Sharp: Nearest neighbor upsampling stretching image

[edwjusti]

Images are being stretched when resized with sharp.kernel.nearest, however the stretching only happens when the resolution is not an exact scale like 8 (1), 16 (2), or 24 (3).

Exact scale (x32) with kernel nearest:

Non exact scale with kernel nearest:

Non exact scale with default kernel:

[lovell]

Hello, please can you share the input image and (standalone) code sample.

[edwjusti]

Here's the original image:

And here's the code:

const imageBuffer = /* ... */;
const size = 250; // causes stretching

const avatar = await sharp(imageBuffer).extract({
  left: 8,
  top: 8,
  width: 8,
  height: 8
}).resize({
  width: size,
  height: size,      
  kernel: sharp.kernel.nearest
}).png().toBuffer();

[lovell]

Thank you, I've been able to reproduce this.

If I extract the 8x8 image:

then use vips at the command line to resize with a scale of 31.25 (=250/8)

$ vips resize extract.png out.png 31.25 --kernel=nearest

the result is:

so it looks like this is happening within libvips itself.

[jcupitt]

Hello, should be fixed in git master, thanks for the report! It was to do with the way centre vs. corner sampling is implemented.

With this 3x3 pixel PNG:

If I run:

$ vips resize 3x3.png 3x3-nearest.png 83.33333333333 --kernel nearest 
$ vips resize 3x3.png 3x3-cubic.png 83.33333333333

I now see:

[lovell]

Thanks @jcupitt !

[lovell]

sharp v0.23.0 / libvips v8.8.1 is now available. Thanks for reporting this @edwjusti.