Ignoring aspect ratio in vipsthumbnail

[kleisauke]

Sometimes it's needed to ignore the aspect ratio and distort the image so it always generates an image exactly the size specified. With imagemagick this can be done with ! flag, see:

convert terminal.gif -resize 64x64\! exact_terminal.gif

^{(terminal.gif can be downloaded here)}

There's not yet an option to do this with vipsthumbnail.

Background information
The resize logic from our image cache and resize proxy (which uses libvips as back-end) has much in common with the vipsthumbnail operator. Unfortunately, we can't use the vipsthumbnail operator (yet) because it doesn't support ignoring the aspect ratio (which we've supported for almost 7 years now).

In order to not break anyone's apps, we'll like to see that this feature is incorporated in the vipsthumbnail operator.

How much demand is there for this feature?
I don't know the exact numbers of developers who'll like to see this feature in vipsthumbnail, but I think there is still some demand for it. See:
lovell/sharp#118
http://stackoverflow.com/questions/42708357/resizing-stretching-with-vipsthumbnail/

To see how much demand there is for this feature, please vote with a 👍 reaction.

Number of requests
For a week, we've monitored the usage of a parameter (&t=absolute) to ignore the aspect ratio. A total of 7,875 uncached requests were made on 2,633 unique images (which can be seen as a small number of requests).

Use case scenarios
We found out that there are some specific use case scenarios where ignoring the aspect ratio will come in handy:

• A single-color image to use as background for different types of devices.
• Placeholder images like X (or to overlay over another image).
• Converting ellipses to circles and vice versa (see this for example, not sure why people do this 😕).

Relevant links (for implementation)
lovell/sharp#192

[jcupitt]

For implementation, perhaps the simplest way to add this is to add a new value for VipsSize:

http://jcupitt.github.io/libvips/API/current/libvips-resample.html#VipsSize

That's the thing that's set by < and > at the end of a 100x200 or whatever size specification.

We could add a new value, perhaps FORCE, which forces the image to exactly those dimensions, ie. breaks aspect. It sounds like vipsthumbnail.c should use ! at the end of a size specification to trigger this.

[jcupitt]

OK, it seems to work. Just needs some tests.

$ vipsthumbnail k2.jpg -s 100x5\!
$ vipsheader tn_k2.jpg 
tn_k2.jpg: 100x5 uchar, 3 bands, srgb, jpegload

You can't mix < > and !, it has to be just one of those modifiers, but hopefully that's OK.

[kleisauke]

Just tested it, and it works! Awesome! 👍

I don't think there is a use case to mix those modifiers, so just one modifier is OK.

Found only 1 bug in combination with auto_rotate. It doesn't seem to force the dimensions when there is EXIF orientation. You can try this image in combination with auto_rotate and VIPS_SIZE_FORCE.

[jcupitt]

OK, autorot seems to work. I'll add some more tests.

[jcupitt]

Added some tests, it all seems to work. I'll leave it a day two for more testing, then merge to master.

Thanks for pushing for this, @kleisauke.

[jcupitt]

Merged to master! I'll close. Thanks for testing!

[kleisauke]

Thanks for incorporating this in vipsthumbnail!

I think libvips have a time-based release cycle of 6 months (just like Ubuntu). So I guess 8.6.0 will be released in October 2017?

[jcupitt]

Yes, it's a while off, probably.