smartcrop: bad extract area

[pieh]

In very specific case resizing with cropping set to sharp.strategy.attention or sharp.strategy.entropy will result with error smartcrop: bad extract area

Repository to reproduce - https://github.com/pieh/sharp-smartcrop-bad-extract-area-repro

This happens when width is set to 1110 and height is unspecified using image from above repository (it is 1600 x 1200)

In readme of repository I placed more outputs where it works fine:

• with crop set to sharp.gravity.center
• with slightly higher and lower widths (1120 and 1100 respectively)
• with specified height (both calculated using source image aspect ratio and full image height)

This was originally reported in gatsbyjs/gatsby#4142 (just to keep paper trail - reproduction repository should be enough)

[lovell]

Hi Michal, thank you for this detailed report and accompanying test repository.

The "attention" and "entropy" crop strategies rely on the resize() operation being provided with both a width and height but this isn't checked or enforced until after passing control to libvips.

I think the best fix here is for sharp to ignore the strategy choice should either width or height be missing as that situation will result in no crop taking place anyway. I'll take a look at this.

[pieh]

When resizing with just width (or height) crop strategy/gravity shouldn't have any effect - it's just scaling without cropping, right? What I find weird is that it works fine (doesn't throw error) with some width (f.e. 1120px or 1100px in my example) and not with others (1110px).

I will add conditional check to not use attention or entropy crop strategy in gatsby for now if it's just scaling and not cropping.

[lovell]

This error occurs when cumulative rounding during the resize operation occasionally produces an image 1px smaller along one edge than the expected target dimensions. The bug here is that there's no conditional guard to prevent a "smartcrop" operation taking place when the image is already small enough.

I've added your example dimensions as a new unit test locally, which currently fails, and expect to push the fix that will get this test to pass within the next 24 hours. Thanks for reporting!

[lovell]

v0.19.1 now available with the fix for this.

[ralrom]

Hi, I am getting the same error with a 1024x1034px image on version 0.20.0. I am also using gatsbyjs and I crop all images to a max width of 1024px

I have a project with quite a few images (about 100) and this is the only image that's causing problems:

[lovell]

@ralrom Please can you open a new issue with a minimal code sample (using only sharp) that exhibits this behaviour.