Cannot embed non-transparent image onto a transparent background

[diegocsandrim]

Hi!

There is a strange behavior when resizing JPEG to PNG in embed mode.

sharp('input.jpg')
  .resize(2000, 1000)
  .background({ r: 0, g: 0, b: 0, a: 0 })
  .embed()
  .toFormat(sharp.format.png)
  .toFile('output1.png', function (err, info) {
    console.log(err || info);
  });

I am expecting to have the input image resized to maximum width or height, keeping aspect ratio, and above a transparent background.
But I am getting a full transparent image!

JPEG input

Rezize output

The same does not happens when input image is a PNG, with the same code:

sharp('input.png')
  .resize(2000, 1000)
  .background({ r: 0, g: 0, b: 0, a: 0 })
  .embed()
  .toFormat(sharp.format.png)
  .toFile('output2.png', function (err, info) {
    console.log(err || info);
  });

PNG input

Rezize output

This is really strange to me, am I making a mistake?

A add a git repo to test it:
https://github.com/diegocsandrim/sharp-test

[lovell]

Hi Diego, I've been able to reproduce this thanks to your detailed report. I think I introduced this regression during sharp's recent switch from libvips' C to C++ binding.

When embedding a JPEG onto a background with transparency, we have to add a transparency layer to the decoded JPEG image. It looks like the opaque and transparent values are currently the wrong way round, so you're seeing an "opaque transparency", also known as "nothing" :)

Luckily it's a quick fix, and I'll add more test cases for this feature to (hopefully) prevent this occurring again.

[lovell]

Commit df33c30 adds automated tests that would have discovered this regression, as well as the fix.

Thanks for reporting this!

[diegocsandrim]

Thank you very much Lovell!

[lovell]

v0.13.1 out now