Re: [Openexr-devel] Storing multiple layers in OpenEXR files

[Florian Kainz]

> We don't have regular expressions nor wildcards to retrieve all channel 
> names starting with "diffuse." and which would retrieve the set 
> {diffuse.R, diffuse.G, diffuse.B}. Or alternatively, a query with 
> "diffuse.*" that would retrieve the first channel in alpahbetical order 
> like "Diffuse.B". And then from that we couls continue scanning through 
> the channel names until the layername differs. Without that, we must 
> first retrieve all the channels and parse their names and build a 
> regrouping structure. Then we can start retrieving the actual data and 
> regrouping them into layers.
>
> Since the regrouping data and channel names were readily available at 
> store time, I just decided that it was simpler to save that information 
> rather than deriving and rebuilding it at load time.

A standard way to represent layers should be as easy to use as
possible; we should not avoid an easy-to-use solution in favor
of another one that just happens to require slightly less work
to implement.

Since a file's channels are already sorted alphabetically, it
would not be a lot of work to add a method to class ChannelList
that finds all channels whose names begin with with a  given
prefix, e.g. "layername.":

    void
    ChannelList::channelsWithPrefix
        (const char prefix[],
         ConstIterator &first,
         ConstIterator &last) const
     {
        first = last = _map.lower_bound (prefix);
        int n = strlen (prefix);

        while (last != end() && strncmp (last.name(), prefix, n) <= 0)
            ++last;
     }

The function might be used like this:

    ChannelList::ConstIterator first, last;
    channels.channelsWithPrefix ("diffuse.", first, last);

    for (ChannelList::constIterator i = first; i != last; ++i)
        doSomethingWithChannel (i.channel());

> In one single string attribute. I llike that idea. And this would 
> certainly save file space.
>
> What about something like this :
> "diffuse{R,G,B},ambiance{R,G,B},Z,tangent{X,Y},normal{X,Y}"

I would prefer:

    "diffuse,ambiance,Z,tangent,normal"

Or, even better, use an attribute whose value is a vector of strings,
and store the names of the layers in the elements of the vector.