I have a group of shapes that each have their own unique gradient fill. I would like to use the union of those shapes to create an alpha mask for an image behind it.

In other words, I want the union of the alpha mask effects on a bitmap/image.

Things I've tried:

  • union of the shapes, but I loose the fill information.

  • flattening shapes to bitmap, but it seems there is no way to mask a bitmap with a bitmap (use as mask is not selectable)?

Here is a screenshot of example shapes I'm talking about:


  • an aplha mask is a greyscale bitmap. Can you merge the shapes so you only have the mask on a white background, then copy'n'paste that into a blank alpha mask layer? This may entail a secondary temporary image/export to a raster format.
    – Yorik
    Commented Nov 6, 2015 at 15:31
  • hi @Yorik. when I merge the shapes loose the unique fills for each shape. how do i prevent that from happening?
    – mattr
    Commented Nov 6, 2015 at 15:40
  • rasterize each shape first, then merge the layers.
    – Yorik
    Commented Nov 6, 2015 at 17:01

1 Answer 1


Let me see if I understand the scenario:
You have several shapes, each with a different gradient fill, which you want to merge into one, keeping the unique fills, and apply that as a mask over an image. In Sketch.

Two things to keep in mind:

  1. Sketch is not a photo editing environment. I don't know if it does alpha channel masks, but I really wouldn't want it to take on that bloat.
  2. Merged shapes are stylistically merged as well. You can't have separate style settings between the constituent parts.
  • hey @plainclothes. Thanks for answering! On #1, Alpha masks are supported in Sketch for single shapes: sketchapp.com/support/documentation/04-shapes/4-masking.html. I simply want the union of that masking effect. On #2, thanks for some clarification.
    – mattr
    Commented Nov 6, 2015 at 19:08
  • In my experience, the alpha thing is very spotty. Does it ever work for you on raster layers? Commented Nov 6, 2015 at 19:22
  • Yeah works fine as single shape on bitmap. Thinking about I guess I could just copy the bitmap - one for each shape - apply the masks individually to each bitmap and just group the results. I'll try that. Thanks again you def got me thinking!
    – mattr
    Commented Nov 6, 2015 at 19:45
  • Glad to be a sounding board ... if I were you, I'd be spinning up Affinity Photo right now (^‿- ) Commented Nov 6, 2015 at 20:11

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