Comparing images and returning a difference image

[kipcole9]

The upcoming image version 0.34 includes Image.compare/3 which brings it closer (but not completely) to ImageMagick. One of the requirements stated has been to build a "difference image" that shows what pixels are different between the two images. Here is an example:

iex> image_1 = Image.open!("./test/support/images/Kamchatka-2019-8754.jpg")

# Image 2 is the same image, only with a cat overlayed upon it.
iex> cat = Image.open! "./test/support/images/cat.png"
iex> image_2 = Image.compose!(i, cat, x: :middle, y: :center) |> Image.flatten!

# Now we can compare the image with the defaults of 
# `metric: :ae, difference_color: :red, saturation: 1.0, brightness: 1.0, difference_boost: 1.5`
iex> {:ok, metric, difference_image} = Image.compare(image_1, image_2)
{:ok, 0.13152029520295203,
 %Vix.Vips.Image{ref: #Reference<0.1501890617.2351562776.180408>}}

# Note how the pixels that were different between the two images are masked in red overlaid on the original image. Its possible to adjust the brightness and saturation of the base image and to boost them for the difference pixels.  Here's an example:
iex> {:ok, metric, difference_image} = Image.compare image_1, image_2, brightness: 0.5, saturation: 0.2, difference_boost: 2.0
{:ok, 0.13152029520295203,
 %Vix.Vips.Image{ref: #Reference<0.1501890617.2351562776.180435>}}

Note how the difference pixels stand out more strongly. You can also see there is a color gradient to the difference pixels. The shading indicates how much different (mathematical difference, not perceptual difference) the pixels are.