What do you think of the merits of having a *single* channel shuffling function,
bool IBA::channelselect (IB &dst, const IB &src, ChannelList &mask)
and let people use this explicitly to shuffle or delete channels from inputs,
rather than have to build the flexibility into every op?
On Jul 17, 2012, at 4:43 PM, Stefan Stavrev wrote:
> I did some more thinking on this and here is my new
> opinion on questions from previous mails and a
> potential solution:
>
> 1. How many channels should the result image have?
>
> Look at this image: http://postimage.org/image/6zokq0ha3/.
> Notice that A, B and C have different number of channels,
> and channels are selected in arbitrary ways. This would
> allow you to take the R channel from some RGB image and
> the G channel from another, apply some operation, and store
> the result in arbitrary channel in R.
>
> I think the image processing operation should not modify the
> number of channels in R, the caller should set that properly.
> The image processing operation only takes some input
> channels, computes output and stores it in some channel/s in R.
>
> 2. Should there be one main function that could take any image
> processing operation and apply it selectively to some channels?
> In other words, should there be one function that will coordinate
> the work with channels masks.
>
> Such function would also need to loop over all pixels and
> all channels and call the image processing function in the
> innermost loop. No need for that extra complexity.
>
> 3. Make ImageBuf iterators aware of channels masks?
>
> Could be done, but I suggest a simpler solution.
>
> 4. Example solution for a simple operation:
>
> https://github.com/StefanStavrev/oiio/commit/61f7013f5756018d04c3104b7d87a319b2ff719e
>
> The main thing to notice is that the two loops for looping
> over pixels and channels are swapped. What I have is:
>
> foreach channel
> foreach pixel
>
> rather than:
>
> foreach pixel
> foreach channel
>
> Much more efficient to first figure out which channels to
> operate on based on the mask, and then loop over
> all pixels for that channel.
>
> The mask used in that example {0, 0, 0, 1, 1, 1, 2 ,2 ,2} means:
> 0, 0, 0 - apply multiply on channel 0 in input images A and B and
> store the result in channel 0 in output image R.
> Same reasoning for 1,1,1 and 2,2,2.
> A triplet 1,2,0 in this case would mean, apply multiply operation
> on channels 1 and 2 from input images A and B, and store the
> result in channel 0 in R.
>
> 5. Finally, let me point out that not all operations can
> be used with channels masks. Well, not in a simple way.
> The operations my example code represents, are those
> that need only one channel from each input image in order
> to produce one result channel.
>
>
> Stefan
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--
Larry Gritz
[email protected]
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