[Numpy-discussion] Array blitting (pasting one array into another)

Joseph Fox-Rabinovitz jfoxrabinovitz at gmail.com
Thu Jun 29 21:34:17 EDT 2017


This is a useful idea certainly. I would recommended extending it to an
arbitrary number of axes. You could either raise an error if the ndim of
the two arrays are unequal, or allow a broadcast of a lesser ndimmed src
array.

- Joe

On Jun 29, 2017 20:17, "Mikhail V" <mikhailwas at gmail.com> wrote:

> Hello all
>
> I often need to copy one array into another array, given an offset.
> This is how the "blit" function can be understood, i.e. in
> every graphical lib there is such a function.
> The common definition is like:
> blit ( dest, src, offset ):
> where dest is destination array, src is source array and offset is
> coordinates in destination where the src should pe blitted.
> Main feature of such function is that it never gives an error,
> so if the source does not fit into the destination array, it is simply
> trimmed.
> And respectively if there is no intersection area then nothing happens.
>
> Hope this is clear.
> So to make it work with Numpy arrays one need to calculate the
> slices before copying the data.
> I cannot find any Numpy or Python method to help with that so probably
> it does not exist yet.
> If so, my proposal is to add a Numpy method which helps with that.
> Namely the proposal will be to add a method which returns
> the slices for the intersection areas of two arbitrary arrays, given an
> offset,
> so then one can "blit" the array into another with simple assignment =.
>
> Here is a Python function I use for 2d arrays now:
>
> def interslice ( dest, src, offset ):
>     y,x = offset
>     H,W = dest.shape
>     h,w = src.shape
>
>     dest_starty = max (y,0)
>     dest_endy = min (y+h,H)
>     dest_startx = max (x,0)
>     dest_endx = min (x+w,W)
>
>     src_starty = 0
>     src_endy = h
>     if y<0:     src_starty = -y
>     by = y+h - H             # Y bleed
>     if by>0: src_endy = h - by
>
>     src_startx = 0
>     src_endx = w
>     if x<0:  src_startx = -x
>     bx = x+w - W             # X bleed
>     if bx>0:  src_endx = w - bx
>
>     dest_sliceY =  slice(dest_starty,dest_endy)
>     dest_sliceX =  slice(dest_startx,dest_endx)
>     src_sliceY = slice(src_starty, src_endy)
>     src_sliceX = slice(src_startx, src_endx)
>     if dest_endy <= dest_starty:
>         print "No Y intersection !"
>         dest_sliceY = ( slice(0, 0) )
>         src_sliceY = ( slice(0, 0) )
>     if dest_endx <= dest_startx:
>         print "No X intersection !"
>         dest_sliceX = ( slice(0, 0) )
>         src_sliceX = ( slice(0, 0) )
>     dest_slice = ( dest_sliceY, dest_sliceX )
>     src_slice = ( src_sliceY, src_sliceX )
>     return ( dest_slice, src_slice )
>
>
> ------
>
> I have intentionally made it expanded and without contractions
> so that it is better understandable.
> It returns the intersection area of two arrays given an offset.
> First returned tuple element is the slice for DEST array and the
> second element is the slice for SRC array.
> If there is no intersection along one of the axis at all
> it returns the corresponding slice as (0,0)
>
> With this helper function one can blit arrays easily e.g. example code:
>
> W = 8; H = 8
> DEST = numpy.ones([H,W], dtype = "uint8")
> w = 4; h = 1
> SRC = numpy.zeros([h,w], dtype = "uint8")
> SRC[:]=8
> offset = (0,9)
> ds, ss = interslice (DEST, SRC, offset )
>
> # blit SRC into DEST
> DEST[ds] = SRC[ss]
>
> So changing the offset one can observe how the
> SRC array is trimmed if it is crosses the DEST boundaries.
> I think it is very useful function in general and it has
> well defined behaviour. It has usage not only for graphics,
> but actually any data copying-pasting between arrays.
>
> So I am looking forward to comments on this proposal.
>
>
> Mikhail
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