Creating a starless image

[admin]

Sometimes creating a starless image can be a great, alternative way of showing the features of a deep space object. This short tutorial shows you how to use this accomplish this.
As a source image, we are use the following, rather beautiful image of IC1848 by Immo Gerber (thanks Immo!);

immo_before.jpg (3.5 MiB) Viewed 11411 times

The module that removes the stars and substitutes them for a plausible background is the 'Heal' module. No need to change any of its default settings.

If you'd like to present your image without stars, then the quality of the result will depend on how well you can create a star mask that covers all stars.
To create a mask, start off with the default mask (Mask, Auto, Stars, Do) and see how well that works. If you find that the mask generator also picks up details that are not stars, you can increase (not decrease - slightly counterintuitive, we know! ) the Sensitivity parameter. If you have detail in a particular color channel, you can tell the mask generator to not look in those color channels. For example, if your nebula is mostly red, blue and purple, you can use the 'Exclude Color' parameter in the Mask generator to disregard red, blue or purple details.

Additionally, you will probably want to do some manual touching up of the mask, either by manually switching off parts of the mask that contains non-star detai, or by fixing up very bright stars that have halos around them. Fortunately, the latter is not that hard - to select the halos, click next to the stars (on the boundary of the halo and the start of empty space), while 'Flood Fill Lighter Pixels' is selected for 'Brush Mode'. This will keep looking for pixels (and filling them) that are brighter than the halo pixel that you clicked, effectively progressively filling every pixel going into the center of the star (since stars appear progressively brighter as you get closer to their cores) - very handy!

Doing all this for the image of IC1848, after a few minutes of manual labor, we end up with the following mask;

immo_mask.png (202.87 KiB) Viewed 11411 times

Notice how the star halos in the mask have been fully selected. As outlined above, we accomplished the latter by clicking next to the stars (on the boundary of the halo and the start of empty space), while 'Flood Fill Lighter Pixels' is selected for 'Brush Mode'.

Once you're satisfied with the mask, return to the Heal module to see the result render;

immo_starless.jpg (465.22 KiB) Viewed 11411 times

The result is almost perfect, though some slight faint halos remain. We'll leave it as an exercise to the reader to remove those.

[gustavo_sanchez]

I tried doing this with one of my images, but I could not get rid of the halos... Any suggestions or tricks? I give up :-S

[admin]

I tried doing this with one of my images, but I could not get rid of the halos... Any suggestions or tricks? I give up :-S

Big halos indeed make this technique a lot trickier. Any suggestions would depend on your image - could you post a JPEG?
It can help to ease off on the Wavelet Sharpening of bigger structures (e.g. only sharpen the smaller scales and/or use a high 'Small Detail Bias') as well as masking out the bigger stars when using this module.
Apart from that, the Tighten algorithm in the Magic module can help reduce halos on the bigger stars.
You could also simply decide to simply leave the bigger stars in! The result would still be quite interesting.

[Rowland]

Thanks Ivo. Great thread. Will try this when I get home.