Thank you for the answer. I understand the attribute means and OIIO design.
For PSD, I think there is a bug the last layer's ImageSpec only has
oiio::UnassociatedAlpha attribute. All layer's ImageSpec should have the
attribute.
For SGI, we need to process as unassociated alpha because of the
compatibility. (e.g. Photoshop SGI plug-in saves unassocia
ted alpha) It would be helpful if I could know the the correct alpha type
that I actually read: associated, unassociated, UNKNOWN.
Akihiro
2017年8月10日(木) 16:35 Larry Gritz :
> What the attribute means is: If the image file has unassociated alpha,
> KEEP it unassociated (preserving the original data values from the file),
> rather than the usual practice of converting to associated alpha, as is the
> normal OIIO convention.
>
> It does not convert an associated alpha file into unassociated alpha. As
> you noted, if you want to force unassociated alpha from an associated alpha
> file, you'll need to do the math to deassociate it yourself.
>
> You mentioned psd and sgi. I think that for PSD, the alpha is ALWAYS
> unassociated, so using this attribute ought to keep it from being
> auto-converted to associated. (Unless you are claiming that there is a
> bug.) The sgi format spec doesn't specify anything about this issue, so we
> assume that the data are always associated alpha (that should be the
> default assumption, except for the few misguided formats that say they
> store unassociated), and therefore the sgi reader doesn't pay any attention
> to this attribute because there's no behavior change expected.
>
> I think it's important to point out that there is a reason why associated
> alpha is the default and is preferred -- there are useful RGBA values that
> can be represented as associated but cannot be represented at all as
> unassociated. So converting associated alpha to unassociated alpha may
> involve irretrievable data loss.
>
>
> On Aug 10, 2017, at 12:02 AM, Akihiro Yamasaki
> wrote:
>
> Hi there,
>
> I'm running into the alpha channel issue. I want to read an image as
> unassociated alpha. So I used following code:
>
> ImageSpec config;
> config.attribute("oiio:UnassociatedAlpha", 1);
> ImageInput *image_in = ImageInput::open(filename, &config);
> image_in->read_image(...);
>
> But I noticed that some files are read as associated alpha image. I looked
> into it and found the following solution:
>
> // After above code
> if (image_in->spec().get_int_attribute("oiio:UnassociatedAlpha") != 1) {
> // convert associated alpha image to unassociated alpha
> deassociateAlpha(...);
> }
>
> Now some format (associated alpha tiff) works correctly, but some format
> (psd, sgi) is still wrong. I expected the ImageSpec always has the
> "oiio::UnassociatedAlpha" attribute if read_image returns
> unassociated alpha image.
>
> Is it an issue of psd/sgi input? Or is there any misunderstanding?
>
> Thanks,
> Akihiro Yamasaki
>
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>
> --
> Larry Gritz
> l...@larrygritz.com
>
>
>
>
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