OpenEXR.OutputFile Fails to Write to BytesIO Buffer

[shiukaheng]

Description:

I encountered an issue when trying to use OpenEXR.OutputFile to write to a BytesIO buffer. The operation fails with the following error: OSError: Cannot open image file "". file write failed. It appears that OpenEXR.OutputFile does not handle in-memory buffers as expected.

Steps to Reproduce:

1. Install OpenEXR and dependencies.
2. Run the following Python code:

   from io import BytesIO
   import OpenEXR
   import Imath
   import numpy as np
   
   def encode_exr(image: np.array) -> bytes:
       width, height, _ = image.shape
       header = OpenEXR.Header(width, height)
       header['channels'] = {
           'R': Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT)),
           'G': Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT)),
           'B': Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT))
       }
       header['compression'] = Imath.Compression(Imath.Compression.DWAA_COMPRESSION)
       
       data_r = image[:, :, 0].tobytes()
       data_g = image[:, :, 1].tobytes()
       data_b = image[:, :, 2].tobytes()
       
       buffer = BytesIO()
       exr = OpenEXR.OutputFile(buffer, header)
       exr.writePixels({'R': data_r, 'G': data_g, 'B': data_b})
       exr.close()
   
       return buffer.getvalue()
   
   if __name__ == '__main__':
       random_image = np.random.rand(720, 1280, 3).astype(np.float32)
       exr_bytes = encode_exr(random_image)

3. Which gives the following error

   Traceback (most recent call last):
     File "script.py", line 30, in <module>
       exr_bytes = encode_exr(random_image)
                   ^^^^^^^^^^^^^^^^^^^^^^^^
     File "script.py", line 22, in encode_exr
       exr = OpenEXR.OutputFile(buffer, header)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   OSError: Cannot open image file "". file write failed
   

Expected Behavior:

OpenEXR.OutputFile should support writing to a BytesIO buffer without raising an error.

I am not very C++ savvy but it seems that the C++ implementation and the python wrapper would handle a byte stream for image writing? Especially when reading from a byte stream is supported.

Appreciate any help. Thanks!

Environment:

• Python version: 3.11.9
• OpenEXR version: 3.2.4
• OS: Linux Mint 21.2

[cary-ilm]

You're right, the current python bindings only support file I/O, even though the underlying library supports streams.

I'm in the process of entirely rewriting the bindings (#1756), I can add support for byte streams.