search

colour-science / colour-demosaicing

CFA (Colour Filter Array) demosaicing algorithms for Python
https://www.colour-science.org
BSD 3-Clause "New" or "Revised" License
285 stars 58 forks source link

Default convolution mode introduces edge effects #8

Open maaleske opened 6 years ago

maaleske commented 6 years ago

The default image extension mode used in scipy.ndimage.convolve is 'reflect', which reflects the image about the edge of the pixels. This introduces artefacts in the edge of the resulting RGB layers when demosaicing:

# Constant ones as image
orig = np.ones((2,2)) 
R, G, B = demosaicing_CFA_Bayer_bilinear(orig)

# R, G and B should also be constant 1 (at least in the measured pixels), but instead
# R: [[2.25, 0.75], G: [[0.5, 1.5], B: [[0.25, 0.75],
#     [0.75, 0.25]]     [1.5, 0.5]]     [0.75, 2.25]]

Passing mode='mirror' - which reflects about pixel centers (doesn't repeat the edge values) - to convolve in demosaicing_CFA_Bayer_bilinear returns the expected values. I figure this should be the default for demosaicing?

KelSolaar commented 6 years ago

Hi @maaleske,

I think this is a good point, basically going from:

d c b a | a b c d | d c b a

to

d c b | a b c d | c b a

Let's do it! I saw that #9 is breaking and I'm a bit unsure why, I will kick a fresh build of develop to see.

KelSolaar commented 6 years ago

So yeah, develop builds, not sure what is happening in your branch, I will have to pull it down.

maaleske commented 6 years ago

There is probably something silly wrong in the tests I wrote. I haven't yet been able to run the test suite myself as I'm having trouble getting OpenImageIO to work. However, I also think that the reason the existing tests are failing because they are testing against images that have this exact issue (demosaiced using the reflected padding). I'm actually wondering whether any demosaicing codes deal with this issue.