Python: PIL replace a single RGBA color

Question

I have already taken a look at this question: SO question and seem to have implemented a very similar technique for replacing a single color including the alpha values:

Answer 1

Try this , in this sample we set the color to black if color is not white .

#!/usr/bin/python
from PIL import Image
import sys

img = Image.open(sys.argv[1])
img = img.convert("RGBA")

pixdata = img.load()

# Clean the background noise, if color != white, then set to black.

for y in xrange(img.size[1]):
    for x in xrange(img.size[0]):
        if pixdata[x, y] == (255, 255, 255, 255):
            pixdata[x, y] = (0, 0, 0, 255)

you can use color picker in gimp to absorb the color and see that's rgba color

Answer 2

The Pythonware PIL online book chapter for the Image module stipulates that putpixel() is slow and suggests that it can be sped up by inlining. Or depending on PIL version, using load() instead.

Answer 3

If you have numpy, it provides a much, much faster way to operate on PIL images.

E.g.:

import Image
import numpy as np

im = Image.open('test.png')
im = im.convert('RGBA')

data = np.array(im)   # "data" is a height x width x 4 numpy array
red, green, blue, alpha = data.T # Temporarily unpack the bands for readability

# Replace white with red... (leaves alpha values alone...)
white_areas = (red == 255) & (blue == 255) & (green == 255)
data[..., :-1][white_areas.T] = (255, 0, 0) # Transpose back needed

im2 = Image.fromarray(data)
im2.show()

Edit: It's a slow Monday, so I figured I'd add a couple of examples:

Just to show that it's leaving the alpha values alone, here's the results for a version of your example image with a radial gradient applied to the alpha channel:

Original:

Result: