Example of large gradient removal procedure

Image credit Patrick Bornet

The image to process

LOAD IMAGE

STEP 1

Run the "Wavelet" command (Processing menu), and set to unity only the largest wavelet scale :

Click OK. The result

SAVE BLUR

STEP 2

Make a synthetic background

LOAD BLUR
SUBSKY
SYNTHE
SAVE SKY

Or run the "Remove gradient..." function of Processing menu

and execute the commands line

SYNTHE
SAVE SKY

The result

STEP 3

Remove the synthetic sky

LOAD IMAGE
SUB SKY 100
VISU 180 50

The result

 

VARIATION

For the present situation, a better method for fit the complex sky is the manual strategy

LOAD BLUR
POINTON

and select some points with the mouse (skip the Milky Way):

SYNTHE 3
SAVE SKY2
POINTOFF

LOAD IMAGE
SUB SKY2 100
VISU 180 50

The final result

Tips: For make a large blur of an image you can also use the commands MEDIANF ot RING_MEDIAN for example

Original sized images

Successively start image and final image

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