[Octave-bug-tracker] [bug #32986] imread() reads .png with alpha channel as an unexpected array format

[Tobias Wolf]

Follow-up Comment #7, bug #32986 (project octave):

This is a whole *bug complex* concerning image alpha, to be frank.

First, imread does not match the Matlab API concerning imread output Array
format and [I, M, A] output variables (as already mentioned below)

Second, imwrite does not follow the Matlab API in *only saving* the Alpha
channel when an image array has [:,:,4] layers if the 'Alpha' parameter has
been given separately like so 

    imwrite(img(:,:,1:3), 'img-alphsep.png', 'Alpha', img(:,:,4))

If I give Octave a [:,:,4] image it saves alpha always.

Third and most importantly, Octave misinterprets the alpha channel polarity
and inverts the alpha channel upon saving and loading. White in the alpha
channel should be taken as fully opaque and black refers to fullz transparent.
Octave mixes this up and actually reverses this channel on both imread and
imwrite.

I've attached a Screenshot illustrating this. Left is Octave, right is
Matylab.

Should I open separate bugs about parts 2 and 3, or is this one bug enough?

(file #23846)
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Additional Item Attachment:

File name: inverted alpha.png             Size:1589 KB


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