Starguide - A Practical Guide to Astro-Photography

PixInsight - Image Post Processing

Morphological Transformation

updated: 2021-05-30

Purpose

The MorphologicalTransform (MT) process is an advanced implementation of several morphological transforms. It does so by applying a function using a structuring element (a mask).

Activation

Menu: Process 🡺 All Processes 🡺 MorphologialTransformation

Process Explorer: Morphology 🡺 ScreenTransferFunction

Input

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

Output

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

Previous Step

Full Final Stretch or any image form chapter Improving the Image

Next Step

Improving the Image (any step)

Tutorial

https://www.youtube.com/watch?v=MhaW1mcybzk  (Part 10  Star Mask Shrink)

Reference: PixInsight v1.5 - Unofficial Reference Guide: Morphology (deepskycolors.com)

Step

Description

 Action

1

Start with the fully stretched image

from any step after Full Stretch

2

Make a Copy of the Image

for later usage

Clone the Image

3

Send the Clone to a New Workspace

to keep it separated from current changes

Moving an Image to a Workspace

4

Create a Mask

Menu: Image 🡺 Extract 🡺 Lightness (CIE L*) 

or  

5

Apply the Mask to the Original Image

Pull the tab of the B/W stretched image right below the tab of the original image to apply the mask

The red image is the MASK which will be used further. Notice the tab on the left side of the windows turned to brown indicating a mask. 

Everything in red is protected (here this is the background). By inverting the mask you can also protect the foreground which is good if you want to reduce noise on the background.

Make sure the background (and not the stars are protected) for the next steps, the background should appear red

Hide the mask with CTRL+K

6

Minimize the B/W Luminance Image

7

Open Morphological Transformation

4

Morphological Filter

Try following settings:

Operator  🡺 description, try Erosion first

Interlacing: 1

Iterations: 1

Amount: 100%

5

Structuring Element

Size: the more pixels your camera or sensor has, the higher size you should select, e.g.  for a 20Mpixel camera select a size of 9

Way: 1

Structure: select a round structure for stars

5

Create a Preview Box + Open the Preview

in an area with some stars. This step is required to reduce the processing amount that would be required for the entire picture

ALT+N

6

Apply the Morphological Transformation on the Preview

Obviously that was too harsh and all stars became little circles...

So - undo the last operation

7

Best Morphological Transformation Settings

Try different options until the image looks good

Operator: Morphological Selection

Amount: 0.55

Star: Diamond

🡺  

8

Remove the Preview and the Mask

Menu 🡺  Mask 🡺  Remove Mask

9

Apply the Morphological Transformation on the Entire Image

9

Save the Image

If you are happy with the result, close all preview images and apply the setting on the main image. Save the image and close everything in Workspace 1

use file name like  “*_i_c_DBE_ABE_BN_CC_HT_MLT_FS_MT.xisf”

or "20170921_M8_FUJXT1_i_c_DBE_ABE_BN_CC_HT_MLT_FS_SCNR_MT.xisf" in this case

using postfix _SM

file name like  “*_SM.xisf”

13

Open Workspace2

and move the image to workspace1 for the next chapter