Adding an image watermark in Go with Net/HTTP

In this guide, we'll walk you through the process of adding text watermarks to your PDF files, using PDFShift's API.

Adding a watermark to your PDF can be done by adding the watermark object to your query. The advantage of providing an image is that you can add a visual stamp on top of each of your pages that are generated through PDFShift.

package main

import (
    "bytes"
    "encoding/base64"
    "encoding/json"
    "fmt"
    "io/ioutil"
    "net/http"
)

func main() {
    // You can get an API key at https://pdfshift.io
    apiKey := "sk_xxxxxxxxxxxx"

    params := map[string]interface{}{
        "source": "https://www.example.com",
        "watermark": map[string]string{
            "image":       "https://placekitten.com/200/300",
            "offset_x":   "center",
            "offset_top": "top",
        },
    }

    // Marshal the parameters into JSON
    jsonParams, err := json.Marshal(params)
    if err != nil {
        fmt.Println("Error marshaling JSON:", err)
        return
    }

    // Create a new HTTP client
    client := &http.Client{}

    // Create a new request
    req, err := http.NewRequest("POST", "https://api.pdfshift.io/v3/convert/pdf", bytes.NewBuffer(jsonParams))
    if err != nil {
        fmt.Println("Error creating request:", err)
        return
    }

    // Set request headers
    req.Header.Set("Content-Type", "application/json")

    // Set basic authentication header
    auth := "api:" + apiKey
    req.Header.Set("Authorization", "Basic "+base64.StdEncoding.EncodeToString([]byte(auth)))

    // Perform the request
    resp, err := client.Do(req)
    if err != nil {
        fmt.Println("Error performing request:", err)
        return
    }
    defer resp.Body.Close()

    // Read response body
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println("Error reading response body:", err)
        return
    }

    // Check response status code
    if resp.StatusCode >= 400 {
        fmt.Printf("Request failed with status code %d: %s\n", resp.StatusCode, string(body))
        return
    }

    // Save the PDF document
    err = ioutil.WriteFile("result.pdf", body, 0644)
    if err != nil {
        fmt.Println("Error saving PDF document:", err)
        return
    }

    fmt.Println("The PDF document was generated and saved to result.pdf")
}

The "image" parameter for the watermark can be used in two different values:

URL : A full URL to an image that will be used as the watermark.
Base64 : A base64 encoded image that will be used as the watermark.

Note that you can also rotate the image by passing the rotate parameter as a degree (or negative degree)

You can also customize the position of the text watermark by applying the following properties:

offset_x: The horizontal position of the text. Can be 'left', 'center', 'right', or a specific value in pixels. Defaults to 'center'.
offset_y: The vertical position of the text. Can be 'top', 'middle', 'bottom', or a specific value in pixels. Defaults to 'center'.

For the offset positions, we accept a value in integer which will be translated to pixels, but you can also pass a unit such as 'px', 'in', 'cm', 'mm', 'pt'.

For example:

params = {
    "source": "https://www.example.com",
    "watermark": {
        "text": "PROTECTED DOCUMENT",
        "offset_x": "5cm",
        "offset_top": "15mm"
    }
}

For further details on the watermark property and its usage, please refer to our dedicated documentation.

We hope this guide was helpful. If you have any questions or noticed any issues on the code above,
feel free to drop us a line.