add democard: image difference view

Author: johnnychen94

docs/examples/spatial_transformation/image_diffview.jl

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+# ---
+# title: Image Difference View
+# cover: assets/image_diffview.png
+# ---
+
+# This demonstration shows some common tricks in image comparision -- difference view
+
+# People with MATLAB experiences would miss the function
+# [`imshowpair`](https://www.mathworks.com/help/images/ref/imshowpair.html), but in JuliaImages
+# it is not that indispensable.
+
+using Images, MosaicViews
+using TestImages
+
+img = float.(testimage("cameraman"))
+## rotate img by 4 degrees and keep axes unchanged
+img_r = imrotate(img, -pi/45, axes(img))
+nothing #hide #md
+
+# `mosaicview` is a convenience tool to show multiple images, especially useful when they have
+# different sizes and colors.
+
+mosaicview(img, img_r; nrow=1, npad=20, fillvalue=colorant"white")
+
+# In some cases, when the differences of two images are relative insignificant, a plain
+# substraction can help amplify the difference.
+plain_diffview = @. img - img_r
+nothing #hide #md
+
+# For gray images, a fancy trick is to fill each image into different RGB channels
+# and make a RGB view
+RGB_diffview = colorview(RGB, channelview(img), channelview(img_r), fill(0., size(img)))
+nothing #hide #md
+
+# or convert the RGB view back to Gray image after that
+Gray_diffview = Gray.(RGB_diffview)
+
+mosaicview(plain_diffview, RGB_diffview, Gray_diffview;
+           nrow=1, npad=20, fillvalue=colorant"white")
+
+
+# --- save covers --- #src
+using FileIO #src
+save("assets/image_diffview.png", RGB_diffview) #src