Add support for extracting thumbnails

[letmaik]

Implements #96.

[letmaik]

@harshithdwivedi @dtakeshta Could you give this a try? You can download the wheels by clicking "Show all checks", then clicking on the right variant, and then on the right click on "Artifacts". E.g. Windows 64bit Python 3.7 is here: https://github.com/letmaik/rawpy/pull/98/checks?check_run_id=401958378

Usage is like that:

            with rawpy.imread('image.nef') as raw:
              thumb = raw.extract_thumb() # returns (format: enum, data: bytes) namedtuple
              assert thumb.format == rawpy.ThumbFormat.JPEG
              with open('thumb.jpg') as fp:
                f.write(thumb.data)

Does this make sense?

[harshithdwivedi]

Hey @letmaik thanks for this!
I just tried this on my windows machine and while it works as expected, there are some performance issues that I noticed when compared to the dcraw binary.

The time taken to extract the jpeg previews from 6 raw files is as follows:
rawpy : 5.94 secs
dcraw binary with -e : ~0.26 sec

I'm not sure if this overhead is added by python or if there's something else that's going on.

Also, in case the embedded raw doesn't exist, will extract_thumb() return a None object?

[harshithdwivedi]

I looked at the libraw api examples, and looks like the sample code here emulates the behaviour of dcraw -e; using which might speed up the process.

https://www.libraw.org/docs/Samples-LibRaw.html#code

[letmaik]

Thanks for trying this. I will have a look at performance, I'm assuming it's to do with that we unpack both the raw and the thumb, whereas dcraw would only do the latter.

If there's no thumbnail or an unsupported format, then an exception is raised. Currently there's no easy way to distinguish between both cases, like in any other libraw error condition. If you need to handle them differently I could expose the error code via named constants in the exception object.

[harshithdwivedi]

If you need to handle them differently I could expose the error code via named constants in the exception object.

Yeah, that'd be great!
Thanks again.

[letmaik]

I added support for bitmap thumbnails (which are returned as RGB ndarray) and expose libraw errors now. A semi-complete example from the docstring:

with rawpy.imread('image.nef') as raw:
  try:
    thumb = raw.extract_thumb()
  except rawpy.LibRawNoThumbnailError:
    print('no thumbnail found')
  except rawpy.LibRawUnsupportedThumbnailError:
    print('unsupported thumbnail')
  else:
    if thumb.format == rawpy.ThumbFormat.JPEG:
      with open('thumb.jpg') as f:
        f.write(thumb.data)
    elif thumb.format == rawpy.ThumbFormat.BITMAP:
      imageio.imsave('thumb.tiff', thumb.data)

Let me know what you think.

[harshithdwivedi]

I just used the updated wheel again and it seems to have a lower performance than the earlier one.
Unpacking 6 CR2 files is taking more than 10 secs now.

Here's the code I have:

    def raw_to_jpeg(self, raw_path, file_name, path_for_jpeg):

        # create the final destination
        new_file_path = pathlib.Path(path_for_jpeg).joinpath(
            "{}.jpg".format(file_name))

        try:
            raw = rawpy.imread(raw_path)
            try:
                thumb = raw.extract_thumb()
                if thumb.format == rawpy.ThumbFormat.JPEG:
                    print("jpeg")
                    with open(new_file_path, 'wb') as f:
                        f.write(thumb.data)
                elif thumb.format == rawpy.ThumbFormat.BITMAP:
                    print("bitmap")
                    cv2.imwrite(new_file_path, thumb.data)
            except (rawpy.LibRawNoThumbnailError, rawpy.LibRawUnsupportedThumbnailError):
                # no embedded thumb, so convert the raw to jpeg
                print("None")
                rgb = raw.postprocess()
                cv2.imwrite(new_file_path, rgb)
        except rawpy._rawpy.LibRawNonFatalError:
            pass

I only see "jpeg" printed on my terminal, so it's certain that the library is extracting the embedded jepg only.

[letmaik]

I haven't looked at the performance issue yet, but my latest changes wouldn't have any impact. This was only about the bitmap support and errors.
BTW, you shouldn't use anything inside rawpy._rawpy. Just use rawpy.LibRawNonFatalError

[harshithdwivedi]

Ok, got it!

[letmaik]

@harshithdwivedi I fixed the performance issue. Can you try again?

[harshithdwivedi]

Perfect! It works like a charm now.