eagerly convert Array type before sending to IO backend

Project.toml

@@ -19,11 +19,12 @@ StackViews = "0.1.1"
 julia = "1"
 
 [extras]
+AxisArrays = "39de3d68-74b9-583c-8d2d-e117c070f3a9"
 ImageDistances = "51556ac3-7006-55f5-8cb3-34580c88182d"
 ImageMagick = "6218d12a-5da1-5696-b52f-db25d2ecc6d1"
 Suppressor = "fd094767-a336-5f1f-9728-57cf17d0bbfb"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 TestImages = "5e47fb64-e119-507b-a336-dd2b206d9990"
 
 [targets]
-test = ["ImageDistances", "ImageMagick", "Suppressor", "Test", "TestImages"]
+test = ["AxisArrays", "ImageDistances", "ImageMagick", "Suppressor", "Test", "TestImages"]

src/showmime.jl

@@ -25,9 +25,7 @@ function Base.show(io::IO, mime::MIME"image/png", img::AbstractMatrix{C};
                    minpixels=10^4, maxpixels=10^6,
                    # Jupyter seemingly can't handle 16-bit colors:
                    mapi=x->mapc(N0f8, clamp01nan(csnormalize(x)))) where C<:Colorant
-    if Base.has_offset_axes(img)
-        img = collect(img)
-    end
+    img = enforce_standard_dense_array(img)
     if !get(io, :full_fidelity, false)
         while _length1(img) > maxpixels
             img = restrict(img)  # big images
@@ -54,6 +52,13 @@ csnormalize(c::AbstractGray) = Gray(c)
 csnormalize(c::Color) = RGB(c)
 csnormalize(c::Colorant) = RGBA(c)
 
+# Unless we have PNG IO backend that works on generic array types, we have to eagerly
+# convert it to dense array types
+# The performance of `convert` depends on the concrete array type `typeof(A)`
+enforce_standard_dense_array(A::AbstractArray) = convert(Array, A)
+enforce_standard_dense_array(A::DenseArray) = A
+enforce_standard_dense_array(A::OffsetArray) = parent(A)
+
 const ColorantMatrix{T<:Colorant} = AbstractMatrix{T}
 
 function _show_odd(io::IO, m::MIME"text/html", imgs::AbstractArray{T, 1}) where T<:ColorantMatrix

test/writemime.jl

@@ -1,6 +1,7 @@
 using ImageShow, ImageCore, FileIO, OffsetArrays
 using ImageCore: PaddedViews
 import ImageBase: restrict
+import AxisArrays: AxisArray
 
 using Test
 
@@ -143,6 +144,15 @@ end
         end
         @test load(fn) == Ac[[1,1,2,2],[1,1,2,2]]
     end
+    @testset "Array types that may not recognized by IO backends" begin
+        A = N0f8[0.01 0.99; 0.25 0.75]
+        A_axis = AxisArray(A)
+        fn = joinpath(workdir, "axis.png")
+        open(fn, "w") do file
+            show(file, MIME("image/png"), Gray.(A_axis), minpixels=5, maxpixels=typemax(Int))
+        end
+        @test load(fn) == A[[1,1,2,2],[1,1,2,2]]
+    end
 end
 try
     # if this fails, it is not our fault