imresize: handle 0-parameter colorant types

src/resizing.jl

@@ -351,7 +351,11 @@ end
 # changes correspond to integer ratios.  We mimic ratio arithmetic
 # without actually using Rational (which risks promoting to a
 # Rational type, too slow for image processing).
-imresize_type(c::Colorant) = base_colorant_type(c){eltype(imresize_type(Gray(c)))}
+function imresize_type(c::Colorant)
+    CT = base_colorant_type(c)
+    isconcretetype(CT) && return CT # special 0-parameter colorant types: ARGB32, etc
+    CT{eltype(imresize_type(Gray(c)))}
+end
 imresize_type(c::Gray) = Gray{imresize_type(gray(c))}
 imresize_type(c::FixedPoint) = typeof(c)
 imresize_type(c) = typeof((c*1)/1)

test/resizing.jl

@@ -181,4 +181,26 @@ end
         @test OffsetArrays.no_offset_view(out) == imresize(img, (128, 128), method=Lanczos4OpenCV())
         end
     end
+
+    @testset "special RGB/Gray types (#97)" begin
+        ori = repeat(distinguishable_colors(10), inner=(1, 10))
+        for T in (
+            RGB, BGR, RGBX, XRGB,
+            ARGB, RGBA,
+            RGB24, ARGB32,
+        )
+            img = T.(ori)
+            out = @inferred imresize(img, ratio=2)
+            @test eltype(out) <: T
+            ref = imresize(ori, ratio=2)
+            @test ref ≈ RGB.(out)
+        end
+        for T in (Gray, AGray, GrayA, Gray24)
+            img = T.(ori)
+            out = @inferred imresize(img, ratio=2)
+            @test eltype(out) <: T
+            ref = imresize(Gray.(ori), ratio=2)
+            @test ref ≈ Gray.(out)
+        end
+    end
 end