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I'm attempting to use the compositor to blur a test render. I'm hoping to make a scene where I fly through a space filled with stars of various sizes, colors and brightnesses.

To visually underscore the brightness of the stars, I'm using the Gaussian Blur node in the Compositor. I was hoping that the brighter stars would have "glare" that changed in size and intensity depending on the brightness of the star and its emission strength.

But, before I do this, I'm running up against an issue that I can't resolve. I'm getting a bizarre looking "square" blur when I crank up the size of the blur beyond a certain point.

Here's my test image before the composite blur is added:

enter image description here

And here is the image after the blur is added:

enter image description here

Here is what happens when I use the "glare" node:

enter image description here

enter image description here

I'd like the blur to be circular.

Exporting the files and adding the blur to individual images in After Effects or Photoshop would not work, because these programs cannot distinguish the between a rendered object that has an emission from a small or large object, or differentiate between an object that has an emission strength of 1 from one that has one of 100 (as Blender can):

enter image description here

Left icosphere: emission strength 2 Middle icosphere: emission strength 20 Right icosphere: emission strength 200

Any thoughts?

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    $\begingroup$ Consider sharing your .blend file so that others can inspect it. You can upload it at blend-exchange.giantcowfilms.com and then paste the resulting link as part of your question. $\endgroup$
    – user1853
    Commented Mar 7, 2019 at 23:47
  • $\begingroup$ AND comment here. Otherwise, we won't be notified. $\endgroup$
    – Lauloque ♦
    Commented Mar 8, 2019 at 9:14
  • $\begingroup$ Try spheres/ icospheres with higher subdivisions $\endgroup$
    – WhatAMesh
    Commented Mar 8, 2019 at 19:34
  • $\begingroup$ Hi, I seemed to have solved the problem by resetting blender to its factory settings. I don't know what the issue is. However, I have another issue: Flicker. I uploaded the file that I'm working with. Between each from, the brightness of the flog glare that I'm using changes dramatically, giving an annoying flicker. Thoughts on how to stabilize this? <img src="https://blend-exchange.giantcowfilms.com/embedImage.png?bid=5988" /> $\endgroup$
    – Brad Hoehne
    Commented Mar 12, 2019 at 20:34

1 Answer 1

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A "true" gaussian blur is not limited by distance - if the 'centre' point is strong enough it would have an effect on even the most distant pixels in the image (the gaussian distribution falls off towards zero as the distance increases but never actually reaches it). For efficiency, the Gaussian blur in Blender is limited by range in each of the X and Y directions - so the value falls off with distance but cuts off at the edge of the range. Since the range is determined only in the X and Y directions this forms a 'square of effect' of the blur from any pixel in your image.

This would not normally cause a problem since relatively low intensity pixels would naturally drop off to very close to zero at the edge of the range when the gaussian blur is taken into effect. However, in your case you have extremely large values for your bright points. This means that the gaussian has not dropped quite so close to zero at the edge of its range to be unnoticeable and so the edge of that 'square of effect' is noticeable.

The only solutions I can suggest are to either limit the brightest stars to lower values or to blur them in multiple stages to 'spread' their impact more evenly - eg, instead of a single Blur of 20x20 you could use one at 10x10 to soften them out followed by another at 17x17 (I picked these values 'by eye') to produce the following :

blur example

(The left-hand side is the 'single' blur (showing the square artifact edge) while the right-hand is the 'double' blur)

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  • $\begingroup$ Thank you Rich. I seem to have found a solution (though I don't know why it works). I reset blender 2.79b to its factory settings. The blur is, for some reason, no longer square. The Bokeh blur is not what I'm after- since that makes the blur the roughly the same size no matter the brightness. I want the size of the glare to give some indication of the brightness of the stars. The dimmest stars will have very little glare, or at least the glare will drop off very quickly, and the brightest, when they are nearby, will nearly wash out the view. $\endgroup$
    – Brad Hoehne
    Commented Mar 12, 2019 at 20:34

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