Local Histogram Equalization

PixInsight - Image Post Processing

Local Histogram Equalization

updated: 2021-05-31

Purpose	Implements local equalization of the histogram. Can be used to enhance local contrast and visibility of structures in the image.
Activation	Menu: Process 🡺 All Processes 🡺 Local Histogram Equalization Process Explorer: IntensityTransformations 🡺 Local Histogram Equalization
Input	Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS.xisf” or similar into sub folder "aligned"
Output	Stretched image, file name “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS_LHE.xisf” or similar into sub folder "aligned"
Previous Step	HDR Multiscale Transform or any image form chapter Improving the Image
Next Step	Curves Transformation
Tutorial	https://youtu.be/FxM5WGrvhoI?t=566 (Part 9 HDR starts around timestamp = 9 m and 44s)

Step

Description

Action

1

Open Local Histogram Equalization

2

Parameters

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.

Amount

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.