Add max min filter demo (#190)

Co-authored-by: Johnny Chen <[email protected]>

.gitignore

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 *.jl.cov
 *.jl.*.cov
 *.jl.mem
+.vscode
 docs/build
 docs/site
 docs/Manifest.toml

docs/demos/filters/min_max_filter.jl

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+# ---
+# title: Max min filters
+# cover: assets/max_min.gif
+# author: Deeptendu Santra
+# date: 2020-10-08
+# ---
+
+# In this tutorial we see how can we can effectively use max and min filter to distinguish
+# between smooth and texture edges  in grayscale images.
+
+# We will use the [`mapwindow`](@ref) function in `ImageFiltering.jl` which provides a general
+# functionality to apply any function to the window around each pixel.
+
+using ImageCore, ImageShow, ImageFiltering
+using TestImages
+
+img = Gray.(testimage("house");)      # Original Image
+
+# We can use the `minimum` function to compute the minimum of the grayscale values in the given
+# matrix or array. For example: 
+minimum([Gray(0.7),Gray(0.5),Gray(0.0)]) # Should return Gray(0.0) i.e black.
+# 
+filter_size = (15, 15)
+## Using the `mapwindow` function, we create an image of the local minimum.
+## `mapwindow` maps the given function over a moving window of given size.
+img_min = mapwindow(minimum, img, filter_size)
+## Similarly for maximum
+img_max = mapwindow(maximum, img, filter_size)
+## The max(min) filter
+img_max_min = mapwindow(maximum, img_min, filter_size)
+## The min(max) filter
+img_min_max = mapwindow(minimum, img_max, filter_size)
+mosaicview(img_min, img_max, img_max_min, img_min_max; nrow=1)
+
+# Now that we are done with the basic filtered images, we proceed to the next part
+# which is edge detection using these filters.
+
+# For edge detection, we need to define thresholds for our image. The threshold is an important tool
+# to binarize a grayscale image. The threshold value for a given pixel practically
+# decides if the pixel visible or not in the output image. However, appyling a global threshold might not consider the
+# variation of colors/brightness within the image. Thus we consider an adaptive type theresholding method
+# here using max/min results.
+
+# The max(min) and min(max) filter effectively follows the smooth edges of the image. Therefore,
+# their average also follows the smooth parts of the image. If we use this image as a threshold for the original
+# image, the smooth parts of the original image will get filtered out, leaving only the texture and/or noise behind.
+# So we can use the average of `img_max_min` and `img_min_max` as the texture threshold.
+
+# The average of min and max filters also gives us the smooth edges, but it also includes the noise in the image.
+# So using average of `img_min` and `img_max` as a threshold will yeild only texture.
+
+img_texture_noise_threshold = (img_max_min + img_max_min) ./ 2
+img_texture_threshold = (img_max + img_min) ./ 2
+mosaicview(img_texture_noise_threshold, img_texture_threshold; nrow=1)
+#
+## The Dynamic Gist is obtained by subtracting the img_texture_threshold from the original image.
+## The filtered image gives us the texture of the image.
+img_dynamic_gist = img - img_texture_threshold
+## The Texture Gist is obtained by subtracting img_texture_noise_threshold from the original image.
+## The filtered image gives us the texture along with the noise of the image.
+img_texture_gist = img - img_texture_noise_threshold
+mosaicview(img_dynamic_gist, img_texture_gist; nrow=1)
+#
+## We extract out the smooth/ramp parts of the image.
+## The darker section of the image consist of the ramp edges. The brighter pixels are mostly noise.
+ramp = img_dynamic_gist - img_texture_gist
+## Filtered-out edges
+edge = img_max - img_min
+## Smoothed-out version of edge
+edge_smoothed = img_min_max - img_max_min
+mosaicview(img, ramp, edge, edge_smoothed; nrow=2)
+
+# # References
+
+# Verbeek, P. W., Vrooman, H. A., & Van Vliet, L. J. (1988). [Low-level image processing by max-min filters](https://core.ac.uk/download/pdf/194053536.pdf). Signal Processing, 15(3), 249-258.
+
+## save covers #src
+using ImageMagick #src
+mkpath("assets")  #src
+ImageMagick.save("assets/max_min.gif", cat(img, edge, edge_smoothed, map(clamp01nan, ramp);dims=3);fps=1) #src