# OpenCV: issue with function argument type in Julia binding

**URL:** <https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207>\
**Category:** New to Julia\
**Tags:** question, array, type\
**Created:** [March 30, 2021, 9:06am UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207 "2021-03-30T09:06:43Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![erwanlecarpentier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erwanlecarpentier/32/23448_2.png) [@erwanlecarpentier](https://discourse.julialang.org/u/erwanlecarpentier)\
**Post date:** [March 30, 2021, 9:06am UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/1 "2021-03-30T09:06:44Z")

</div>

Hi!  
I followed [this tutorial](https://docs.opencv.org/master/d8/da4/tutorial_julia.html) to build an OpenCV binding for Julia. Now I am stuck with some functions I do not know how to use. I would like to use the `OpenCV.split` function whose definition is below in `cv_cxx_wrap.jl`:

```julia
function split(m::InputArray, mv::Array{InputArray, 1})
	return cpp_to_julia(jlopencv_cv_cv_split(julia_to_cpp(m),julia_to_cpp(mv)))
end
split(m::InputArray; mv::Array{InputArray, 1} = (Array{InputArray, 1}())) = split(m, mv)

```

where `InputArray = Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}`.  
My issue is that I get a `no method matching` error, I think I do not pass the right argument type as the second argument of `split`.  
Here is a MWE with all the types I tried:

```julia
using OpenCV

noisy_img = rand(collect(UInt8, 0:255), (3, 320, 210))

out = Array{OpenCV.InputArray}[]
out = Array{OpenCV.Mat{UInt8}, 1}[]
out = Array{Array{UInt8,1}, 1}[]

out = OpenCV.CxxMat[]
out = AbstractArray{UInt8,3}[]
out = OpenCV.Mat{UInt8}[]
out = Array{UInt8,1}[]

out = Array{Array{UInt8,1}}(undef, 3)
out = Array{OpenCV.InputArray}(undef, 3)
out = Array{OpenCV.Mat{UInt8}}(undef, 3)

out = Array{OpenCV.Mat{UInt8}, 1}()

OpenCV.split(noisy_img, out) # Split RGB channels

```

Here is the error I get:

```julia
MethodError: no method matching split(::Array{UInt8,3}, ::Array{OpenCV.Mat{UInt8},1})
Closest candidates are:
  split(::Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}; mv) at /home/wahara/build/OpenCV/src/cv_cxx_wrap.jl:2307
  split(::Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}, !Matched::Array{Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}},1}) at /home/wahara/build/OpenCV/src/cv_cxx_wrap.jl:2304
top-level scope at functions.jl:138

```

I am new to Julia and my issues with OpenCV could come from the fact that I do not really know to handle types in Julia. Any help would be appreciated!

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [March 30, 2021, 2:30pm UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/2 "2021-03-30T14:30:47Z")

</div>

Welcome! You’ve run into a common confusion, which is the fact that parametric types (including Arrays) in julia are [invariant](https://docs.julialang.org/en/v1/manual/types/#man-parametric-composite-types).

Here’s a much simpler example:

```julia
julia> function foo(x::Vector{Union{Int, Float64}})
         println(x)
       end
foo (generic function with 1 method)

julia> x = [1, 2, 3]
3-element Vector{Int64}:
 1
 2
 3

julia> foo(x)
ERROR: MethodError: no method matching foo(::Vector{Int64})
Closest candidates are:
  foo(::Vector{Union{Float64, Int64}}) at REPL[1]:1

```

A `Vector{Int}` is not a subtype of `Vector{Union{Int, Float64}}`. That’s because a `Vector{Union{Int, Float64}}` is its own kind of thing entirely: it’s a container in which _each individual element_ can be an `Int` or a `Float64` separately. A `Vector{Int}` does not fit that description.

