metadata: expose VIPS_META_PAGE_HEIGHT, VIPS_META_N_PAGES, requires libvips v8.7.0

[lovell]

Multi-page images e.g. TIFF are treated by libvips as a "toilet-roll" image. sharp could expose this information via the existing metadata feature.

If an image has the VIPS_META_PAGE_HEIGHT integer property set, add two new properties to metadata() response:

• pageHeight: the value of VIPS_META_PAGE_HEIGHT
• pages: the (total) height divided by VIPS_META_PAGE_HEIGHT, i.e. the number of pages

[woolite64]

Does libvips have a way to count the number of TIFF directories? There are some situations (e.g. concatenated documents) where height may vary by page.

[lovell]

libvips requires that all the heights (and a few other properties) are the same when reading multiple "pages" of a TIFF file.

https://github.com/jcupitt/libvips/blob/5176b4a17ec9ca5151f1b8b503b2f237f3c83b27/libvips/foreign/tiff2vips.c#L2173-L2197

[woolite64]

If I'm reading correctly, it looks like that requirement only holds when you are retrieving more than one page at a time. In the test cases for PR #1204, we're successfully loading different sized pages from the same file.

Before the snippet you linked, there is a call to rtiff_n_pages -- it appears to count the number of TIFF directories without requiring uniformity.

https://github.com/jcupitt/libvips/blob/5176b4a17ec9ca5151f1b8b503b2f237f3c83b27/libvips/foreign/tiff2vips.c#L495-L512

[lovell]

Good point, thank you, for multi-page TIFF input this only makes sense after the n property of tiffload is exposed also (possibly with a more descriptive name).

[lovell]

Better yet, we can use VIPS_META_N_PAGES, available in the future libvips v8.7.0, for the value of pages.

[woolite64]

Thanks @lovell, you beat me to it. jcupitt was quick with that commit.

How do you manage development against the latest libvips changes? Willing to test and PR this, but I'm not sure where to start.

[lovell]

Features that require the future libvips v8.7.0 will need to wait until (a) it's released and (b) the next "major" version of sharp that places a minimum dependency on that version of libvips, which is likely to be sharp v0.21.0.

[lovell]

v0.21.2 now available with this feature.