actually in this case, the resize is not changing the result,
running the conversion without the resize on the input image (exr with
pixelAspectRatio 2 - according to both RV and oiiotool.)
I get a jpeg image that is not recognized by RV as having
PixelAspectRatio of 2.
oiiotool recognizes that the image has a PixelAspectRatio attribute with
a value of 2.
I guess other packages are expecting something different than that
attribute, RV shows the density as 2400x1200 for the file written from
nuke. and as 1 for the file converted by oiiotool.
On 01/29/2015 09:58 AM, Nathan Rusch wrote:
Sorry, the old Ctrl + Enter hotkey got me again...
I think an important thing here is to look at the command that was
being run:
oiiotool in.exr --ch R,G,B --resize 50% --attrib PixelAspectRatio
2.0 -o nonsquare.jpg
There is no non-uniform scaling being applied, and changing the pixel
aspect ratio of the image should not change its physical resolution at
all. As such, the result of this should be an image that, when viewed
with proper pixel aspect ratio correction, should appear to be twice
as wide as the original.
Now, about those metadata tags...
If you look at the EXIF tag definitions for XResolution and
YResolution, the language is a bit strange:
The number of pixels per <ResolutionUnit> in the <ImageWidth>
direction. [...]
If you reorient your thinking so the "resolution" of the image is
actually whatever <ResolutionUnit> is (defaults to inches), it starts
to make some sense. For a 512x512 square image at 72 dpi, the
"resolution" is actually 7.111111 x 7.1111111.
Now, if the <XResolution> and <YResolution> values of that same image
are 2400 and 1200, respectively, and we take the language of the EXIF
tags as-is, those 7.111111 inches of resolution are not going to be
uniformly mapped to screen pixels in both dimensions(remember, "pixels
per <ResolutionUnit>"). Instead, the result will be a "pixel image"
twice as wide as it is tall.
In practice, the resolution tags are treated as a ratio, and the
image's pixel resolution is read directly from the file.
Finally, even if you disagree with all of this (which I wouldn't
really fault you for), the fact is that Nuke, RV, and Adobe products
currently all agree on how they should be handled, so I think it might
be best to try and stay consistent.
-Nathan
-----Original Message----- From: Larry Gritz
Sent: Wednesday, January 28, 2015 11:04 PM
To: OpenImageIO developers
Subject: Re: [Oiio-dev] aspect ratio from oiiotool
(WARNING: this whole explanation depends on your viewing with a
fixed-width font.)
I think maybe this is a disagreement between the different apps on
what aspect ratio means.
I very well could have screwed this up, so let me explain my thinking,
and people can tell me if it makes sense or if I botched it.
First of all, when we talk about the FRAME aspect ratio of a whole
film image, we say the aspect is 1.85, or 16/9, or 2.35, or whatever,
all of which are varying degrees of wider than they are tall. "Wider
than tall" means a frame aspect ratio of greater than 1.0, "taller
than wide" means a frame aspect ratio of less than 1.0. Right? So I'm
gonna assume that the same is true of pixel aspect ratio.
OK, here's my cartoon of a 2x2 image with square pixels. Let's make up
some densities, say the image is supposed to print 1 cm wide and 1 cm
tall, so XResolution = 2, YResolution = 2, ResolutionUnit = "cm".
+---+---+ ^
| * | * | |
+---+---+ 1cm
| * | * | |
+---+---+ v
<- 1cm ->
We agree that this is a 1.0 aspect ratio, I assume. (I do hope you're
viewing with a fixed width font)
So let's say we want to cut the density in half horizontally, giving
us wide pixels.
+-------+ ^
| * | |
+-------+ 1cm
| * | |
+-------+ v
<- 1cm ->
There are still 2 vertical samples per cm, but only 1 horizontal
sample per pixel. In other words, XResolution = 1, YResolution = 2.
What is the PixelAspectRatio?
Here's the shape of just one pixel:
+-------+
| |
+-------+
Remembering what we said about the aspect ratio of a whole frame, I
would argue that the aspect ratio of a pixel that is wider than it is
tall also should be a number greater than 1. For the above pixel, its
PixelAspectRatio is 2.0, i.e. YResolution/XResolution. Also known as
ydensity/xdensity, because note that in this terminology, "resolution"
means "dots per length", like printer's resolution, NOT the faux
"resolution" we use to describe the number of pixels in a whole image.
Nuke wrote an image that it says is 1047x858, with 2400 horizontal
pixels per inch (let's say; the units are undefined), so the image is
0.43625 inches wide, and at a y density of 1200 pixels per inch, it
should be 0.715 inches tall:
<-0.436"->
^ +--------+
| | |
| | |
0.715"| |
| | |
| | |
| | |
| | |
v +--------+
Is that what you expect? It's a tall skinny image?
Or, do you expect a wide image? If you expect wide, then I'm going to
go out on a limb and claim that Nuke is totally botching the meaning
of the density fields, and thus the aspect ratio. Maybe rv is also
getting it backwards, either coincidentally having made the same
mistake, or else purposely backwards in order to match Nuke's broken
output.
Somebody let me know if I'm totally borked in my thinking about this.
Maybe I'm the one who got it all wrong.
-- lg
PS. https://www.youtube.com/watch?v=Bt9zSfinwFA
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--
Ran Sariel
CTO / Pipeline supervisor
The Embassy VFX Inc.
177 West 7th Ave, 4th Floor
Vancouver, BC
Phone: (604) 696-6862 ext. 244
[email protected]
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