What you probably want is something like “A vector whose element type is some subset of the set `{Int, Float64}`”. In julia, that’s spelled:

```julia
Vector{T} where {T <: Union{Int, Float64}}

```

or

```julia
Vector{<:Union{Int, Float64}}

```

For example:

```julia
julia> function foo2(x::Vector{T}) where {T <: Union{Int, Float64}}
         println(x)
       end
foo2 (generic function with 1 method)

julia> foo2(x)
[1, 2, 3]

julia> function foo3(x::Vector{<:Union{Int, Float64}})
         println(x)
       end
foo3 (generic function with 1 method)

julia> foo3(x)
[1, 2, 3]

```

---

<div class="post-metadata">

**Author:** ![erwanlecarpentier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erwanlecarpentier/32/23448_2.png) [@erwanlecarpentier](https://discourse.julialang.org/u/erwanlecarpentier)\
**Post date:** [March 30, 2021, 2:55pm UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/3 "2021-03-30T14:55:43Z")

</div>

Thanks a lot for the answer, I did not know that, very useful!  
However, I think my issue is different and I did not type the proper definition of `InputArray` which is

```julia
const dtypes = Union{UInt8, Int8, UInt16, Int16, Int32, Float32, Float64}
const InputArray = Union{AbstractArray{T, 3} where {T <: dtypes}, CxxMat}

```

So I think the problem you mentioned is not the case here.  
Actually, I found a way to get a different error (so I think the type issue of my original question is solved) which is the following:

```julia
using OpenCV

noisy_img = rand(collect(UInt8, 0:255), (3, 320, 210))
out = OpenCV.InputArray[]
OpenCV.split(noisy_img, out) # Split RGB channels

```

And I get

```julia
MethodError: no method matching jlopencv_core_Mat_mutable_data(::OpenCV.CxxMatDereferenced)
Closest candidates are:
  jlopencv_core_Mat_mutable_data(!Matched::Union{CxxWrap.CxxWrapCore.SmartPointer{T2}, T2} where T2<:OpenCV.CxxMatAllocated) at /home/wahara/.julia/packages/CxxWrap/OcN1Z/src/CxxWrap.jl:618
cpp_to_julia(::OpenCV.CxxMatDereferenced) at mat_conversion.jl:20
cpp_to_julia at mat_conversion.jl:103 [inlined]
split(::OpenCV.Mat{UInt8}, ::Array{Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}},1}) at cv_cxx_wrap.jl:2305
top-level scope at functions.jl:136

```

This is a similar issue as in [this opened issue](https://github.com/opencv/opencv_contrib/issues/2788) about the function `findContours` of the OpenCV Julia binding.

Alternatively, I am still curious why I get a type error in the following:

```julia
using OpenCV

noisy_img = rand(collect(UInt8, 0:255), (3, 320, 210))
out = OpenCV.Mat{UInt8}[]
OpenCV.split(noisy_img, out) # Split RGB channels

```

As I have `typeof(noisy_img) = Array{UInt8,3}` and `typeof(out) = Array{OpenCV.Mat{UInt8},1}`, I would have thought it would work but I get the following:

```julia
MethodError: no method matching split(::Array{UInt8,3}, ::Array{OpenCV.Mat{UInt8},1})
Closest candidates are:
  split(::Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}; mv) at /home/wahara/build/OpenCV/src/cv_cxx_wrap.jl:2307
  split(::Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}, !Matched::Array{Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}},1}) at /home/wahara/build/OpenCV/src/cv_cxx_wrap.jl:2304
top-level scope at functions.jl:135

```

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [March 30, 2021, 3:02pm UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/4 "2021-03-30T15:02:15Z")

</div>

> [@erwanlecarpentier](#):
>
> So I think the problem you mentioned is not the case here.

Mm, I’m pretty sure it’s still relevant. The error you posted is:

```julia
MethodError: no method matching split(::Array{UInt8,3}, ::Array{OpenCV.Mat{UInt8},1})

```

and the candidate is:

```julia
split(::Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}}, !Matched::Array{Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}},1}) at /home/wahara/build/OpenCV/src/cv_cxx_wrap.jl:2304

```

from which the relevant part is:

```julia
!Matched::Array{Union{...}}

```

which is exactly what I described above. You’ve used the `<:` operator to define `InputArray`, but your `split` function is still defined using `Array{InputArray, 1}` as its argument type, whereas it should be `Array{<:InputArray, 1}` .

