Expanding Repa array shapes

Question

I'm writing a program that generates images, which I would like to bring into a Repa array. I'm currently using the type:

data Colour = Colour Double Double Double

to represent pixels, and I have a (probably inefficient but functional) function which converts a Colour into a DIM1 array:

colourToRepa :: Colour -> Array U DIM1 Double
colourToRepa (Colour r g b) = R.fromListUnboxed (Z :. (3::Int)) [r,g,b]

An image in my program at the moment is just a [Colour] value with a (Int, Int) representing dimensions.

I can use this to build a Array V DIM2 Colour easily enough, but is there a way (using colourToRepa or otherwise) to expand this array into a Array U DIM3 Double?

With lists I could just use map but Repa's map preserves the shape of the array (I want to go from a DIM2 to a DIM3).

Answer 1

Remember that shapes are stored separately to data in repa, so you would be reshaping the array from DIM1 to DIM3, without changing the extent of the array.

We have a DIM1 (flat) array:

> let x :: Array DIM1 Double ; x = fromList (Z :. (9::Int)) [1..9]
> x
Array (Z :. 9) [1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0]

It's extent is:

> extent x
Z :. 9

Using reshape we can 'cast' the shape of the array from 1D to 3D:

> let y :: Array DIM3 Double ;
      y = reshape (Z :. (3::Int) :. (3::Int) :. (1::Int)) x
> y
Array (Z :. 3 :. 3 :. 1) [1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0]

And its type changes:

> :t y
y :: Array DIM3 Double

Answer 2

Depending on precisely how you want to add an extra dimension can also use extend, for example:

extraDim :: Source a Double =>
            Array a (Z :. Int :. Int) Double ->
            Array D (Z :.Int :. Int :. Int) Double
extraDim a = extend (Any :. i :. All) a
               where (Z :. i :. j) = extent a