I'm trying to simulate a cloud of data points around a graph and I found that the air spray filter applied to a copy of that graph will give me almost what I want, especially when changing the morphology effect operator from dilate to erode.


This doesn't look too bad, although now the wannabe data points are maybe a bit too weak depending on output resolution. In any case, especially when magnified, you can see another problem:


Some of the pixels have a really low alpha value which doesn't make sense in my scenario. I'd like all pixels to have an alpha value of 1.

I tried playing around with the two blend effects the air spray filter contains but to no avail. I then tried to force the alpha channel to 1 using the colour matrix effect with the following setting.

1 0 0 0 0
0 1 0 0 0
0 0 1 0 0 
0 0 0 0 1

But of course this sets everything that has alpha value 0 to 1, too, with a rather devastating effect.

background turns black

Is there a way to turn only the orange pixels non-transparent but not the background?

I know, when exporting this to PDF it's unlikely that the air spray effect will survive without being rendered as a raster image anyway so I probably could go through that cycle once and then do something to the raster image. But if there is a more elegant way to do this, that'd be great.

  • Don't you think you would achieve a better effect by simply using the spray can tool (Shift-F3)?
    – Moss
    Commented Nov 22, 2013 at 17:17
  • @Moss But how would I make that follow an exact path?
    – Christian
    Commented Nov 23, 2013 at 21:01
  • By tracing over the path when you use the spray can tool. It doesn't need to be totally exact does it? I mean, it is a fuzzy cloud of points. Do you need to be able to manipulate the path and have the points follow? Or does it need to be an automatic process?
    – Moss
    Commented Nov 25, 2013 at 5:45
  • @Moss Yes, it needs to be pretty exact ... and also, I'm all thumbs with a mouse.
    – Christian
    Commented Nov 26, 2013 at 12:39

1 Answer 1


Might be I just found an answer to my own question. It's not pretty though so actually good solutions are still welcome. You can multiply the alpha channel with some insanely high value, again using the colour matrix effect:

1    0    0    0    0
0    1    0    0    0
0    0    1    0    0 
0    0    0 1000    0

Zero alpha will of course remain so while everything else will be cropped to 1.

crop without visible transparent pixels

Why there aren't some pixels whose alpha channel was so close to zero that now they are ugly transparent pixels I don't know. Well, maybe there are but as long as this isn't visible, I guess I don't care.

But as I said, this is no solution but an ugly hack so if you know a better way, please feel free to share it here.

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