Add PDF background color option

[calebmer]

Fixes #3321.

I followed existing code for options.subifd and options.create.background to pass a background option to vips for rendering PDFs. I haven't tested that this works with a vips build containing PDF support. Instead of using sharp's global vips support I override the contents of the @img/sharp-libvips-* packages with my own custom builds derived from https://github.com/lovell/sharp-libvips which makes testing custom sharp C code a little more difficult.

If you’re not sure this is the right way to pass the background option to vips_pdfload() (source) I can try to jump through some hoops to see if this change works in my local setup.

[lovell]

Brilliant, thank you very much for working on this Caleb.

[GuM75]

Hello,

this is my first time, commenting in github.

I just updated to the latest version 0.34.1 to try the new pdfBackground option, but it doesn't seem to work.

I think I found out why. In the file sharp/lib/input.js in line 251 the following function is called:

    // PDF background colour
    if (is.defined(inputOptions.pdfBackground)) {
      this._setBackgroundColourOption('pdfBackground', inputOptions.pdfBackground);

This function sets the pdfBackground in the sharp.options object, but it needs to be set in inputDescriptor.
What worked for me was changing the if-clause to

    // PDF background colour
    if (is.defined(inputOptions.pdfBackground)) {
      const value = inputOptions.pdfBackground;
      if (is.object(value) || is.string(value)) {
        const colour = color(value);
        inputDescriptor['pdfBackground'] = [
          colour.red(),
          colour.green(),
          colour.blue(),
          Math.round(colour.alpha() * 255)
        ];
      } else {
        throw is.invalidParameterError('background', 'object or string', value);
      }
    }

But maybe there is a better way to fix this, maybe by adding another function in color.js that returns the color object.

[lovell]

@GuM75 Yes, you are correct, thanks for reporting. It's relatively complex to test this kind of change as the prebuilt binaries do not support PDF.

Commit 5b5dfba should fix this by refactoring the way in which background colours are created/set internally. This will be in v0.34.2.