implement FiniteDiffs submodule with new operators

New operators:

- gradient and adjoint gradient: `fgradient`
- divergence: `fdiv`
- laplacian: `flaplacian`

To better organize the symbols, I deprecate two entrypoints:

* `ImageBase.fdiff` => `ImageBase.FiniteDiffs.fdiff`
* `ImageBase.fdiff!` => `ImageBase.FiniteDiffs.fdiff!`

Author: johnnychen94

Project.toml

@@ -14,6 +14,7 @@ julia = "1"
 [extras]
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
+ImageFiltering = "6a3955dd-da59-5b1f-98d4-e7296123deb5"
 ImageIO = "82e4d734-157c-48bb-816b-45c225c6df19"
 ImageMagick = "6218d12a-5da1-5696-b52f-db25d2ecc6d1"
 OffsetArrays = "6fe1bfb0-de20-5000-8ca7-80f57d26f881"
@@ -23,4 +24,4 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 TestImages = "5e47fb64-e119-507b-a336-dd2b206d9990"
 
 [targets]
-test = ["Aqua", "Documenter", "Test", "ImageIO", "ImageMagick", "OffsetArrays", "Statistics", "StackViews", "TestImages"]
+test = ["Aqua", "Documenter", "Test", "ImageFiltering", "ImageIO", "ImageMagick", "OffsetArrays", "Statistics", "StackViews", "TestImages"]

src/ImageBase.jl

@@ -5,11 +5,6 @@ export
     # originally from ImageTransformations.jl
     restrict,
 
-    # finite difference on one-dimension
-    # originally from Images.jl
-    fdiff,
-    fdiff!,
-
     # basic image statistics, from Images.jl
     minimum_finite,
     maximum_finite,
@@ -26,7 +21,6 @@ using Reexport
 using Base.Cartesian: @nloops
 @reexport using ImageCore
 using ImageCore.OffsetArrays
-using ImageCore.MappedArrays: of_eltype
 
 include("diff.jl")
 include("restrict.jl")

src/deprecated.jl

@@ -8,4 +8,9 @@
 @deprecate maxfinite(A; kwargs...) maximum_finite(A; kwargs...)
 @deprecate maxabsfinite(A; kwargs...) maximum_finite(abs, A; kwargs...)
 
+# These two symbols are exported by previous ImageBase versions and now organized in the
+# FiniteDiffs submodule.
+@deprecate fdiff ImageBase.FiniteDiffs.fdiff
+@deprecate fdiff! ImageBase.FiniteDiffs.fdiff!
+
 # END 0.1 deprecation

src/diff.jl

@@ -1,5 +1,34 @@
-# TODO: add keyword `shrink` to give a consistant result on Base
-#       when this is done, then we can propose this change to upstream Base
+# Because there exists the `FiniteDiffs.jl` package with quite different purposes,
+# this module is not expected to be reexported.
+module FiniteDiffs
+
+using ImageCore
+using ImageCore.MappedArrays: of_eltype
+
+"""
+Although stored as an array, image can also be viewed as a function from discrete grid space
+Zᴺ to continuous space R (or C if it is complex value). This module provides the discrete
+version of gradient-related operators by viewing image arrays as functions.
+
+This module provides:
+
+- forward/backward difference [`fdiff`](@ref) are the Images-flavor of `Base.diff`
+- gradient operator [`fgradient`](@ref) and its adjoint via keyword `adjoint=true`.
+- divergence operator [`fdiv`](@ref) is the negative sum of the adjoint gradient operator of
+  given vector fields.
+- laplacian operator [`flaplacian`](@ref) is the divergence of the gradient fields.
+
+Every function in this module has its in-place version.
+"""
+FiniteDiffs
+
+export
+    fdiff, fdiff!,
+    fdiv, fdiv!,
+    fgradient, fgradient!,
+    flaplacian, flaplacian!
+
+
 """
     fdiff(A::AbstractArray; dims::Int, rev=false, boundary=:periodic)
 
@@ -19,7 +48,7 @@ Take vector as an example, it computes `(A[2]-A[1], A[3]-A[2], ..., A[1]-A[end])
 
