RGB arrays mirror reversed when saved to 16-bit png using Freeimage plugin

[tomwallis]

I am using io.imsave to output a numpy array to a 16-bit .png file (using the Freeimage plugin). If the array is MxNx3 (RGB) or MxNx4 (RGBA), the saved file is mirror reversed (both L-R and T-B). Works fine for greyscale (MxN).

For more details see my question on stackoverflow.

I'm posting here as an issue, as suggested by ali_m on stackoverflow.

[stefanv]

Provided code snippet:

import numpy as np
from skimage import io, img_as_uint, img_as_ubyte

im = np.random.uniform(size=(256, 256, 3))
im[:128, :128] = 1
im = img_as_ubyte(im)
io.use_plugin('freeimage')
io.imsave('test_0.png', im)

vs

import numpy as np
from skimage import io, img_as_uint, img_as_ubyte

im = np.random.uniform(size=(256, 256, 3))
im[:128, :128] = 1
im = img_as_uint(im)  # problem here, this works if ubyte
io.use_plugin('freeimage')
io.imsave('test_1.png', im)

[stefanv]

test_0.png:

test_1.png:

[stefanv]

Likely culprit: freeimage_plugin.py:648:_array_to_bitmap()

I see comments like:

# swizzle the color components and flip the scanlines to go to
# FreeImage's BGR[A] and upside-down internal memory format"

[tomwallis]

Thanks for adding my code snippets. Sorry, this is my first github issue report.

Those comments don't sound good. I'm not sure why I would want my color components "swizzled" and my scanlines flipped.

[stefanv]

Having looked at the code a bit, this is because FreeImage uses a different internal representation. I think I've almost got it sorted.

[stefanv]

Please see if #1102 solves the problem for you.

[tomwallis]

Seems to! Thanks stefanv.