Cython, numpy speed-up

Question

I am trying to write an algorithm that calculates the mean value of certain neighboring elements of a 2D array.

I would like to see if it is possible to speed it up using Cython, but it is the first time I use it myself.

Python version :

import numpy as np

def clamp(val, minval, maxval):
    return max(minval, min(val, maxval))


def filter(arr, r):
    M = arr.shape[0]
    N = arr.shape[1]

    new_arr = np.zeros([M, N], dtype=np.int)

    for x in range(M):
        for y in range(N):
            # Corner elements
            p1 = clamp(x-r, 0, M)
            p2 = clamp(y-r, 0, N)
            p3 = clamp(y+r, 0, N-1)
            p4 = clamp(x+r, 0, M-1)

            nbr_elements = (p3-p2-1)*2+(p4-p1-1)*2+4

            tmp = 0

            # End points
            tmp += arr[p1, p2]
            tmp += arr[p1, p3]
            tmp += arr[p4, p2]
            tmp += arr[p4, p3]

            # The rest
            tmp += sum(arr[p1+1:p4, p2])
            tmp += sum(arr[p1+1:p4, p3])
            tmp += sum(arr[p1, p2+1:p3])
            tmp += sum(arr[p4, p2+1:p3])

            new_arr[x, y] = tmp/nbr_elements

    return new_arr

and my attempt of a Cython implementation. I found out that max/min/sum was faster if you re-implemented them, rather than using the python version

Cython version :

from __future__ import division
import numpy as np
cimport numpy as np

DTYPE = np.int
ctypedef np.int_t DTYPE_t

cdef inline int int_max(int a, int b): return a if a >= b else b
cdef inline int int_min(int a, int b): return a if a <= b else b

def clamp(int val, int minval, int maxval):
    return int_max(minval, int_min(val, maxval))

def cython_sum(np.ndarray[DTYPE_t, ndim=1] y):
    cdef int N = y.shape[0]
    cdef int x = y[0]
    cdef int i
    for i in xrange(1, N):
        x += y[i]
    return x


def filter(np.ndarray[DTYPE_t, ndim=2] arr, int r):
    cdef M = im.shape[0]
    cdef N = im.shape[1]

    cdef np.ndarray[DTYPE_t, ndim=2] new_arr = np.zeros([M, N], dtype=DTYPE)
    cdef int p1, p2, p3, p4, nbr_elements, tmp

    for x in range(M):
        for y in range(N):
            # Corner elements
            p1 = clamp(x-r, 0, M)
            p2 = clamp(y-r, 0, N)
            p3 = clamp(y+r, 0, N-1)
            p4 = clamp(x+r, 0, M-1)

            nbr_elements = (p3-p2-1)*2+(p4-p1-1)*2+4

            tmp = 0

            # End points
            tmp += arr[p1, p2]
            tmp += arr[p1, p3]
            tmp += arr[p4, p2]
            tmp += arr[p4, p3]

            # The rest
            tmp += cython_sum(arr[p1+1:p4, p2])
            tmp += cython_sum(arr[p1+1:p4, p3])
            tmp += cython_sum(arr[p1, p2+1:p3])
            tmp += cython_sum(arr[p4, p2+1:p3])

            new_arr[x, y] = tmp/nbr_elements

    return new_arr

I made a test script:

import time
import numpy as np

import square_mean_py
import square_mean_cy

N = 500

arr = np.random.randint(15, size=(N, N))
r = 8

# Timing

t = time.time()
res_py = square_mean_py.filter(arr, r)
print time.time()-t

t = time.time()
res_cy = square_mean_cy.filter(arr, r)
print time.time()-t

Which prints

9.61458301544
1.44476890564

that is a speed-up of approx. 7 times. I have seen a lot of Cython implmentations that yield a lot better speed-up, and so I was thinking that maybe some of you see aa potential way of speeding up the algorithm?

Answer 1

There are a few issues with your Cython script:

1. You are not giving Cython some key information, such as the types of x, y, M and N which are used in ranges.
2. I have cdef ed the two functions cython_sum and clamp since you don't need them at Python level.
3. What is im that appears in filter function? I am assuming you meant arr .

Fixing those I will rewrite/modify your Cython script like so:

from __future__ import division
import numpy as np
cimport numpy as np
from cython cimport boundscheck, wraparound 

DTYPE = np.int
ctypedef np.int_t DTYPE_t

cdef inline int int_max(int a, int b): return a if a >= b else b
cdef inline int int_min(int a, int b): return a if a <= b else b

cdef int clamp3(int val, int minval, int maxval):
    return int_max(minval, int_min(val, maxval))

@boundscheck(False)
cdef int cython_sum2(DTYPE_t[:] y):
    cdef int N = y.shape[0]
    cdef int x = y[0]
    cdef int i
    for i in range(1, N):
        x += y[i]
    return x

@boundscheck(False)
@wraparound(False)
def filter3(DTYPE_t[:,::1] arr, int r):
    cdef int M = arr.shape[0]
    cdef int N = arr.shape[1]

    cdef np.ndarray[DTYPE_t, ndim=2, mode='c'] \
    new_arr = np.zeros([M, N], dtype=DTYPE)
    cdef int p1, p2, p3, p4, nbr_elements, tmp, x, y

    for x in range(M):
        for y in range(N):
            # Corner elements
            p1 = clamp3(x-r, 0, M)
            p2 = clamp3(y-r, 0, N)
            p3 = clamp3(y+r, 0, N-1)
            p4 = clamp3(x+r, 0, M-1)

            nbr_elements = (p3-p2-1)*2+(p4-p1-1)*2+4

            tmp = 0

            # End points
            tmp += arr[p1, p2]
            tmp += arr[p1, p3]
            tmp += arr[p4, p2]
            tmp += arr[p4, p3]

            # The rest
            tmp += cython_sum2(arr[p1+1:p4, p2])
            tmp += cython_sum2(arr[p1+1:p4, p3])
            tmp += cython_sum2(arr[p1, p2+1:p3])
            tmp += cython_sum2(arr[p4, p2+1:p3])

            new_arr[x, y] = <int>(tmp/nbr_elements)

    return new_arr

Here is the timing on my machine:

arr = np.random.randint(15, size=(500, 500))

Original (Python) version: 7.34 s
Your Cython version: 1.98 s
New Cython version: 0.0323 s

That is almost 60 times speed up over your Cython script and over 200 times speed-up over the original Python script.