Reduce Star Sizes In GIMP

Reduce star sizes in GIMP! This is a definite way to immediately add punch to your image. By reducing the star sizes you can remove distractions. To me, the stars – whilst pretty – can detract from some images by blanketing them with hundreds of points of light which takes your eyes from the deep sky object. Which is the main focal point of the image, right?

So in this tutorial I will be showing you a way you can use the free image processor GIMP to reduce the star sizes.

If you’re new to editing in GIMP anyway, I have a full tutorial that can be found at the bottom of this page.

How To Reduce Star Sizes In GIMP

Before we start I’ll tell you that in my own workflow I usually perform a star reduction towards the end of processing. By the time I’m reducing stars my deep sky object (DSO) and background sky is where I want them to be, or very close. I’ve performed noise reduction by this stage also, as well as the initial crop to remove stacking artefacts etc.

In this tutorial I’ll be using my image of the Sadr & Butterfly region that was captured with a modified Canon 600D and a Baader 7nm Hydrogen Alpha filter. I’ve already got the nebula quite close to the end of editing here just to keep this tutorial focused and on topic.

Be sure to click on the photos to see a bigger image – they open in a new window!

Beginning Steps

This is where I’m at. The image is at a stage now to begin star reduction processing.

To begin with, you should duplicate the layer. This is to see a before and after, but also for further blending if desired at the end. To do this go to Layer – Duplicate Layer

Now rename these layers. The bottom layer we’ll call “Star Reduction Base”, and the top layer we’ll call “Star Reduction”.

 Selecting The Stars

By now you’ll have done all the initial steps needed. So it’s time to select the stars themselves to be removed. This is very easily done by using the Colour Select tool found Tools – Selection Tools – Colour Select.

By using Colour Select you’re able to isolate specifics. Not only does it affect colours but also luminance. It’s luminance we’re after. Pay attention to the Threshold setting on the left. This determines how much will be selected.

If your Threshold is too high then you’re likely to select the DSO as well as the stars. Again as it’s based on luminosity. You’ll notice this as you’ll just see the selection around areas of the DSO like this.

So what we have to do is lower the Threshold. What you want to do is use the lowest number you can before it begins selecting the DSO. That way you can select all the stars you can, but not any of the object. In this example a Threshold of about 21 is just right.

Even so if you’ve selected even small bits of the DSO, which still can happen, you’re able to use the Free Select Tool.

Holding Control (or command I think on a Mac) and using left click & drag, you’re able to deselect areas. Draw around the area to be removed from the selection, close the selection by returning back to the first part you clicked. Then press Enter/Return to complete.

Now we’ve selected our stars we want to expand our selection and feather it to remove the hard edge.

Expanding the selection means we’re able to select a bit of the background sky. This further helps the blending process and makes the transition that much more smoother. It can be a subtle difference but has a good impact on the overall effect.

With the selection still active, select the Grow tool by going Select – Grow.

From here we want to expand our selection. This can be image specific depending on scale and resolution. For my example a scale of about 3 is about right. The smaller the stars the more the grow will be noticeable.

Now it’s time to feather. What feathering does is softens the hard edge of a selection. This means the effect is more gentle. Like a rolling bank into a lake rather than a sheer drop. If you want to see this in action, skip this step and test it out. Then come back and feather and see the difference!

Go to Select – Feather.

From here you’ll see the pop up. A larger feather can give a more pleasing effect, but the bigger your feather amount the less stars could be affected. This is because feathering can change the selection. In my example a Feather of about 5 is more than enough.

Ignore that this says 10! Blunder during screenshotting!

Now we’re finally ready to reduce the stars.

Reduce Star Sizes

Finally all the preparation work has been done. It’s fine to actually now reduce your star sizes in GIMP. To do this we’re going to use a filter called Value Propagate.

Go to Filters – Distorts – Value Propagate.

This allows you to add more white or more black to the image. To reduce the star sizes we need to change the mode to more black.

Click Mode and from the drop down menu select More Black (Smaller Value). You’ll immediately see the difference!

If this affect is too much for you, you can change the Propagating Rate to reduce the amount of black added to the selection. Reduce that down to your taste. Then sit back and admire how much more your image is popping!

Picture saved with settings embedded.
Picture saved with settings embedded.

If this is still too much for you, then it’s here where having the background layer comes in. On the Star Reduction layer (the top one that is) change the opacity slider down. This helps you show more of the original image with the full size stars and allows you to blend the reduced stars in.

From here all the stars have been reduced. If you want to reduce big bright stars more, then you can repeat this process. However use a smaller threshold and also layer masks. Repeat the value propagate multiple times if required. Fill it with a white mask, and use a black paint brush on the white mask to show the smaller stars as desired.

Now you can proceed on with final image editing and move on.

Thanks very much for reading. I do hope this tutorial has been useful for you. If it has, let me know! There’s also a video tutorial on my YouTube channel.


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