Gamma increase produces artefacts

[almcl]

Not sure if this is a bug or (more likely) operator error but.

Today I separated the stars from a post tracking image with a view to quietening down the background. The extracted stars looked rather dimmer in the 'stars only' field, so I launched the develop module intending to apply enough gamma to restore their brightness. Bearing in mind that the star image should only contain stars and no background, I was surprised to see a mottled pattern appearing in the background as the gamma slider was increased. (Images below, and yes, it's not a very good image - M46 is very low on the horizon at my latitude.)

Should this happen? And is there any way to prevent it?

Part of the star field:

The extracted star field.jpg (55.08 KiB) Viewed 7334 times

After applying a gamma increase:

After applying gamma 2.jpg (86.12 KiB) Viewed 7334 times

[admin]

Hi,

I can't think of any way a linear stretch would introduce noise or mottle.

How exactly do you extract the star field?

[almcl]

via the work flow here: http://forum.startools.org/viewtopic.php?f=7&t=244.

Using a star mask then the Heal module:
-- Heal
Parameter [Quality] set to [Ultra]
Parameter [New Darker Than Old] set to [Yes]
Parameter [Grow Mask] set to [0 pixels]
Parameter [Neighbourhood Samples] set to [0]
Parameter [New Must Be Darker Than] set to [Off]
Parameter [Neighbourhood Area] set to [200 pixels]
Mask used (BASE64 PNG encoded)

The mask was

download.png (56.39 KiB) Viewed 7312 times

The the layer module to extract the stars:
--- Layer
Parameter [Layer Mode] set to [Subtract]
Parameter [Cap Mode] set to [Clip]
Parameter [Brightness Mask Mode] set to [Off]
Parameter [Filter Type] set to [Gaussian (Fg)]
Parameter [Blend Amount] set to [100 %]
Parameter [Mask Fuzz] set to [1.0 pixels]
Parameter [Filter Kernel Radius] set to [1.0 pixels]
Parameter [Offset X] set to [-0.5 pixels]
Parameter [Offset Y] set to [0.0 pixels]
Parameter [Brightness Mask Power] set to [1.00]

[admin]

Hmmmm....

Could it be the Offset X by half a pixel? (this is a bug that has been fixed in the latest version)

[almcl]

Hadn't noticed that update! I was using 1.4.341.

I'll download and see what result comes from that.

[almcl]

Yippee! That's cracked it!

No artefacts in the star field when stretched now in .344!

Thanks for the brilliant support, as ever Ivo!

[admin]

Yippee! That's cracked it!

No artefacts in the star field when stretched now in .344!

Thanks for the brilliant support, as ever Ivo!

Great! Not sure where that glitch snuck in (it only happened when the image X or Y resolution is an odd number).