Varying Saturation relative to Brightness in Affinity Photo

[Jim Smith]

Is there a way to reduce the colour saturation of all darker parts of an image while maintaining the colour saturation of all brighter parts of the image?

Thanks in advance, Jim

[carl123]

An HSL adjustment layer can target specific colours allowing you to reduce the saturation level of those colours but it has its limits

An example (uploaded image?) of what you want to do may yield a better answer

[Jim Smith]

28 minutes ago, carl123 said:

An HSL adjustment layer can target specific colours allowing you to reduce the saturation level of those colours but it has its limits

An example (uploaded image?) of what you want to do may yield a better answer

I want to make images of what various astronomical objects look like through small telescopes. Colours of things like nebulae rarely show to the naked eye because they are too faint. Stars, however, can be seen to vary in colour from orange to blue. 

I want to try processing images like the one attached to make the nebulae much less saturated while keeping the star colours.

[RichardMH]

A little confused. The darker parts of the image are the black sky.

[Jim Smith]

54 minutes ago, RichardMH said:

A little confused. The darker parts of the image are the black sky.

I want to remove the colour from the dark areas. Anything that is so dark that it is black won't matter.

[carl123]

Still confused...

Do you mean something like this - just leaving the brightest stars to shine through?

 

 

[Klobrillenmann]

I understood "de-saturate according to brightness"
Basically I would to use a black and white copy of the image as a mask for an adjustment layer.

• Duplicate image layer, change it into a black/white image. I would do it with Recolour (can de-saturate to b/w and adjust brightness)
• convert the black/white result to a mask (right click on layer -> rasterize to mask)
• select the mask, then in the Channels window right click "Mask Alpha" and store it in a Spare Channel. 
• Apply an adjustment layer to your image, could be HSL or b&w  (the adjustment has a white mask which will get the spare channel data)
• select the adjustment layers mask and right click on the spare channel, then select "Load to Adjustment Alpha)
•  the spare channel data (which is an alpha, created from a mask) is copied to the mask of the adjustment.
• change Adjustment and its mask as you need it. (i.e. invert mask)

[Jim Smith]

2 hours ago, Klobrillenmann said:

select the mask, then in the Channels window right click "Mask Alpha" and store it in a Spare Channel. 

Thanks. I got to your third point but can't find "Mask Alpha" to right-click.

[Jim Smith]

2 hours ago, carl123 said:

Do you mean something like this - just leaving the brightest stars to shine through?

Not really. I want the brightness of everything to stay the same. I want the colour saturation of the nebulae to be reduced so that they appear less colourful. I want the colour saturation of the stars to stay the same as the original image so that they still have slightly different colours.

[Lisbon]

9 hours ago, Jim Smith said:

...reduce the colour saturation of all darker parts...

Hi Jim!

One option is to add a curves adjustment layer in saturation blending mode.

Another is to reduce the saturation with a vibrance adjustment in conjunction with blend ranges.

[Klobrillenmann]

Quote

Thanks. I got to your third point but can't find "Mask Alpha" to right-click.

 

In the screenshot you provided earlier, you didn´t select the previously created mask.
When a mask is selected, the channel view has an alpha available.

 

[RichardMH]

Something like this? Very quickly from 3D LUT creator using the Sat/Luma curve.

[Jim Smith]

4 hours ago, Lisbon said:

One option is to add a curves adjustment layer in saturation blending mode.

 

Yes. That seems to do it. Thanks!

[NotMyFault]

wouldn't a simple  HSL adjustment with a blend range do the trick?

[Lisbon]

2 hours ago, Jim Smith said:

Yes. That seems to do it. Thanks!

Youre welcome.