---

<div class="post-metadata">

**Author:** ![erwanlecarpentier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erwanlecarpentier/32/23448_2.png) [@erwanlecarpentier](https://discourse.julialang.org/u/erwanlecarpentier)\
**Post date:** [March 30, 2021, 3:26pm UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/5 "2021-03-30T15:26:12Z")

</div>

Oh right! This makes a difference indeed. So I redefined the `split` function as you just wrote with `Array{<:InputArray, 1}` and now I never have this `!Matched` issue again. Precisely, I can run the following code where any definition of the `out` variable does not produce the `!Matched` issue.

```julia
using OpenCV

noisy_img = rand(collect(UInt8, 0:255), (3, 320, 210))

out = OpenCV.CxxMat[]
out = AbstractArray{UInt8,3}[]
out = OpenCV.Mat{UInt8}[]
out = Array{UInt8}[]
out = Array{OpenCV.Mat{UInt8}, 1}()
out = OpenCV.InputArray[]
out = Array{OpenCV.InputArray, 1}()

OpenCV.split(noisy_img, out)

```

However, in all the cases, I end-up with the other error I mentioned with `out = OpenCV.InputArray[]` and which is:

```julia
MethodError: no method matching jlopencv_core_Mat_mutable_data(::OpenCV.CxxMatDereferenced)
Closest candidates are:
  jlopencv_core_Mat_mutable_data(!Matched::Union{CxxWrap.CxxWrapCore.SmartPointer{T2}, T2} where T2<:OpenCV.CxxMatAllocated) at /home/wahara/.julia/packages/CxxWrap/OcN1Z/src/CxxWrap.jl:618
cpp_to_julia(::OpenCV.CxxMatDereferenced) at mat_conversion.jl:20
cpp_to_julia at mat_conversion.jl:103 [inlined]
split(::Array{UInt8,3}, ::Array{Union{OpenCV.CxxMat, AbstractArray{T,3} where T<:Union{Float32, Float64, Int16, Int32, Int8, UInt16, UInt8}},1}) at cv_cxx_wrap.jl:2305
top-level scope at functions.jl:131

```

1. About the definition of `split` with `Array{<:InputArray, 1}`. This function is defined in `cv_cxx_wrap.jl` which is part of the [Julia OpenCV Binding](https://docs.opencv.org/master/d8/da4/tutorial_julia.html). Do you think we should raise the issue in [opencv\_contrib](https://github.com/opencv/opencv_contrib) and suggest your modification?
2. About the second error (`MethodError: no method matching jlopencv_core_Mat_mutable_data(::OpenCV.CxxMatDereferenced)`), any thoughts?

Thanks again for your help!

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [March 30, 2021, 3:36pm UTC](https://discourse.julialang.org/t/opencv-issue-with-function-argument-type-in-julia-binding/58207/6 "2021-03-30T15:36:30Z")

</div>

> [@erwanlecarpentier](#):
>
> Do you think we should raise the issue in [opencv\_contrib](https://github.com/opencv/opencv_contrib) and suggest your modification?

I think so. I don’t know how the OpenCV bindings are actually generated, but defining a function whose argument type is `Array{Union{<anything>}}` is nearly always wrong (and certainly wrong in this case).

> [@erwanlecarpentier](#):
>
> About the second error ( `MethodError: no method matching jlopencv_core_Mat_mutable_data(::OpenCV.CxxMatDereferenced)` ), any thoughts?

Sorry, no–I don’t know anything about the OpenCV Julia stack.

By the way, depending on what you are actually trying to do, you might find the [JuliaImages](https://juliaimages.org/stable/) ecosystem easier to use from Julia. Having used opencv from Python in the past, I vastly prefer using the JuliaImages tools and have never wanted to try using opencv from Julia. YMMV, of course.