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

[Chris Cox]

Per pixel metadata is probably overkill, but some people have specified
object/surface identification with masks -- using one mask for each surface
or object.  When querying a pixel for information, you look in the matching
list of masks for non-zero values and add to the list that should be
associated with that pixel.

Chris



On 4/14/11 3:13 PM, "Florian Kainz" <address@hidden> 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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