specialize imrotate for 0, 90, 180, 270 degrees

Project.toml

@@ -1,6 +1,6 @@
 name = "ImageTransformations"
 uuid = "02fcd773-0e25-5acc-982a-7f6622650795"
-version = "0.8.0"
+version = "0.8.1"
 
 [deps]
 AxisAlgorithms = "13072b0f-2c55-5437-9ae7-d433b7a33950"
@@ -17,14 +17,14 @@ StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"
 
 [compat]
 AxisAlgorithms = "1.0"
+ColorTypes = "0.7.3, 0.8"
 ColorVectorSpace = "0.2, 0.3, 0.4, 0.5, 0.6, 0.7"
 Colors = "0.7, 0.8, 0.9"
-ColorTypes = "0.7.3, 0.8"
 CoordinateTransformations = "0.5"
 FixedPointNumbers = "0.5, 0.6"
+IdentityRanges = "0.3"
 ImageCore = "0.7, 0.8"
 Interpolations = "0.9, 0.10, 0.11, 0.12"
-IdentityRanges = "0.3"
 OffsetArrays = "0.10, 0.11"
 StaticArrays = "0.10, 0.11, 0.12"
 julia = "1"

src/warp.jl

@@ -112,7 +112,7 @@ By default, rotated image `imgr` will not be cropped. Bilinear interpolation wil
 ```julia
 julia> img = testimage("cameraman")
 
-# rotate with bilinear interpolation but without cropping 
+# rotate with bilinear interpolation but without cropping
 julia> imrotate(img, π/4)
 
 # rotate with bilinear interpolation and with cropping
@@ -124,8 +124,28 @@ julia> imrotate(img, π/4, Constant())
 
 See also [`warp`](@ref).
 """
-function imrotate(img::AbstractArray{T}, θ::Real, args...) where T
-    θ = floor(mod(θ,2pi)*typemax(Int16))/typemax(Int16) # periodic discretezation
-    tform = recenter(RotMatrix{2}(θ), center(img))
-    warp(img, tform, args...)
+function imrotate(img::AbstractMatrix{T}, θ::Real, args...) where T
+    # 1. discretize periodic for numerical stability to make sure
+    # imrotate(img, θ+2pi) == imrotate(img, θ)
+    # 2. typemax(Int16) is 32767, we choose 32760 to make sure the
+    # discretization result of pi/2 is exactly pi/2 (or 90°)
+    max_num_angles = 32760
+    θ = round(Int, 180*floor(mod(θ, 2pi)/pi*max_num_angles)/max_num_angles)
+    tform = recenter(RotMatrix{2}(θ/180*pi), center(img))
+    if θ == 0
+        return img
+    elseif θ == 90
+        idx = StepRange.(axes(img))
+        perm_img = PermutedDimsArray(img, (2, 1))
+        return view(perm_img, idx[2], reverse(idx[1]))
+    elseif θ == 180
+        idx = map(i->1:1:length(i), axes(img))
+        return view(img, reverse(idx[1]), reverse(idx[2]))
+    elseif θ == 270
+        idx = StepRange.(axes(img))
+        perm_img = PermutedDimsArray(img, (2, 1))
+        return view(perm_img, reverse(idx[2]), idx[1])
+    else
+        return warp(img, tform, args...)
+    end
 end

test/warp.jl

@@ -427,15 +427,26 @@ ref_img_pyramid_grid = Float64[
                 @test_nowarn imrotate(img, π/4, Linear())
                 @test_nowarn imrotate(img, π/4, axes(img))
                 @test_nowarn imrotate(img, π/4, axes(img), Constant())
-                @test isequal(channelview(imrotate(img,π/4)), channelview(imrotate(img, π/4, Linear()))) # TODO: if we remove channelview the test will break for Float
+                @test isequal(channelview(imrotate(img, π/4)), channelview(imrotate(img, π/4, Linear()))) # NaN != NaN
             end
+
+            # check special rotation degrees
+            img = rand(Gray{N0f8}, 100, 50)
+            @test size(imrotate(img, pi/2)) == (50, 100)
+            @test size(imrotate(img, pi)) == (100, 50)
+            @test size(imrotate(img, 3pi/2)) == (50, 100)
+            @test size(imrotate(img, 2pi)) == (100, 50)
+            rotated_img = imrotate(imrotate(imrotate(imrotate(img, pi/2), pi/2), pi/2), pi/2)
+            @test rotated_img == img
+            rotated_img = imrotate(imrotate(img, pi), pi)
+            @test rotated_img == img
         end
 
         @testset "numerical" begin
             for T in test_types
                 img = Gray{T}.(graybar)
                 for θ in range(0,stop=2π,length = 100)
-                    @test isequal(channelview(imrotate(img,θ)), channelview(imrotate(img,θ+2π))) # TODO: if we remove channelview the test will break for Float
+                    @test isequal(channelview(imrotate(img,θ)), channelview(imrotate(img,θ+2π))) # NaN != NaN
                 end
             end
         end