Histogram Transformation


The main tool you will use for stretching your images in Pixinsight


This is not the same as other programs and does not usually require the use of curves to perform a simple stretch and all channels are on view with pixel readouts giving you a high degree of control


This is the panel of the tool when opened and a image selected

A) Output histogram--New histogram of your image after application of your alterations ( C )

B) zoom controls for input histogram ( C )-- You can zoom in on the histogram to give finer control your you alterations

C) Input histogram-- Histogram of image you have selected at F

D) Graphic style of Histogram and Plot resolution ( Please note resolution picked here only relates to display on tool, they do not relate too or alter image depth )

E) Information Panel--Pixel information of cursor placed on histogram will be displayed here

F) Image selector--Select your image to be used

G) Channel selector-- you can choose to work on combined channel or individual channels, very handy for colour problems

H) Shadows clipping ( Black point to some ) --Shows amount of histogram clipped , more important to keep a eye on P which shows how many pixels are being clipped

I)Highlights clipping-- Shows amount of histogram clipped, In normal use Never I said Never clip any Highlights

J) Midtones Balance readout

k) Realtime preview-- great for showing your alterations in real time

L) Slider controls--Use these to make adjustments to the histogram

m) Histogram Display settings

N) Readout mode buttons

O) Automatic adjustments can autoclip for highlights and shadows with the settings preset in Q Personally I choose to use my eye and skill

P) Amount of clipped pixels

R) Output zoom controls

There are some further drop panel controls to do with extending the dynamic range , must find out what to do with this !

S) Midtones transfer curve


How to use Histogram Transformation


Click on image to enlarge


To Start with I will show you how to do a simple stretch

But before that a couple of rules

1) Never Never clip the highlights as you will loose information that may be required later, there is no reason to do so

2) Even though you can clip a few pixels in the shadows over doing it will make the background black and the sky is not black

With that in Mind we do the following


1) Select your Image to work on if F

2) I like to activate the real time preview to see the effect of our stretch which I did using K

3) I Then Grabbed the midtones slider with the mouse and slide to the left, the amount you can do this is obviously suited to each image i.e. the amount of noise / taste ETC

4) Then I moved the shadow slider to the right and clipped some pixels , but only 53 , that 53 out of 6 million!, you can get away with clipping more than this but never move past the start of the histogram curve

5) You can zoom in on the histogram if you wish to , but personally I do two stretch's one to get us roughly where I want to be and use a second ( shown above ) to do the fine work , the choice is yours

6 ) Once happy close down the real time preview and apply it to your image , hey presto a stretched image with no curves


Other things you can do with the histogram tool

As you can see individual colour channels you can balance this by clipping a single channel or stretch one if you think your image is weak in one

Return visits to the histogram throughout processing will reveal whether there is any problems with colour etc


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