Why is accessing image data so slow?

Question

This script in the Python console:

>>> pixels = D.images[-1].pixels
>>> len(pixels)
8294400

>>> for i in range(64):
...     dummy = pixels[i]
...
>>>

takes over two seconds to run the for loop.

However, running pixels = [0 for i in range(len(pixels))] before the for loop causes the script to finish instantly.

What's going on with the Blender Python image pixel access, and how can I speed it up?

Answer 1

.pixels is a python object and pretty slow, especially if you change pixels. That is 'cause the entire buffer / python object is re-created for every single change (if you do it in a python loop). Not sure why it's that slow on read-only actions though...

You can speed it up tremendously if you cast it to a native python type (tuple) and use that copy instead:

img = bpy.data.images['Untitled']

pixels = img.pixels[:] # create a copy (tuple) for read-only access
# -- OR --
pixels = list(img.pixels) # create an editable copy (list)

# Use the tuple object, which is way faster than direct access to Image.pixels
for i in range(0, len(pixels), 4):
    pixels[i] = 1.0 - pixels[i] # invert red channel

# Write back to image.
# Slice notation here means to replace in-place, not sure if it's faster...
img.pixels[:] = pixels

# Should probably update image
img.update()