Aha, I steered you wrong.
If you are using the ImageCache, the attribute name is "unassociatedalpha".
The "oiio:UnassociatedAlpha" is the configuration attribute to use if you are
using ImageBuf or directly with ImageInput.
Here is a Python example with the usual auto-association:
#!/usr/bin/env python
import OpenImageIO as oiio
buf = oiio.ImageBuf("yang.png")
p = buf.getpixel (9,43)
print ("pixel @9.43: ", p)
$ ./testpng.py
('pixel @9.43: ', (0.19607844948768616, 0.3137255012989044, 0.5490196347236633,
0.5568627715110779))
And when we set the ImageCache attribute to NOT auto-associate (give us the raw
values):
#!/usr/bin/env python
import OpenImageIO as oiio
cache = oiio.ImageCache.create (shared=True)
cache.attribute ("unassociatedalpha", 1)
buf = oiio.ImageBuf("yang.png")
p = buf.getpixel (9,43)
print ("pixel @9.43: ", p)
$ ./testpng.py
('pixel @9.43: ', (0.3529411852359772, 0.5647059082984924, 0.988235354423523,
0.5568627715110779))
As far as I know, these are all correct. I assume that the
non-auto-premultiplied value, having B > A, means it's a properly
unpremultiplied file after all.
> On Apr 14, 2016, at 11:03 AM, Yang Yang <[email protected]> wrote:
>
> Indeed, different packages handles png in different way.
> I have sent an example png to you.
>
> Thanks
>
> 2016-04-14 11:09 GMT-05:00 Larry Gritz <[email protected]
> <mailto:[email protected]>>:
> All we really know is that several packages do not agree among them what the
> values are. It's hard to tell which, if any, are correct.
>
> Can you email me the file?
>
>
>
>> On Apr 14, 2016, at 8:59 AM, Yang Yang <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Thanks for quickly reply. :-)
>>
>> I load an png in imageview in IrfanView or Photoshop SE. And both show the
>> image is 24bpp. So I assume the image itself is premultiplied or there is no
>> alpha channel info.
>> However, when I use imagecache to open the png and then load it into opencv.
>> It gives me 4 channel, which alpha channel seems contain transparency info.
>> But at the same time, I load it using Qt's QImage, it gives me 4 channel,
>> but the alpha channel is simply a binary mask.
>> So I am wondering what I can do to let OpenImageIO give me the same result
>> like QImage.
>> For my purpose, the QImage gives me correct result, while OpenImageIO seems
>> not correct. I use eariler version of 1.6. I am not sure it is known issue
>> or just I use it wrong.
>>
>> Larry, If I use imagecache->attribute ("oiio::UnassociatedAlpha", 1), the
>> rgb chanel will not premultiplied, and 0 means it will premultiplied.
>> Is this correct?
>> I set it both before and after call get_imagespec (I don't know it should
>> happen before or after it), and then call get_pixels.
>> I tried both ways, seems no impact on end result.
>>
>> Thanks.
>>
>> Yang
>>
>> 2016-04-13 19:09 GMT-05:00 Larry Gritz <[email protected]
>> <mailto:[email protected]>>:
>> Yes, though there are some subtleties.
>>
>> PNG spec is clear that PNG files store alpha as unassociated. (Bad.)
>>
>> OIIO is clear that the values delivered to the app should be associated
>> alpha, and so file format readers that encounter unassociated alpha are
>> expected to multiply the color channels by alpha in the process of copying
>> the values to the app's buffer.
>>
>> Every once in a while, you want to bypass this conversion (usually to
>> prevent loss of precision), and the way you can do that for an ImageCache is:
>>
>> imagecache->attribute ("unassociatedalpha", 1);
>>
>> (You must do this BEFORE reading anything from the file.)
>>
>> That will help you get the unassociated alpha when you want it. But Yang
>> didn't describe that -- he said he wanted premultiplied, and that should be
>> what OIIO is giving him. Maybe he described it backwards, and he wanted the
>> original unassociated? If so, see the above advice. The other possibility is
>> that the data is stored in the file already associated (in violation of the
>> PNG spec), and he's complaining about the fact that it's being
>> double-corrected, and in that case the answer also is to set the above
>> attribute, and just know that the value you're getting is the (unchanged)
>> associated alpha.
>>
>> The one thing we can't do is distinguish between unassociated alpha
>> correctly stored in the PNG, and associated alpha incorrectly stored in the
>> PNG. Because PNG stipulates that alpha is always unassociated (i.e. colors
>> not "premultiplied"), there is no PNG header field that indicates whether
>> the alpha is associated or not, because the spec says it always must be.
>>
>>
>>
>>> On Apr 13, 2016, at 4:36 PM, Troy Sobotka <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>> PNG, by specification, can only use unassociated alpha.
>>>
>>> With respect,
>>> TJS
>>>
>>>
>>> On Wed, Apr 13, 2016, 3:17 PM Yang Yang <[email protected]
>>> <mailto:[email protected]>> wrote:
>>> HI All,
>>>
>>> I am trying to read a bunch of png images supposed to have premultiplied
>>> alpha.
>>> However, it always give me unpremultiplied alpha.
>>>
>>> I don't know how can i configure Image cache to give me correct result.
>>>
>>> Could anyone give me suggestion?
>>>
>>> Thanks,
>>>
>>> Yang
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>>
>> --
>> Larry Gritz
>> [email protected] <mailto:[email protected]>
>>
>>
>>
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>
> --
> Larry Gritz
> [email protected] <mailto:[email protected]>
>
>
>
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