Ensure TIFF RowsPerStrip is multiple of 8 for JPEG compression

[kmilos]

Fixes #5586

[kmilos]

This works only for RGB mode 4:4:4 data (no subsampling). AFAICT, we don't support YCbCr subsampling directly when saving to TIFF, but the tag might be copied over from another info structure, so might be an idea to handle it?

However, there's something weird going on wrt to saving YCbCr mode instead of RGB, even if using single strip (i.e. 8.2.0):

OSError: encoder error -9 when writing image file

junk.tiff: Warning, fractional scanline discarded.
JPEGLib: Application transferred too few scanlines.

[kmilos]

Thanks, that leaves YCbCr mode TIFF saving which it seems never worked w/ JPEG compression?

I also tried setting ifd[530] = (1,1) (YCbCr subsampling to 4:4:4) as the default is (2,2) which doesn't correspond to the data, but the encoder doesn't like that - I think unfortunately TIFFSetField interface is different for this tag (2 ints) compared to the expected count+pointer (e.g. ExtraSamples)? Setting atts[530] = 1 didn't work either.

[kmilos]

YCbCr saving doesn't even work uncompressed (i.e. no libtiff involved), even if I add correct tags 530, 531, and 532. I guess this is a separate issue.

Uncompressed YCbCr 4:4:4 seems to work after all (if correct tags 530 and 532 are added). Just that most viewers don't seem to support it...

However, there is a clear bug w/ libtiff_encoder, as it tries to save tag 530 2-tuple with a "default" type LONG (i.e. count+offset), which doesn't fit into the expected 4 bytes value/offset sub-field as 2 SHORTs would.

[radarhere]

Suggested change

	rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, im.size[1])
	rows_per_strip = ((rows_per_strip + 7) // 8) * 8

Because rows_per_strip has already been set to be im.size[1] at minimum, and the calculation here only ever rounds up, the min part is unnecessary.

[kmilos]

I believe it is needed, precisely because it rounds up. This is to support the rare case of small images where im.size[1]<8 and you end up as a single strip. Libtiff/jpeg codec have no problem handling the single/last strip that is not factor of 8, as it is padded internally.

[radarhere]

Ok, thanks

[radarhere]

Suggested change

	rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, im.size[1])
	rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, rows_per_strip)

What about this?

[kmilos]

That won't work for the corner case where you have u multi-strip w/ e.g. rows_per_strip=2 and im.size[1]=6. For JPEG, you want to end up as single strip of 6 rows.