swirl operation

docs/examples/operations/swirl.jl

@@ -9,6 +9,7 @@
 # and pass it to `warp`. This swirl example comes from
 # [the Princeton Computer Graphics course for Image Warping (Fall 2000)](https://www.cs.princeton.edu/courses/archive/fall00/cs426/lectures/warp/warp.pdf)
 # and [scikit-image swirl example](https://scikit-image.org/docs/dev/auto_examples/transform/plot_swirl.html).
+# To apply the swirl effect, please use the existing `swirl` function in ImageTransformations.jl directly.
 
 using ImageTransformations
 using OffsetArrays, StaticArrays

src/ImageTransformations.jl

@@ -23,14 +23,16 @@ export
     warpedview,
     InvWarpedView,
     invwarpedview,
-    imrotate
+    imrotate,
+    swirl
 
 include("autorange.jl")
 include("interpolations.jl")
 include("warp.jl")
 include("warpedview.jl")
 include("invwarpedview.jl")
 include("resizing.jl")
+include("swirl.jl")
 include("compat.jl")
 include("deprecated.jl")
 

src/swirl.jl

@@ -0,0 +1,37 @@
+"""
+    swirl(img, rotation, strength, radius, x0 = OffsetArrays.center(img))
+
+Create whirlpool effect on `img`
+
+# Arguments
+- `img`: to specify input image array
+- `rotation`: to specify rotation angle(clockwise)
+- `strength`:to specify amount of swirl 
+- `radius`:to specify extent of swirl 
+- `x0` (optional): to specify the center point of the swirl. The default value is `OffsetArrays.center(img)`.
+# Examples
+
+```julia
+using ImageTransformations, TestImages
+
+
+img = imresize(testimage("cameraman"), (256, 256));
+
+preview = swirl(img, 0, 10, 50, 100) # swirl with rotation 0, strength 10, radius 50, x0 center 100
+```
+"""
+function swirl(img, rotation, strength, radius, x0 = OffsetArrays.center(img))
+    r = log(2)*radius/5
+
+    function swirl_map(x::SVector{N}) where N
+        xd = x .- x0
+        ρ = norm(xd)
+        θ = atan(reverse(xd)...)
+
+        θ̃ = θ + rotation + strength * exp(-ρ/r)
+
+        SVector{N}(x0 .+ ρ .* reverse(sincos(θ̃)))
+    end
+
+    warp(img, swirl_map, axes(img))
+end

test/runtests.jl

@@ -7,14 +7,15 @@ ambs = detect_ambiguities(ImageTransformations, CoordinateTransformations, Base,
 @test isempty(setdiff(ambs, refambs))
 
 # helper function to compare NaN
-nearlysame(x, y) = x ≈ y || (isnan(x) & isnan(y))
-nearlysame(A::AbstractArray, B::AbstractArray) = all(map(nearlysame, A, B))
+nearlysame(x, y, atol = 0.0) = isapprox(x, y; atol = atol) || (isnan(x) & isnan(y))
+nearlysame(A::AbstractArray, B::AbstractArray; atol = 0.0) = all(map((a, b, atol)-> nearlysame(a, b, atol), A, B, atol))
 
 tests = [
     "autorange.jl",
     "resizing.jl",
     "interpolations.jl",
     "warp.jl",
+    "swirl.jl",
     "deprecated.jl" # test deprecations in the last
 ]
 

test/swirl.jl

@@ -0,0 +1,20 @@
+@testset "Swirl Operation" begin
+
+    img = [0 0 0 0 0
+           0 0 1 0 0
+           0 1 0 0 0
+           0 0 1 0 0
+           0 0 0 0 0]
+    expected = [ NaN NaN       0.0        0.0      NaN
+                 0.0 0.431797  0.476449   0.431797 NaN
+                 0.0 0.476449  0.0        0.393894 0.0
+                 NaN 0.431797  0.0825553  0.0      0.0
+                 NaN 0.0       0.0        NaN      NaN]
+    res = swirl(img, 1, 10, 1)
+    replace!(res, NaN=>0.0)
+    replace!(expected, NaN=>0.0)
+    @test nearlysame(res, expected)
+
+    @test nearlysame(swirl(img, 1, 10, 1), swirl(img, 1, 10, 1, OffsetArrays.center(img)))
+
+end