Sorting subset of buffer

[cdeterman]

Not sure of a mailing list so I hope posting a use question here is appropriate. I have been able to use boost::compute::sort successfully on a general OpenCL buffer. However, I am often dealing with 'padded' buffers that are interpreted as matrices (row-wise storage). For example, the matrix

8 2 7
6 5 4
1 3 9

Is actually stored as

8 2 7 0 0 0
6 5 4 0 0 0
1 3 9 0 0 0 

So the internal flat buffer would be

8 2 7 0 0 0 6 5 4 0 0 0 1 3 9 0 0 0

How could I use sort on just the 'non-padded' elements?

Likewise, the column wise may also be padded (less common but might as well include here)

8 2 7 0 0 0
6 5 4 0 0 0
1 3 9 0 0 0 
0 0 0 0 0 0
0 0 0 0 0 0 
0 0 0 0 0 0

[jszuppe]

If the padding is constant, then you should be able to develop something similar to strided_iterator, which would jump over the padding to the next row (and ignore column padding). I don't see a way to do this using included fancy iterators.

[cdeterman]

@jszuppe any thoughts on how to actually use strided_iterator to that effect? My understanding, from what I can see in the test examples here is that it takes every 'x' element (e.g. second, third, etc.).

[jszuppe]

I meant you would have to implement your own iterator class similar to strided iterator.

[cdeterman]

@jszuppe Are there any guidelines for how to approach this or should I just try to adapt from strided_iterator.hpp? This would be my first attempt at contributing to boost compute.

[jszuppe]

I think strided_iterator.hpp is a good starting point. You need to change index expression. I don't have much free time to show you how to do it. Maybe I'll find a moment this week.

[elwongest]

I know this question has been asked a long time ago, but having just got to using this very cool library, maybe this will help.
If your strided buffer is in host memory, you can move it to an "unstrided" device vector by using enqueue_write_buffer_rect instead of copy. You can "restride" it back to the host using enqueue_read_buffer_rect.
I use these for pixel format conversion in and out of FFmpeg AVFrames