Equalize histogram...

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priort
Posts: 21
Joined: November 8th, 2019, 12:19 pm

Equalize histogram...

Post by priort » December 1st, 2019, 8:07 pm

I am getting some great results with this transform. I have many years of jpg files from various sources and many are overexposed or washed out and the equalize transform really adds back some wonderful contrast...sometimes too much and so opacity is fine other times I mess around with the curve options and I have gotten some nice results but its a ton of trial and error with the curves in that module for me...are there any examples for this written up anywhere for this version or other versions....I think PW is unique. I see histo normalization and equalize but never with the ability to tweak with a curve....

jsachs
Posts: 2166
Joined: January 22nd, 2009, 11:03 pm

Re: Equalize histogram...

Post by jsachs » December 1st, 2019, 8:44 pm

The Equalize transformation is simply a another way to create a brightness curve that is then applied to the input image. It works by trying to force the histogram of the output image to have a given shape, based on histogram matching algorithms. Since there is no "correct" shape for a histogram, there is always going to be some trial and error. Generally a flat histogram will have approximately equal amount of each gray level. A histogram that is higher on both ends and lower in the middle will have more contrast. A histogram that is higher in the middle and lower at the end will be low contrast.

Histogram equalization was originally used as a way to enhance local contrast in medical images such as x-rays as it tends to bring out detail in low contrast areas.
Jonathan Sachs
Digital Light & Color

davidh
Posts: 698
Joined: June 9th, 2009, 2:16 am

Re: Equalize histogram...

Post by davidh » December 2nd, 2019, 2:10 pm

A useful and easy method that worked for me when I wanted to better understand the effects of the Equalize transformation:

1. I outlined the shape of the left histogram with the curve adding as many points as necessary, but not too many. I usually did not use more than 10 but it depends on the shape of the histogram, especially on the number of peaks and sharp spikes.

The goal is to make the histogram on the right look the same or almost the same as the original one on the left. The input and output image will then look the same (or almost) and the curve on the right will be diagonal ideally going from lower left to upper right corners. However, he curve will hardly ever be straight and start and finish in the corners. I have never achieved it, but there is no need to do it.

2. Then I dragged individual points on the curve up, down, right or left a fraction and watched the effects in the output image as well as the changes of the histogram on the right.
To prevent a part of the curve slope from becoming too steep and contrasty, more one point usually needs to be moved.

It is still trial and error but not a ton of as it it helps to see and understand the effect of even small changes in different parts of the output image.

Often it helps to just move a part of the shadow or midtones curve to the right a bit to get a nice improvement without meddling into lights.

Your overexposed images might profit from the oposite - moving a right part or the whole of the outlined curve a bit to the left. However, this will not restore the burnout lights.

priort
Posts: 21
Joined: November 8th, 2019, 12:19 pm

Re: Equalize histogram...

Post by priort » December 2nd, 2019, 4:24 pm

Thanks Jonathan, I have been messing with the curves that can be applied...I guess like a transfer function. I did understand at least for the most part the equalize concept and I was playing around with the different curve options that I was trying to master....starting from the horizontal line....dropping either end point etc and then on to experimenting with multipoint curves...it was a bit confusing at first that dropping the left end point lightened the dark and the opposite for the other endpoint....so basically I was trying to figure out how to control this and then understand multipoint curves so that I was a bit more able to target what I wanted rather than just keep moving things around until it looked better or the way I was hoping for.....you can really get some fantastic contrast....

priort
Posts: 21
Joined: November 8th, 2019, 12:19 pm

Re: Equalize histogram...

Post by priort » December 2nd, 2019, 4:28 pm

Thanks for the tips David I will walk through that approach and see how it goes..I appreciate you taking the time to share your experience.

priort
Posts: 21
Joined: November 8th, 2019, 12:19 pm

Re: Equalize histogram...

Post by priort » December 2nd, 2019, 4:32 pm

davidh wrote:
December 2nd, 2019, 2:10 pm
A useful and easy method that worked for me when I wanted to better understand the effects of the Equalize transformation:

1. I outlined the shape of the left histogram with the curve adding as many points as necessary, but not too many. I usually did not use more than 10 but it depends on the shape of the histogram, especially on the number of peaks and sharp spikes.

The goal is to make the histogram on the right look the same or almost the same as the original one on the left. The input and output image will then .
Thanks for the tips David I will walk through that approach and see how it goes..I appreciate you taking the time to share your experience.

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