re: unassociated alpha - I ran into something similar when pushing PNG's to iOS devices from XCode. Apparently Apple uses a modified libpng and added a special chunk denoting that the file (against the png spec) is premultiplied and XCode did that premult step automagically when bundling. That took awhile to figure out. This test.png is NOT one of those files though. It comes from Unity. Thanks again for digging into it.
On Fri, Feb 8, 2019 at 3:02 PM Larry Gritz <[email protected]> wrote: > I can answer that question, but then it leads right back to something ELSE > that's probably my fault. :-) > > OK, so PNG is, by specification, "unassociated alpha" (what some people > call "un-premultiplied"), and OIIO by convention always passes <ahem, > CORRECT> "associated alpha", which means that when reading unassoc files it > will automatically multiply RGB by A before giving the values to the app. > > Your file has alpha = 0, which is supposed to mean (for PNG) that the > appearance is black everywhere. > > This auto-conversion of unassociated to associated that happens in OIIO's > PNG reader can be disabled with a special configuration attribute, so the > correct command line should have been this: > > oiiotool -iconfig oiio:UnassociatedAlpha 1 steveparker.png -ch > R,G=R,B=R,A=R -o out.tif > > ... Except something is going wrong there as well. Ugh. I'm trying to > understand why. Stand by. > > > > On Feb 8, 2019, at 2:52 PM, Stephen Parker <[email protected]> > wrote: > > Indeed. That was my first reaction too. But then why doesn't this work: > > oiiotool test.png --ch "R=R,B=R,G=R" -o out.png > > oiiotool test.png --ch "R=R,B=R,G=R,A=1.0" -o out.png > > I figured it was user error. > > > > > On Fri, Feb 8, 2019 at 2:39 PM Larry Gritz <[email protected]> wrote: > >> Doesn't the fact that everything interesting ends up in out-0.png tell us >> that the alpha was black all along, and it was the red channel that you're >> after? >> >> >> On Feb 8, 2019, at 2:03 PM, Stephen Parker <[email protected]> >> wrote: >> >> ... actually, disregard the -swap as -separate is all that's needed. >> >> convert test.png -separate out.png >> >> On Fri, Feb 8, 2019 at 1:51 PM Stephen Parker <[email protected]> >> wrote: >> >>> Try: >>> >>> convert test.png -separate -swap 4,0 out.png >>> >>> You should get an image written out for each channel. "out-0.png" can be >>> opened in Preview.app. >>> >>> -s >>> >>> >>> On Fri, Feb 8, 2019 at 1:38 PM Larry Gritz <[email protected]> wrote: >>> >>>> On my mac, ImageMagick "convert test.png test.tif" also results in what >>>> appears to be a black image. >>>> >>>> And using ImageMagick's "identify -verbose", I see these stats: >>>> >>>> Alpha: >>>> min: 0 (0) >>>> max: 0 (0) >>>> mean: 0 (0) >>>> standard deviation: 0 (0) >>>> kurtosis: -3 >>>> skewness: 0 >>>> entropy: 0 >>>> >>>> Are you *sure* that the alpha channel isn't really 0 everywhere? >>>> >>>> >>>> >>>> On Feb 8, 2019, at 11:55 AM, Larry Gritz <[email protected]> wrote: >>>> >>>> Hmmm... it's not just "--ch", that image looks "black" (all 0.0 values) >>>> in all channels to all OIIO tools. (For example, `oiiotool -stats >>>> test.png`) I'm suspecting something in the reader rather than an issue with >>>> channel selection. >>>> >>>> Looking into it. >>>> >>>> -- lg >>>> >>>> >>>> On Feb 8, 2019, at 11:31 AM, Stephen Parker <[email protected]> >>>> wrote: >>>> >>>> Hi Larry, >>>> >>>> I'm on the release branch. Here is the PNG in question (attached). >>>> >>>> thanks >>>> >>>> On Fri, Feb 8, 2019 at 11:12 AM Larry Gritz <[email protected]> wrote: >>>> >>>>> Which version of OIIO? >>>>> And I don't suppose you can send me an example PNG that has the >>>>> problem? Or is it weird for every png you try? >>>>> >>>>> >>>>> >>>>> On Feb 8, 2019, at 10:59 AM, Stephen Parker < >>>>> [email protected]> wrote: >>>>> >>>>> Hello, >>>>> >>>>> I'm having some trouble using oiiotool's copy channel operator. It >>>>> might be user error but I wanted to check and see if it's intended >>>>> behavior. I've only glanced at the code so far. >>>>> >>>>> My test image is a PNG (4-channel RGBA), but only has information in >>>>> the Alpha. When attempting to copy the Alpha into the other 3 channels and >>>>> write out an RGB image (no Alpha), I get black. Imagemagick's 'convert' >>>>> did >>>>> what I expected. >>>>> >>>>> invoked as: >>>>> >>>>> oiiotool input.png --ch "R=A,G=A,B=A" -o output.png >>>>> >>>>> I also tried using the numeric channel labels to no avail. >>>>> >>>>> -sp >>>>> _______________________________________________ >>>>> Oiio-dev mailing list >>>>> [email protected] >>>>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>>>> >>>>> >>>>> -- >>>>> Larry Gritz >>>>> [email protected] >>>>> >>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> Oiio-dev mailing list >>>>> [email protected] >>>>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>>>> >>>> <test.png>_______________________________________________ >>>> Oiio-dev mailing list >>>> [email protected] >>>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>>> >>>> >>>> -- >>>> Larry Gritz >>>> [email protected] >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Oiio-dev mailing list >>>> [email protected] >>>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>>> >>>> >>>> -- >>>> Larry Gritz >>>> [email protected] >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Oiio-dev mailing list >>>> [email protected] >>>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>>> >>> _______________________________________________ >> Oiio-dev mailing list >> [email protected] >> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >> >> >> -- >> Larry Gritz >> [email protected] >> >> >> >> >> _______________________________________________ >> Oiio-dev mailing list >> [email protected] >> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >> > _______________________________________________ > Oiio-dev mailing list > [email protected] > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org > > > -- > Larry Gritz > [email protected] > > > > > _______________________________________________ > Oiio-dev mailing list > [email protected] > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >
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