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Live blur to alpha channel only tutorial

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Consider you have a nice pixel layer (or any other object like shapes, groups etc) with a mask layer. There are multiple ways to this destructively, e.g. by blur brush, using refine mask & feather, or apply a destructive Gaussian blur.

Now what if you want to feather the mask non-destructively?
I have been searching for years to find any possible method. The live blur filter affects either all channels, or you can protect alpha, but this is just the opposite of what you want.

By accident i stumbled over an odd behavior, which delivers what is desired: limiting the blur live filter to alpha channel only. The trick is grouping the layers, add a live blur filter, and invert the inherent mask. The blur does not affect the RGB channels, but still affect the alpha channel.

Steps to reproduce:

  • assuming you have already a group, containing your layers and one nested mask layer. Blend mode needs to be set to passthrough.
  • add a live blur filter to the group. set radius to taste.
  • move blur filter to masking position.
  • invert inherent mask (cmd/ ctrl I) of blur layer

I cannot guarantee that this trick will work forever. It could be an bug and may change in future releases.

If this is rated as bug, please implement the feature request to have an official way to achieve this function. See https://forum.affinity.serif.com/index.php?/topic/141381-option-alpha-only-for-all-filters-adjustments/&tab=comments#comment-835914


PS: this trick opens unlimited creative options.

  • you can add multiple blurs, e.g. radial blur + Gaussian gives a nice paper scissors effect from a sharp freehand selection
  • add maximum / minimum blur to grow / shrink mask
  • voronoi gives a kind of dissolve effect
  • you can use vector shapes / curves instead of masks


Never the less, it is the first way to limit the effect of live filters to alpha channel.


tutorial blur on alpha channel.afphoto

Mac mini M1 A2348

LG34WK950U-W, calibrated to DCI-P3 with LG Calibration Studio / Spider 5

iPad Air Gen 5 (2022) A2589

Special interest into procedural texture filter, edit alpha channel, RGB/16 and RGB/32 color formats, stacking, finding root causes for misbehaving files, finding creative solutions for unsolvable tasks, finding bugs in Apps.


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