look at histogram of ImlogL.

 

[bulrichl]

The OP wants a logarithmic scale of the y axis of the histogram which is the frequency (or abundance?) of values. This goal is not at all achieved by your formula.

Bernd

 

[fredvanner]

The OP wants a logarithmic scale of the y axis of the histogram which is the frequency (or abundance?) of values.

Ah! I misunderstood the question. That is indeed harder to do (I can do it in MatLab, but not PI!).

 

[bulrichl]

@Juan Conejero:
I had wished to set the scale of the y axis of a histogram to logarithmic as well. Indeed this thread would be well suited in the category "Wish List".

Bernd

 

[fredvanner]

I had wished to set the scale of the y axis of a histogram to logarithmic as well

Me too - in fact I have sometimes exported an image into MatLab for just this reason (so I really should have understood the original question!)

 

[DrJimSok]

Oh sorry if my original 'ask' wasn't clear... oops !! Seems like an easy fix too... just add a button to the GUI (like w/ HistTransform), quickly compute (or have stored already in cache) the logs, and switch the display... of course, the GUIs are pretty feature righ already, so "just add a button" is more complex to do than to say !! Yucks...