Technique request - depth of field blur

[PixelSnader]

Hey, I was wondering if anyone had figured out a way to use a mask, or a z-depth image to blur an image?

In Photoshop you can use a black and white image to partially blur an image, to get a narrow depth-of-field effect:

 

I've already tried to use a full-image blur with a mask, but that gives 80s-esque halo effects. The halo is there because the whole image (BG+dress) is blurred, while the dress should not be taken in to consideration for the BG-blur. It is quite noticable around the dress here:

Anyone have a usable method for this effect in Affinity?

[John Rostron]

Having selected your unblurred target (such as the dress and occupant), you need to cut it out (to the clipboard) using ctrl-X or cmnd-X.  Then activate the gaussian blur filter making sure that you tick Preserve Alpha. Then paste your clipboard back nto your image using ctrl-V.or cmnd-V

Ticking Preserve Alpha ensures that the blurring is not affected by the transparent hole.

John

[digit42]

Great tip - thank you!