Document fixes for font render issues.

This patch adds some documentation to guide people through the inevitable font rendering issues on Linux.

On Linux, Skia has issues rendering certain fonts, it especially completely messes up font kerning. Mitigations exist in the outlined options, yet we decided not to apply any by default, in order not to break any existing templates.

Also added a new text_rendering option to ChromicPDF.Template to allow easily setting that CSS rule on * elements.

References

Tests

Setup

Below you can see test rendering of the same input markup with the various options applied.

<!DOCTYPE html>
<html>
  <head>
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;700&display=swap" rel="stylesheet">

    <style>
      :root {
        font-family: Inter;
        font-size: 8.5pt;
      }

/*
    * {
      text-rendering: geometricPrecision !important;
    }
*/
    </style>
  </head>
  <body>
    <p style="font-size: 0.75rem;">Hello World</p>
  </body>
</html>

iex> ChromicPDF.start_link(); {:ok, pdf} = ChromicPDF.print_to_pdf({:file, "test.html"}, evaluate: %{expression: "new Promise(resolve => setTimeout(resolve, 2000))" }); File.write!("test.pdf", Base.decode64!(pdf))

Results

Desc	Image	PDF
nothing		test-current.pdf
`--font-render-hinting=medium`		test-font-render-hinting-medium.pdf
`--font-render-hinting=none`		test-font-render-hinting-none.pdf
`text-rendering: geometricPrecision`		test-text-rendering-geometric-precision.pdf

bitcrowd / chromic_pdf