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

Mikhail V mikhailwas at gmail.com
Thu Jun 29 20:16:04 EDT 2017

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")
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.


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