On Jan 30, 2015, at 11:44 AM, Nathan Rusch <[email protected]> wrote:
>> If the pixels are wider than they are tall, then the JPEG metadata should >> have xDensity (what OIIO reports as "XResolution", using the TIFF >> terminology) SMALLER than the xDensity (OIIOs "YResolution"). > > There are no Density tags in the EXIF spec, but XResolution and YResolution > do exist. Sorry, I was using confusing terminology. To make it worse, the JPEG header has an xDensity and yDensity, and ALSO an Exif block that contains XResolution and YResolution, which are described as the same thing. I think we're all kind of punting on what happens when these seemingly coupled values contradict each other, and assuming that it's the Exif data that really counts. Now then... > >> The "working version" rendered from Nuke, as you reported, said that the >> XResolution = 2400 and YResolution = 1200. That is supposed to mean >> pixels that are taller than they are wide, because the density of pixels is >> higher horizontally than vertically. > > Based on the language in the EXIF spec, I'm not sure this is true. As I > mentioned in my previous message, the spec says this about the Resolution > tags: > > The number of pixels per <ResolutionUnit> in the <ImageWidth> direction. > [...] Right. If there are MORE pixels per ResolutionUnit, that means they are spaced closer together, i.e. smaller. > Using this language as a guide, an XResolution of 2400 and a YResolution of > 1200 makes sense for an image with a pixel aspect of 2.0, and I believe this > is how other applications (Adobe, RV, Nuke) handle "non-square pixels" in > JPEG files. If "XResolution" is number of pixel per ResolutionUnit (inch, cm, or undefined), that means that HIGHER resolution (aka density) means THINNER pixels, and LOWER resolution (density) means WIDER pixels. Once again, XResolution < YResolution means short wide pixels, XResolution > XResolution means tall skinny pixels. More pixels per inch horizontally than vertically means individual pixels are tall, not wide. So XResolution=2400, YResolution=1200 is a tall skinny pixel, which I think we would all describe as aspect ratio 0.5, whereas aspect ratio 2.0 would be a short wide pixel (perhaps with XResolution=1200, YResolution=2400), just like we describe a short wide FRAME as aspect ratio > 1.0. No? -- Larry Gritz [email protected] _______________________________________________ Oiio-dev mailing list [email protected] http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org
