Efficiency of Jpeg compression in TiffSaver

[iwbh15]

The jpeg compression does not write blocks of 8x8 pixels and can be improved.

See comment in https://github.com/ome/ome-codecs/pull/29#discussion_r1270301260: "... Tif images saved with JPEG compression (at high quality settings) are larger than the original uncompressed image. ..."

I tested different combinations of JPEG quality, interleaved mode and chroma subsampling and found that the size of the saved tiff files was larger than the original in almost all combinations.

To test a possible improvement, I made a change in loci.formats.tiff.TiffSaver in the writeImage() function. My change forces 8 lines per strip when saving, resulting in jpeg compression in blocks of 8x8 pixels (what is normally used in jpeg). I have tested the different combinations with this modified code. Here is a comparison of the file size in bytes. (All files that are larger than the original are marked in bold.) File used for testing: lena-std.tif (786572 bytes) (available at e.g. https://imagej.nih.gov/ij/images/)

interleaved	jpeg-quality	chroma-subsampling	Unmodified TiffSaver	Modified TiffSaver
no	0.95	no	1668225	364087
no	0.95	yes	1668225	364087
no	0.7	no	1121273	169081
no	0.7	yes	1121273	169081
yes	0.95	no	1083658	202473
yes	0.95	yes	914820	163099
yes	0.7	no	625754	89538
yes	0.7	wC	599917	85053

This test clearly shows that the storage of jpeg-compressed tiff files can be improved.

My code change is NOT as a solution. It was only for my testing and is meant as a hint.

I added a short statement in https://github.com/ome/bioformats/blob/d566f70845be514ab9c88d910ae42ab7a5f0bac0/components/formats-bsd/src/loci/formats/tiff/TiffSaver.java#L313C7-L318C64

Original code

        ...
	compression = ifd.getCompression();
	tileWidth = (int) ifd.getTileWidth();
	tileHeight = (int) ifd.getTileLength();
	int tilesPerRow = (int) ifd.getTilesPerRow();
	int rowsPerStrip = (int) ifd.getRowsPerStrip()[0];
	int stripSize = rowsPerStrip * tileWidth * bytesPerPixel;
        ...

My version

        ...
	compression = ifd.getCompression();
	
	if (compression.getCodecName().equals("JPEG"))      
		ifd.putIFDValue(IFD.ROWS_PER_STRIP, new long[] {8});

	tileWidth = (int) ifd.getTileWidth();
	tileHeight = (int) ifd.getTileLength();
	int tilesPerRow = (int) ifd.getTilesPerRow();
	int rowsPerStrip = (int) ifd.getRowsPerStrip()[0];
	int stripSize = rowsPerStrip * tileWidth * bytesPerPixel;
        ...