Well, you already have R, G, B, A, right? So what is there to merge? Do you not
simply want to *extract* the R,G,B,A that you have? You would do that with
oiiotool allchannels.exr -ch R,G,B,A -o justrgba.exr
(or justrgba.tif, if your point is to extract it to a different format)
Layers (or for that matter, "parts" in multi-part exr, which is the modern way
to do all of this, layers being just a way to try to group channels before
parts were available) are generally used in two ways:
1. Things that don't meaningfully combine at all -- for example one layer/part
for RGBA, and another for motion vectors.
2. Things that combine by addition -- for example, one layer for RGBA, a second
layer containing just the specular light, and a third containing just the
diffuse light (with RGBA = specular + diffuse).
Layers or parts are never, to the best of my knowledge, used to store things
that are supposed to composite (in the "over" sense), and there is no way to
represent an ordering. In fact, in what I believe was a grave mis-design,
OpenEXR stores channels in *alphabetical* sorted order, without the option of
specifying any other ordering of the channels within the file.
Presence of a Z channel does not make the file deep; it's fine to have a flat
file with a Z channel. Although if the file is deep, it is in theory required
to have a Z channel. The thing that makes it deep is the presence of multiple
(and potentially differing numbers of) samples in each pixel. Flat files have
just one sample per pixel. That's the difference.
If you had Z/Zback channels in each layer, then you could certainly do a depth
merge of them, since these channels would imply a front-to-back order (per
pixel, not of whole layers). But I've never heard of people doing this. You'd
just use a deep file if this is what you were trying to store in the file.
> On Sep 16, 2020, at 12:47 PM, Radu Arjocu <[email protected]> wrote:
>
> I am talking about "multi-layer" which is a single "part" with many channels
> that are logically grouped togheter in layers. Right now I'm interested in
> the EXR case, with its rules of naming layers -- which I think I understand.
> For simplicity, let's assume a single EXR view and the following channels:
>
> R, G, B, A, shape1.R, shape1.G, shape1.B, shape1.A, shape2.R, shape2.G,
> shape2.B, shape2.A
>
> The layers would be: the (default) nameless layer and respectively the
> "shape1" and "shape2" layers.
>
> So, what I want to know is how to merge these 3 layers into 1 layer
> (containing only RGBA), for viewing the end result!? What layer is in front
> of another, or at the back? That's what I meant with "the order in which to
> merge [the layers]" -- determining which layer is more at the front or at the
> back.
>
> As I understand OpenEXR, the depth channels (Z & ZBack) correspond to the
> deep pixels (for all channels of a pixel), and don't reveal which layer is at
> the front or back. Am I missing something? Can the Z & ZBack channels apper
> on non-deep (/flat) EXRs and are they used to tell the depth of layer
> channels on a per pixel basis -- so my example would need Z, shape1.Z and
> shape2.Z?
>
> Radu Arjocu
>
> From: Oiio-dev <[email protected]> on behalf of Larry
> Gritz <[email protected]>
> Sent: Wednesday, September 16, 2020 20:23
> To: OpenImageIO developers <[email protected]>
> Subject: Re: [Oiio-dev] How to merge layers of multi-layer EXRs?
>
> Are you talking about a "multi-part" exr (what OIIO calls multi-subimage), or
> are you talking about "multi-layer" which is a single "part" with many
> channels but that just follows a convention for how to name those channels so
> that they can be grouped easily by an application?
>
> If you just want a single RGBA image in the end, then are you just talking
> about simply extracting the RGBA channels for export to another format?
>
> "flatten" just composites the multiple depth samples within each pixel of a
> deep image to a single per-channel value. But it does not affect which
> channels are in which "part" of a multi-part image.
>
>
>> On Sep 16, 2020, at 7:45 AM, Radu Arjocu <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Hello,
>>
>> Being relatively new to the OpenEXR format, I have a couple of questions
>> regarding multi-layer EXR:
>>
>> 1) What is the (optimal) way to go if I want to merge/compose all the layers
>> of an EXR (say flat image for simplicity) into a single layer (RGBA-only
>> channels), so that I can convert it to another format and rescale it as
>> needed for quick viewing ? -- just like viewing thumbnails, but probably at
>> a higher resolution.
>>
>> I understand that I have to manually determine what the layers are and their
>> respective channels, but I don't know (especially) the order in which to
>> merge them. NOTE: I use the terminology "flat image" to refer to non-deep
>> images (since "flattening" is used by OIIO to denote this operation) and
>> "merge"/"compose" for rendering multi-layers into a single layer; but other
>> SW use "flattening" to refer to the merge/compose operation.
>>
>> 2) How do I treat "special" channels like effects, vectors etc. in the
>> scenario at point 1?
>>
>> 3) Does using the `flatten()` function for deep and multi-layer images
>> (EXRs) also merge the layers or not? I assume not, but I cannot verify this,
>> since I don't have an EXR sample. I would greatly appreciate if someone has
>> a sample like this.
>>
>> Thank you,
>> Radu Arjocu
>> <https://protect2.fireeye.com/v1/url?k=c25e4a02-9cd1f486-c2592a0e-86801c117383-76c7f1f405a319a7&q=1&e=975968e2-8694-4935-af42-44e261a41663&u=http%3A%2F%2Fwww.masstech.com%2F>
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>> <https://protect2.fireeye.com/v1/url?k=82a4ba62-dc2b04e6-82a3da6e-86801c117383-ba9b89b8c2796bf7&q=1&e=975968e2-8694-4935-af42-44e261a41663&u=http%3A%2F%2Flists.openimageio.org%2Flistinfo.cgi%2Foiio-dev-openimageio.org>
> --
> Larry Gritz
> [email protected] <mailto:[email protected]>
>
>
>
>
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