Re: [Openexr-user] Per-pixel Object Identifiers?

[Scott Squires]

Yes, this would be useful.

I'm assuming it might also be useful to have a simple version of this where a 
ObjectID is simply saved as a UINT for each pixel without being
forced to go through the lookup/remap process.
That's doable now but if you're standardizing on ObjectID formats for EXR it 
would it should probably be covered as well.

Scott

On Apr 14, 2011, at 3:13 PM, Florian Kainz wrote:

> 
> Hi,
> 
> At ILM we want to implement a workflow where a computer graphics artist
> can bring up an OpenEXR image of, say, a scene from Rango on his or her
> screen, point to a pixel, and be find out that the object seen at that
> pixel is called "Beans/dress/button3."
> 
> This will require storing per-pixel object identifiers in an OpenEXR file.
> In order to avoid re-inventing the wheel, I would like to find out if any
> OpenEXR user have done something like this already. If you have done it,
> would you be willing to share how you did it?
> 
> Also, is there any interest in per-pixel object identifiers outside ILM?
> 
> Florian
> 
> 
> Unless somebody has a better idea, we'll probably do something like this:
> 
>    Add a channel called objectID, of type UINT, to the image.
>    If the image has multiple views, then add an objectID channel
>    to every view.
> 
>    Add an attribute called objectID to the header.  The type of the
>    attribute is a map from unsigned integers to lists of strings.
>    If the image has multiple views, then add multiple attributes
>    with names of the form <view name>.objectID.
> 
>    For each unsigned integer value that occurs in one or more pixels
>    in the objectID channel, a corresponding map entry in the objectID
>    attribute contains a list of all objects that are visible in those
>    pixels.  More than one object may be visible in a given pixel because
>    of transparency, motion blur, reflections, or anti-aliasing.
> 
>    To find out which object or objects cover a given pixel, application
>    software first looks up the value stored in the objectID channel for
>    that pixel, then it looks up the corresponding list of object names
>    in the objectID attribute.
> 
>    In images with lots of transparency or motion blur any given object
>    name may occur multiple times in the map from unsigned integers to
>    string lists.  In order to save disk space the objectID attribute
>    could be compressed by using a two-stage lookup, where the attribute
>    contains two maps, one from pixel values lists of integer object
>    identifiers, and one from object identifiers to object names.
> 
>    With 32-bit UINT pixel values this scheme could run out of object
>    identifiers for images with more than four Gigapixels, but in VFX
>    production images that large are rare.
> 
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