Are the allocations from `imfilter!` expected?

Hello,

I was trying to use imfilter! with an array gx preallocated before calling the function with the hope of not allocating memory. I see 2.31 MiB allocated. Why is it happening? (should I expect it? I did not expect any allocations if the output array was preallocated)

thank you

A minimal working example:

using Images
using TestImages
using BenchmarkTools

function to_gray(image)
    x = Array{ColorTypes.Gray{Float32}, 2}(image)
    return Array{Float32}(x)
end

img = testimage("mandrill");
img = to_gray(img)
direction_x =  centered([-1 0 1])
gx = imfilter(img, direction_x);

@benchmark imfilter($img, $direction_x)
@benchmark imfilter!($gx, $img, $direction_x)

Prints the following:

BenchmarkTools.Trial: 
  memory estimate:  3.31 MiB
  allocs estimate:  58
  --------------
  minimum time:     4.126 ms (0.00% GC)
  median time:      4.597 ms (0.00% GC)
  mean time:        5.200 ms (7.27% GC)
  maximum time:     10.802 ms (28.68% GC)
  --------------
  samples:          960
  evals/sample:     1


BenchmarkTools.Trial: 
  memory estimate:  2.31 MiB
  allocs estimate:  56
  --------------
  minimum time:     4.101 ms (0.00% GC)
  median time:      4.419 ms (0.00% GC)
  mean time:        4.839 ms (5.47% GC)
  maximum time:     10.787 ms (25.44% GC)
  --------------
  samples:          1032
  evals/sample:     1

Start by reproducing the provided minimal working example and comparing the allocation measurements for imfilter and imfilter!. Trace the imfilter! entry point to determine where the remaining allocations originate, then document whether they are expected or define a focused change and validation for eliminating them.