I certainly understand the argument you're making for resolution as it
relates to pixel density from a logical perspective. I was going to put
together a really long, detailed response explaining how I think the EXIF
tags are being interpreted by other apps, even in the face of logic, but
first I did some more reading and found something that I think more or less
explains all of this:
JPEG does not define any way of indicating pixel aspect ratio, but JFIF
does... sort of.
Remember (and this is something I had to remind myself), JPEG is only a
codec. MySweetPicture.jpg is actually a JFIF file (or should be, anyway).
From the JFIF spec (http://www.w3.org/Graphics/JPEG/jfif3.pdf):
The JFIF APP0 marker provides information which is missing from the JPEG
stream:
version number, X and Y pixel density (dots per inch or dots per cm),
pixel aspect ratio
(derived from X and Y pixel density), thumbnail.
"... (derived from X and Y pixel density)..."
Unfortunately, it doesn't mention *how* it is derived from the density
values, so I started poking around in the JPEG headers (jpeg-8d). And lo and
behold, I found this:
/* These three values are not used by the JPEG code, merely copied */
/* into the JFIF APP0 marker. density_unit can be 0 for unknown, */
/* 1 for dots/inch, or 2 for dots/cm. Note that the pixel aspect */
/* ratio is defined by X_density/Y_density even when density_unit=0. */
UINT8 density_unit; /* JFIF code for pixel size units */
UINT16 X_density; /* Horizontal pixel density */
UINT16 Y_density; /* Vertical pixel density */
"Note that the pixel aspect ratio is defined by X_density/Y_density even
when density_unit=0."
I then looked over the sample jpegWriter plugin code that ships with Nuke,
and sure enough, it sets XDensity and YDensity in the output file's
jpeg_compress_struct:
cinfo.Y_density = 1200;
cinfo.X_density = (UINT16)(iop->format()->pixel_aspect() *
cinfo.Y_density + .5)
So, it appears Nuke is actually doing things "correctly," but the density
values themselves are somewhat misleading.
-Nathan
-----Original Message-----
From: Larry Gritz
Sent: Friday, January 30, 2015 11:56 AM
To: OpenImageIO developers
Subject: Re: [Oiio-dev] aspect ratio from oiiotool
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
_______________________________________________
Oiio-dev mailing list
[email protected]
http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org
