Question

I’m trying to process an RGBA buffer (list of chars), and run “unpremultiply” on each pixel. The algorithm is color_out=color*255/alpha.

This is what I came up with:

def rgba_unpremultiply(data):
    for i in range(0, len(data), 4):
        a = ord(data[i+3])
        if a != 0:
            data[i] = chr(255*ord(data[i])/a)
            data[i+1] = chr(255*ord(data[i+1])/a)
            data[i+2] = chr(255*ord(data[i+2])/a)
    return data

It works but causes a major drawback in performance.

I’m wondering besides writing a C module, what are my options to optimize this particular function?

Answer 1

This is exactly the kind of code NumPy is great for.

import numpy

def rgba_unpremultiply(data):
    a = numpy.fromstring(data, 'B') # Treat the string as an array of bytes
    a = a.astype('I') # Cast array of bytes to array of uints, since temporary values needs to be larger than byte
    alpha = a[3::4] # Every 4th element starting from index 3
    alpha = numpy.where(alpha == 0, 255, alpha) # Don't modify colors where alpha is 0
    a[0::4] = a[0::4] * 255 // alpha # Operates on entire slices of the array instead of looping over each element
    a[1::4] = a[1::4] * 255 // alpha
    a[2::4] = a[2::4] * 255 // alpha
    return a.astype('B').tostring() # Cast back to bytes