[R] call im2rec in R

[thirdwing]

No description provided.

[kimtang]

This is indeed needed. Currently we can create the rec files using python but to be able to stay completely in R is an advantage. All we need is a To-Do list to outline the work to be done to be able to create the rec files from R.

[thirdwing]

I think this shouldn't be too hard. I always used the C++ version of im2rec from https://github.com/apache/incubator-mxnet/blob/master/tools/im2rec.cc

[kimtang]

Yes, it should not be that difficult but im2rec depends on opencv and this is not available in R. Can someone please confirm?
As far as I can see we need to export the functions MXRecordIOWriterCreate,MXRecordIOReaderReadRecord,MXRecordIOWriterWriteRecord and MXRecordIOReaderFree to R. And this is done in mxnetR is by registering these functions using something like MXNET_REGISTER_IO_ITER. See ~\src\io\iter_image_recordio_2.cc (line 599).

If we are able to adapt this then it should be fairly simple to write the im2rec function in R.

[thirdwing]

First, OpenCV is optional. It is not available in the pre-built pkg. However, you can add OpenCV support when compiling from source.

Second, I don't think we need to implement im2rec in R, since we have a C++ implementation. We just need to call it from R. This is almost done in https://github.com/thirdwing/mxnet/tree/iss7273

[areobe]

This breaks compiling the R package even if USE_OPENCV=0 is used in config.mk. Since using OpenCV is optional, I think the R package and imports should respect the settings in config.mk.

[hadjipantelis]

I agree with @areobe. This killed the R bindings on a system with no OpenCV.

[anirudhacharya]

@sandeep-krishnamurthy please label - "Feature Request"

[leezu]

Let's track the bugs introduced by #7389 separately. I opened #17282

Currently this feature request of this issue has been implemented, though breaking other use-cases that were previously supported.