Support for partial decoding of JPEG 2000 and pyramid TIFs

Hi!

I'm using PIL to serve images using the IIIF protocol and have a performance question. Has there been any work done, or interest shown, in partial decoding of JPEG2000-images (and multi-resolution TIFs)?

There are two things that speed up the decompression of an image a lot when using opj_decompress: (1) reducing the size and quality layers and (2) cropping.

This is useful when creating tiles and/or thumbnails on the fly from large image collections, i.e when you don't want to create and save a huge number of pre-scaled/cropped tiles and sizes. As far as I can tell you can set Jpeg2KImageFile.reduce and .layers to decrease the final size which will result in faster decode time, but there is no way to decode only the cropped region. Am I correct, or is there any way to tell PIL to tell OpenJPEG to do partial decoding?

Best regards,
Martin

Start by tracing the Jpeg2KImageFile handling of reduce and layers, then compare it with the cropping options exposed by OpenJPEG's opj_decompress. The work is complete when Pillow can request cropped-region decoding for JPEG 2000 images and address the corresponding partial-decoding behavior for multi-resolution TIFFs.