[Python] Changing pixel colors in Python: How to do it faster?

I have an RGB image and am trying to set every pixel on my RGB to black where the corresponding alpha pixel is black as well. So basically I am trying to "bake" the alpha into my RGB. I have tried this using PIL pixel access objects, PIL ImageMath.eval and numpy arrays:

PIL pixel access objects:


def alphaCutoutPerPixel(im):
pixels = im.load()
for x in range(im.size[0]):
for y in range(im.size[1]):
px = pixels[x, y]
r,g,b,a = px
if px[3] == 0: # If alpha is black...
pixels[x,y] = (0,0,0,0)
return im


PIL ImageMath.eval:


def alphaCutoutPerBand(im):
newBands = []
r, g, b, a = im.split()
for band in (r, g, b):
out = ImageMath.eval("convert(min(band, alpha), 'L')", band=band, alpha=a)
newBands.append(out)
newImg = Image.merge("RGB", newBands)
return newImg


Numpy array:


def alphaCutoutNumpy(im):
data = numpy.array(im)
r, g, b, a = data.T
blackAlphaAreas = (a == 0)
# This fails; why?
data[..., :-1][blackAlphaAreas] = (0, 255, 0)
return Image.fromarray(data)


The first method works fine, but is really slow. The second method works fine for a single image, but will stop after the first when asked to convert multiple. The third method I created based on this example (first answer): Python: PIL replace a single RGBA color But it fails at the marked command:

data[..., :-1][blackAlphaAreas] = (0, 255, 0, 0)
IndexError: index (295) out of range (0<=index<294) in dimension 0


Numpy seems promising for this kind of stuff, but I dont really get the syntax on how to set parts of the array in one step. Any help? Maybe other ideas to achieve what I describe above quickly?

Cheers

Continue reading...