 # Examples
 
-```jldoctest; setup=:(using ImageBase: fdiff)
+```jldoctest; setup=:(using ImageBase.FiniteDiffs: fdiff)
 julia> A = [2 4 8; 3 9 27; 4 16 64]
 3×3 $(Matrix{Int}):
  2   4   8
@@ -106,3 +135,95 @@ _fdiff_default_dims(A::AbstractVector) = 1
 maybe_floattype(::Type{T}) where T = T
 maybe_floattype(::Type{T}) where T<:FixedPoint = floattype(T)
 maybe_floattype(::Type{CT}) where CT<:Color = base_color_type(CT){maybe_floattype(eltype(CT))}
+
+
+"""
+    fdiv(Vs...)
+
+Compute the divergence of vector fields `Vs`.
+"""
+function fdiv(V₁::AbstractArray{T}, Vs::AbstractArray{T}...) where T
+    fdiv!(similar(first(V₁), maybe_floattype(eltype(V₁))), V₁, Vs...)
+end
+function fdiv!(out::AbstractArray, V₁::AbstractArray, Vs::AbstractArray...)
+    # This is the optimized version of `sum(v->fgradient(v; ajoint=true), (V₁, Vs...))`
+    # by removing unnecessary memory allocations.
+    all(v->axes(v) == axes(out), (V₁, Vs...)) || throw(ArgumentError("All axes of vector fields Vs and X should be the same."))
+
+    # TODO(johnnychen94): for better performance, we can eliminate this `tmp` allocation by fusing multiple `fdiff` in the inner loop.
+    out .= fdiff(V₁; dims=1, rev=true)
+    tmp = similar(out)
+    for i in 1:length(Vs)
+        out .+= fdiff!(tmp, Vs[i]; dims=i+1, rev=true)
+    end
+    return out
+end
+
+"""
+    flaplacian(X::AbstractArray)
+
+The discrete laplacian operator, i.e., the divergence of the gradient fields of `X`.
+"""
+flaplacian(X::AbstractArray) = flaplacian!(similar(X, maybe_floattype(eltype(X))), X)
+
+"""
+    flaplacian!(out, X)
+    flaplacian!(out, ∇X::Tuple, X)
+
+The in-place version of the laplacian operator [`flaplacian`](@ref).
+
+!!! tips Non-allocating
+    This function will allocate a new set of memories to store the intermediate
+    gradient fields `∇X`, if you pre-allcoate the memory for `∇X`, then this function
+    will use it and is thus non-allcating.
+"""
+flaplacian!(out, X::AbstractArray) = fdiv!(out, fgradient(X)...)
+flaplacian!(out, ∇X::Tuple, X::AbstractArray) = fdiv!(out, fgradient!(∇X, X)...)
+
+
+"""
+    fgradient(X::AbstractArray; adjoint=false) -> (∂₁X, ∂₂X, ..., ∂ₙX)
+
+Computes the gradient fields of `X`. If `adjoint==true` then it computes the adjoint gradient
+fields.
+
+Each gradient vector is computed as forward difference along specific dimension, e.g.,
+[`∂ᵢX = fdiff(X, dims=i)`](@ref fdiff).
+
+Mathematically, the adjoint operator ∂ᵢ' of ∂ᵢ is defined as `<∂ᵢu, v> := <u, ∂ᵢ'v>`.
+
+See also the in-place version [`fgradient!(X)`](@ref) to reuse the allocated memory.
+"""
+function fgradient(X::AbstractArray{T,N}; adjoint=false) where {T,N}
+    fgradient!(ntuple(i->similar(X, maybe_floattype(T)), N), X; adjoint=adjoint)
+end
+
+"""
+    fgradient!(∇X::Tuple, X::AbstractArray; adjoint=false)
+
+The in-place version of (adjoint) gradient operator [`fgradient`](@ref).
+
+The input `∇X = (∂₁X, ∂₂X, ..., ∂ₙX)` is a tuple of arrays that are similar to `X`, i.e.,
+`eltype(∂ᵢX) == eltype(X)` and `axes(∂ᵢX)  == axes(X)` for all `i`.
+"""
+function fgradient!(∇X::NTuple{N, <:AbstractArray}, X; adjoint=false) where N
+    all(v->axes(v) == axes(X), ∇X) || throw(ArgumentError("All axes of vector fields ∇X and X should be the same."))
+    for i in 1:N
+        if adjoint
+            # the negative adjoint of gradient operator for forward difference is the backward difference
+            fdiff!(∇X[i], X, dims=i, rev=true)
+            @. ∇X[i] = -∇X[i]
+        else
+            fdiff!(∇X[i], X, dims=i)
+        end
+    end
+    return ∇X
+end
+
+
+
+if VERSION < v"1.1"
+    isnothing(x) = x === nothing
+end
+
+end # module

