PDF page overlay with Go

Recently, I had a need to overlay 2 PDFs in Go.

Some background: I had a semi-complex PDF generated off-line with a Python program and I needed to overlay some simple drawing on top, on-demand using some parameters sent from a website. Since I wanted to deploy on Cloud Run (free for my low-traffic app), in order to avoid issues with cold start, I decided to develop the server in Go. The drawing part was simple: I used fpdf, which worked well.

However, I had a bit of trouble to find a way to overlay the generated PDF on top of the complex pre-generated PDF. Also, when I asked Github Copilot, it kept referring me to the Unidoc library, which is not free. At some point, I realized I could simply use pdfcpu and its watermark feature: It supports both images and PDF so it worked for me.

Here is the code:

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import (
    pdfcpu_api "github.com/pdfcpu/pdfcpu/pkg/api"
	pdfcpu_model "github.com/pdfcpu/pdfcpu/pkg/pdfcpu/model"
	pdfcpu_types "github.com/pdfcpu/pdfcpu/pkg/pdfcpu/types"
)

func overlayPdfs(input, inputOverlay, output string) error {
	conf := pdfcpu_model.NewDefaultConfiguration()
	unit := conf.Unit

	wm, err := pdfcpu_api.PDFWatermark(inputOverlay, "", true, false, unit)
	if err != nil {
		return err
	}
	wm.Scale = 1
	wm.ScaleAbs = true
	wm.Diagonal = pdfcpu_model.NoDiagonal

	return pdfcpu_api.AddWatermarksFile(input, output, []string{}, wm, conf)
}

The wm.ScaleAbs = true line is important: Without it, the code worked for documents in US Letter format but not A4. It turned out that without it, the size of the document was converted to integers: In points (1/72 of an inch), it makes no differences for letters (8.5 × 11 inches), but there was a tiny issue with A4.

Afterwards, I found out about the gofpdi library, but since what I had worked, I didn’t switch.

Here is my website that uses this: https://xyl.vellut.com (it is a tool to generate music sheets for toy xylophones, which may have different colors depending on the models).