Re: [Openexr-devel] Efficient read of a single image channel, in a multi-channel image

[Brendan Bolles]

On Jan 24, 2007, at 4:30 PM, Florian Kainz wrote:

> There is no particularly efficient way to read a single channel.
> The channels in the file are interleaved per scan line or per tile.
> This allows efficient random access to individual scan lines or tiles,
> as well as sequential reading of the entire image with no seek
> operations, but it precludes fast extraction of a single image  
> channel.


(saw this older post)

Thanks for clarifying, Florian.  I have a feature request for the  
distant future then: add the ability for files to be stored with  
channels in blocks instead of this interleaved method.  That is, if  
you think there would in fact be a speed advantage for reading a  
single channel this way.


I think for files that have any channels beyond RGBA, it is much more  
common to want to read just one channel at a time.  In fact, for a 50- 
channel file I can't think of anytime I'd want to read all 50  
channels simultaneously.  You generally have input nodes with a few  
channels requested per node.  Each node is getting called  
independently, so there's no way to load all the channels at once and  
then pass them to the different inputs.  At least in the software I'm  
using.


Brendan