Blur a section of an image?

[tielur]

Is there a way to blur only the section of an image? Like the ability to blur only a cropped part of an image?

[kipcole9]

One of the great things about libvips is that is quite "functional" in its design. So images are immutable (unless you specifically take special action). And they compose well. Therefore to blur a region of an image is, in libvips terms, equal to:

• Crop the region
• Blur the crop
• Compose the region back over the top of the base image

That would be something like the following (not tested):

def blur_region(image, left, top, width, height) do
  with {:ok, region} <- Image.crop(image, left, top, width, height),
       {:ok, blurred} <- Image.blur(region) do
    Image.compose(image, blurred, x: left, y: top)
  end
end

I have a few ideas no how to generalise this kind of operation where an image is represented as a set of composable regions but I've not found a model I'm entirely happy with yet.

[tielur]

Simple enough! I'll play around with this. Thank you

[kipcole9]

There is another approach whereby you blur a copy of the image and then create an image mask. The mask can be an arbitrary shape (helpful if you want circular or elliptical blurring of faces) and then you compose the mask as an alpha channel. I can work up an example of that over the weekend.