im curious about this

what is the advantage of progressive blurring ?

i tried to blur text by 15 and then a copy of the same text by 5, 10 and 15
i can see see a small difference, but is very subtle

am i missing something ?

"Applying multiple, successive gaussian blurs to an image has the same effect as applying a single, larger gaussian blur, whose radius is the square root of the sum of the squares of the blur radii that were actually applied. "

So the 5/10/15 blur gives the same result as one single blur with radius:
[code]
Sqrt(5²+10²+15²)=Sqrt(25+100+225)=Sqrt(350)~=18.7
[code]

Reality check:
- take random picture (a car)
- duplicate
- blur one layer at 18.7, one at 5/10/15
- top layer in "Difference" mode
- New layer from visible and look at histogram:

The result:

- 98.3% of the pixels have a difference of 2 or less
- 99.6% of the pixels have a difference of 3 or less
- 99.9% of the pixels have a difference of 4 or less
- no pixel has a difference above 19.

Some randomness may have been added by the round-off errors... (which is confirmed by the moiré patterns you get when you threshold that "difference" picture).

Yes to what kind of image to apply

It is for big png cut as example something as http://photocomix2.deviantart.com/favourites/#/d5e82jm or huge poser renders as in most of the pack here http://photocomix2.deviantart.com/favourites/48052473

In your case will not do any difference because there ,in your example will be only a effect on the outer edges , that will be identical of that of a single blur, this because your image has as inside basically a solid color that blurred or sharpened would look identical.

But if instead was applied as example a wrinkled skin (as on a old man face ) difference would be more evident ...same for dragon scales
(less for renders of girl with pesky skin but anyway also there a progressive lowering of the blur till 0 just inside the edges , and then a inversion of the effect , from blur to sharpen does a difference

again the supposed target are large png cut , as huge poser renders

let say for the edges pixels and the merging with the the BG the effect will be identical what change is that the blur fade progressively INSIDE the render and then will start the opposite effect.

Again the transition will be few pixels in images that will be scaled down after the "collage" but still make the render more pop out because the inside is sharpened and the very peryphery slightly blurred, without sharp transition that may look weird



@ofnuti believe you overlook the same point :
there is not difference for the effect from the egdes to the outside but will be difference in the effect inside the render
this because the area to blur changes each time and at last step selection is changed again, enlarged again and then inverted

Again you are right, as Erisian for the effect from the render edges to the OUTSIDE but what change is the propagation INSIDE and each time the radius of the area edit differ of 1 or 2 pixels
1 quite blurred,
subtle blur,
no blur no change,
Dark halo sharpening

"Dark Sharpening" on faces human or animal will make usually pop out more lips and eyes, so the expression pop more up, important if will be a final downscaling

and sure will be possible optimize avoiding to blur twice overlapping area.. but sound complicated to me gaussian blur is quite fast so no sure if would be worth the effort