Re: [Openexr-devel] Multi-layer multi-pass files

[Ton Roosendaal]

Hi,

On 19 Dec, 2006, at 22:46, Chris Cox wrote:
> If we're going to support layers in OpenEXR files, I would much rather  
> see something more concrete that isn't based on parsing comments and  
> channel names.

The naming schedule I propose follows the notes in ImfChannelList.h:

    // Support for image layers:
    //
    // In an image file with many channels it is sometimes useful to
    // group the channels into "layers", that is, into sets of channels
    // that logically belong together.  Grouping channels into layers
    // is done using a naming convention:  channel C in layer L is
    // called "L.C".
    //
    // For example, a computer graphic image may contain separate
    // R, G and B channels for light that originated at each of
    // several different virtual light sources.  The channels in
    // this image might be called "light1.R", "light1.G", "light1.B",
    // "light2.R", "light2.G", "light2.B", etc.
    //
    // Note that this naming convention allows layers to be nested;
    // for example, "light1.specular.R" identifies the "R" channel
    // in the "specular" sub-layer of layer "light1".
    //
    // Channel names that don't contain a "." or that contain a
    // "." only at the beginning or at the end are not considered
    // to be part of any layer.

There seem to be API functions to parse layers/channels too, but I  
didn't try this.

What only differs in our implementation is the specific definition of a  
"Pass", to distinguish it from a 'nested layer'.

> Where are you going to put the user assigned names for layers and  
> channels?  In another comment?

No, all names are stored in the channel itself, as mentioned above. The  
user only assigns names to the Layers. Pass/channel names are generated  
based on defaults.

> Where are you putting the compositing modes and other attributes of  
> each layer?
> Do you have any way to represent layer data that isn't pixels (ie:  
> "this should be solid 1.0,0,0", or "this is a reference to another  
> node in the graph").

That's outside of the scope for how we like to use OpenEXR.

> And how does this map to other applications that support layers and  
> extra channels, such as Photoshop?

Well, if you would do a similar parsing of layer/channel names (also  
for saving) that would be very nice! :) Afaik photoshop only has  
"Layers" with RGBA, right?
What kind of support Photoshop has now for OpenEXR?

-Ton-


> Chris
>
>
> --------------------
>
> > Currently there is no standard naming scheme for layers.  Would you
> > like
> > to suggest one?
>
> While working on multilayer files I decided it would be very hard to
> get a generic parser for a multilayer formatted exr file. There are too
> many dependencies also with our internal conventions for reading and
> saving, so I've tied it more to what is possible now in Blender (i.e.
> the separation in "Layers", "Passes" and "Channels").
>
> In short the parsing goes as follows;
>
> - check for a comment in the file with string "Blender MultiChannel"
>
> - read all channels from the file, and it put it in a list
>
> - split up channel names, with the following assumptions:
>      (reading string from right to left)
>    - channel type identifier is the last character, preceded with a "."
>    - search back to another "." to get the pass name
>    - the remainder is the layer name.
>
> - create a list of Layers, each with a list of Passes, with an array of
> Channels,
>    and read everything.
>
> This allows any "random" name, for as long it has three dots to
> separate :)
> Examples:
>
> Front Shot 21.01.Diffuse.R
> Light.Group.Blue.Spec.R
> Character John Hair.Speed.X
>
> (Note that these names get close to the 32 character limit! I've
> hardcoded Layer names to a maximum of 19 chars now. Pass names are max
> 10).
>
> As for a standard; our image system will show any named layer/pass
> combo. But for the Blender compositor UI I had to standardize pass
> names. Available now in Blender are:
>
> "Combined" : RGBA (the default combined result of Passes in a Layer)
> "Depth": Z.
> "Vector": XYZW
> "Normal": XYZ
> "UV": UVA (we store alpha for UV pass, to allow antialising)
> "Color": RGBA
> "Diffuse": RGB
> "Spec": RGB
> "Shadow": RGB
> "AO": Ambient occlusion, RGB
> "Reflect": RGB
> "Refract": RGB
> "Radio": RGB
> "IndexOB": X
>
> That list is easy to extend of course.
>
> -Ton-
>
>
>
> _______________________________________________
> Openexr-devel mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/openexr-devel
------------------------------------------------------------------------ 
--
Ton Roosendaal  Blender Foundation address@hidden  
http://www.blender.org