Selective Gaussian Blur Aux Input

[QuinB]

Can anybody help us, please, with information on how to use the

Aux. Input

option on the Selective Gaussian Blur filter?


That you're meant to choose a layer as some sort of mask seems obvious, but basic experiments don't seem to get very far and the manual entry stating "Aux. input: If there is another image on screen, you can select it as auxiliary input." is not as informative as hoped.

There are some very informative articles on using Selective Gaussian Blur for Gimp 2.8 (or perhaps earlier), but nothing for the new option, so almost anything to help explain the how and why would be useful

[denzjos]

The selective gaussian blur seems to blur one picture with another. It depends on the colors from the image one choose in the selective blur how the blur is made and what is the result. I have seen that the contours of the image one choose for blurring is visible in the result.

     

[QuinB]

Well, that's a step further then I got!
I opted for a black and white test mask and found a small effect, as you might expect from the way the filter works, at sharp borders, but had not investigated colour.
Still haven't got any solid rules as to its effect, but tomorrow's another day.
Much obliged for the input, aux or otherwise.

[Ofnuts]

From what I understand of the source code:

• Selective blur is a Gaussian blur that is applied only in low-contrast areas,  i.e., in areas where the difference between a pixels and its neighboring pixels is under some threshold.
  
• In normal operation the contrast is computed on the layer that is blurred
  
• When an aux input layer is provided, the contrast is computed on that layer instead of being computed on the blurred layer
  

[QuinB]

Very nice!
Thanks for checking that for us.
As I frequently say, "If Anybody Knows, Ofnuts Knows".