Filtre anti-violet

This is a cosmetic filter designed to reduce the effect of purple fringing around stars that may be visible when using an achromat refractor especially when broadband imaging. It operates on stretched colour images.

Astuce

This filter is designed to work on non-linear images. It is therefore imperative to use it once the histogram has been stretched.

The filter offers two modes of operation where its effects can be global or limited to the stars only.

It reduces purple fringing by applying a luminance mask to the blue channel based on the green channel, and by allowing you to reduce the blue channel intensity. When in starmask mode, it limits the effects to areas around identified stars.

Global

This mode is useful when working with a starmask or with images containing just stars, e.g. a globular cluster. This is not so useful for nebula or galaxy images where the colour might be affected.

To use, you should reduce the Blue Adjustment slider until the effects are optimal. This is generally around 0.15.

You may also increase the Threshold slider to restrict the effects to those pixels where the luminance is above this value. In this mode, the optimal value will be quite low.

Masque d'étoile

This mode is useful for combined images where you want to restrict the effects of this filter to just the stars.

To use, you should tick Use Starmask. If you've not already defined a starmask using the Dynamic PSF function, there will be a delay of a few seconds whilst the starmask is generated. You may now temporarily reduce the Blue Adjustment to zero. Your stars should now be yellow. If you still see purple fringing, you should increase the Threshold slider until the fringing is entirely covered by yellow. Now you can increase the Blue Adjustment slider until your stars are not yellow but your purple fringing is reduced or eliminated, typically around 0.12-0.15.

Comparison before/after application of the unpurple filter. We can see very clearly that the stars have a better color, without the purple ring.

Ligne de commande Siril

unpurple [-starmask] [-blue=value] [-thresh=value] [-mask]

Applique un filtre cosmétique pour réduire les effets de franges violettes sur les étoiles.

If the -starmask parameter is given, a star mask will be used to identify areas of the image to affect. If a Dynamic PSF has already been run, this will be used for the starmask, otherwise one will be created automatically. The -mod= parameter should be given a value somewhere around 0.14 to reduce the amount of purple. The -thresh= will specify the size modifier for each star in the starmask and should be large enough to cause the stars to be entirely processed without remaining purple fringing. The value should between 0 and 1, typically around 0.5.

If the -starmask parameter is not given, the purple reduction will be applied across the entire image for any purple pixels with a luminance value higher than the given -thresh=. In this case, the -thresh= value should be reasonably low. This mode is useful for starmasks or other images without nebula or galaxy. This command is mask-aware and will obey the mask if and only if the -mask flag is provided

Liens : psf