Extract masked colour from image and save with transparent background

[slashdotdash]

I'm trying to detect the edges of a shape within an image and then extract an outline of only the detected edges into a new image with a transparent background.

Example

Here's an example input image to demonstrate the outcome I am trying to acheive, it's a jigsaw piece.

First I detect the edges of the jigsaw piece using the evision package, adding the green outline and cropping the image to the size of detected edges. See Find and Draw Contours in an Image

Then I want to extract only the outline as a new image with a transparent background. I am able to do this manually using the Preview app in macOS by selecting the green outline with the "instant alpha" tool and copying it into a new image.

Unfortunately I have so far been unable to do this last step using either the evision or image library. Any help would be much appreciated.

The purpose of wanting to have the image outlines with a transparent background is to allow them to overlayed for comparison. The actual use case is product comparison, rather than for jigsaw pieces!

[kipcole9]

@slashdotdash, not too difficult in image I think. I'm on a flight for a bit so will work up an example.

The basic idea is to:

1. Convert the image to greyscale, increase the contrast a bit to improve the edges. Something like grey = original_image |> Image.to_colorspace(:bw) |> Image.contrast!(2.0)
2. Run a canny edge detector like canny = Vix.Vips.Operation.canny!(grey)
3. Convert the image from a float back to 8-bit with mask = Vix.Vips.Operation.linear!(canny, [64], [0], uchar: true)
4. Potentially bur the edge with mask = Image.blur!(mask)
5. Add this edge image as an alpha band to the original image with Image.add_alpha(original_image, mask)

There is also a Vix.Vips.Operation.sobel you might care to try.

Anyway, I'll work up an example in the next few hours using your test piece.

[kipcole9]

You can also use Image.trim/2 to trim the image either before or after processing.

[kipcole9]

I've put together a Livebook with an example of what I think you're trying to do. I need to do a a lot more tutorials like this so let me know if (a) it does what you expect and (b) if you have any improvements you would suggest.

[slashdotdash]

Wow! Thank you for the prompt response and providing a working Livebook to demonstrate it. This is exactly what I was trying to achieve.

Providing tutorials as Livebooks is a fantastic way of demonstrating features in a way that can be played around with and modified to fit a desired use case.

[kipcole9]

Cool, glad you found it helpful. I learn a lot doing these too :-)