PixInsight - Image Post Processing

Local Histogram Equalization

updated: 2021-05-31


 Implements local equalization of the histogram. Can be used to enhance local contrast and visibility of structures in the image.


Menu: Process 🡺 All Processes 🡺  Local Histogram Equalization

Process Explorer: IntensityTransformations 🡺 Local Histogram Equalization


Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS.xisf” or similar into sub folder "aligned"


Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS_LHE.xisf” or similar into sub folder "aligned"

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https://youtu.be/FxM5WGrvhoI?t=566  (Part 9  HDR starts around timestamp = 9 m and 44s)





Open Local Histogram Equalization



Kernel Radius

Radius of the area around the pixel used to evaluate the histogram. Lower values strengthen the effect but are sensitive to noise and suffer from ringing around bright features like stars. Higher values weaken the effect but are more stable and do not exhibit ringing. For most images, values from 32 to 128 are suitable.

Contrast Limit

Maximum allowed slope of the transfer curve applied to individual pixels. A value of 1.0 means no change at all. High values make the method very similar to classical histogram equalization and tend to promote noise. Suggested values are from 1.5 to 3. Setting this value too high will introduce a lot of noise in the image, so keep it below 3.


Allows you to control the amount of delivered change by blending the result with the original image. The value of 0.75 means that the resulting image will be a blend of 1/4 of the original and 3/4 of the processed image.

Histogram Resolution

The resolution of the histogram used to evaluate the equalization transfer curve. Higher values makes computation more precise but slower. Besides, they can work poorly with low values of the kernel radius parameter. For most images, use 8-bit resolution. In case you're using high kernel radii and you encounter posterized results, try higher resolutions.

Circular Kernel

If switched on, the histogram is evaluated from a circular area around each pixel. This is recommended to make the effect more uniform. If switched off, the histogram is evaluated from a square area.


Create a Luminance Mask

this will create a B/W luminance image.

Then hide the mask (CTRL+K)


Make a Preview

of a region of interest in your image

and to reduce processing time during the testing



Open a Real-Time Preview on the Preview WIndow


Check Before and After

using this control in the Live-Preview window:
 Before    ...      After




Final Settings


Close the Real-Time Preview Image


Apply the Local Histogram Equalization to the Main Image

drag the little blue triangle on your main image


Remove the Preview and the Mask

Menu 🡺  Mask 🡺  Remove Mask


 Local Histogram Equalization


Save the Image

file name like  “*_i_c_DBE_ABE_BN_CC_ HT_MLT_FS_SM.xisf”

or "20170921_M8_FUJXT1_i_c_DBE_ABE_BN_CC_ HT_MLT_FS_SCNR_SM.xisf" in this case

using postfix _SM

file name like  “*_SM.xisf”

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