test/deprecated.jl

@@ -14,4 +14,11 @@
         @test maxabsfinite(A) == maximum_finite(abs, A)
         @test maxabsfinite(A, dims=1) == maximum_finite(abs, A, dims=1)
     end
+
+    @testset "fdiff entrypoints" begin
+        A = rand(Float32, 5)
+        @test ImageBase.fdiff(A) == ImageBase.FiniteDiffs.fdiff(A)
+        out = similar(A)
+        @test ImageBase.fdiff!(out, A) == ImageBase.FiniteDiffs.fdiff!(out, A)
+    end
 end

test/diff.jl

@@ -1,8 +1,12 @@
+using ImageBase.FiniteDiffs
+# TODO(johnnychen94): remove this after we delete `Imagebase.fdiff` and `ImageBase.fdiff!` entrypoints
+using ImageBase.FiniteDiffs: fdiff, fdiff!
+
 @testset "fdiff" begin
     # Base.diff doesn't promote integer to float
-    @test ImageBase.maybe_floattype(Int) == Int
-    @test ImageBase.maybe_floattype(N0f8) == Float32
-    @test ImageBase.maybe_floattype(RGB{N0f8}) == RGB{Float32}
+    @test ImageBase.FiniteDiffs.maybe_floattype(Int) == Int
+    @test ImageBase.FiniteDiffs.maybe_floattype(N0f8) == Float32
+    @test ImageBase.FiniteDiffs.maybe_floattype(RGB{N0f8}) == RGB{Float32}
 
     @testset "API" begin
         # fdiff! works the same as fdiff
@@ -107,3 +111,44 @@
         @test fdiff(A, dims=1) == fdiff(float.(A), dims=1)
     end
 end
+
+@testset "fgradient" begin
+    for T in generate_test_types([N0f8, Float32], [Gray, RGB])
+        for sz in [(7, ), (7, 5), (7, 5, 3)]
+            A = rand(T, sz...)
+
+            ∇A = fgradient(A)
+            for i in 1:length(sz)
+                @test ∇A[i] == fdiff(A, dims=i)
+            end
+            ∇A = map(similar, ∇A)
+            @test fgradient!(∇A, A) == fgradient(A)
+
+            ∇tA = fgradient(A, adjoint=true)
+            for i in 1:length(sz)
+                @test ∇tA[i] == .-fdiff(A, dims=i, rev=true)
+            end
+            ∇tA = map(similar, ∇tA)
+            @test fgradient!(∇tA, A, adjoint=true) == fgradient(A, adjoint=true)
+        end
+    end
+end
+
+@testset "fdiv/flaplacian" begin
+    ref_laplacian(X) = imfilter(X, Kernel.Laplacian(ntuple(x->true, ndims(X))), "circular")
+    for T in generate_test_types([N0f8, Float32], [Gray, RGB])
+        for sz in [(7,), (7, 7), (7, 7, 7)]
+            A = rand(T, sz...)
+            out = flaplacian(A)
+            @test out ≈ ref_laplacian(A)
+
+            ∇A = fgradient(A)
+            foreach((out, ∇A...)) do x
+                fill!(x, zero(T))
+            end
+            flaplacian!(out, ∇A, A)
+            @test out == flaplacian(A)
+            @test ∇A == fgradient(A)
+        end
+    end
+end

test/runtests.jl

@@ -1,11 +1,11 @@
 using ImageBase, OffsetArrays, StackViews
+using ImageFiltering
 using Test, TestImages, Aqua, Documenter
 
 using OffsetArrays: IdentityUnitRange
 include("testutils.jl")
 
 @testset "ImageBase.jl" begin
-
     @testset "Project meta quality checks" begin
         # Not checking compat section for test-only dependencies
         Aqua.test_ambiguities(ImageBase)
@@ -17,7 +17,7 @@ include("testutils.jl")
             project_toml_formatting=true
         )
         if VERSION >= v"1.2"
-            doctest(ImageBase,manual = false)
+            doctest(ImageBase, manual = false)
         end
     end