DOC: Stamps and watermarks (#1082)

Closes #307
Closes #410

docs/user/add-watermark.md

@@ -1,24 +1,66 @@
-# Adding a Watermark to a PDF
+# Adding a Stamp/Watermark to a PDF
+
+Adding stamps or watermarks are two common ways to manipulate PDF files.
+A stamp is adding something on top of the document, a watermark is in the
+background of the document.
+
+In both cases you might want to ensure that the mediabox/cropbox of the original
+content stays the same.
+
+## Stamp (Overlay)
 
 ```python
 from PyPDF2 import PdfWriter, PdfReader
 
 
-# Read the watermark
-watermark = PdfReader("watermark.pdf")
+def stamp(content_page, image_page):
+    """Put the image over the content"""
+    # Note that this modifies the content_page in-place!
+    content_page.merge_page(image_page)
+    return content_page
 
-# Read the page without watermark
-reader = PdfReader("example.pdf")
-page = reader.pages[0]
 
-# Add the watermark to the page
-page.merge_page(watermark.pages[0])
+# Read the pages
+reader_content = PdfReader("content.pdf")
+reader_image = PdfReader("image.pdf")
 
-# Add the page to the writer
+# Modify it
+modified = stamp(reader_content.pages[0], reader_image.pages[0])
+
+# Create the new document
 writer = PdfWriter()
-writer.add_page(page)
+writer.add_page(modified)
+with open("out-stamp.pdf", "wb") as fp:
+    writer.write(fp)
+```
+
+![stamp.png](stamp.png)
+
+## Watermark (Underlay)
+
+```python
+from PyPDF2 import PdfWriter, PdfReader
+
+
+def watermark(content_page, image_page):
+    """Put the image under the content"""
+    # Note that this modifies the image_page in-place!
+    image_page.merge_page(content_page)
+    return image_page
+
 
-# finally, write the new document with a watermark
-with open("PyPDF2-output.pdf", "wb") as fp:
+# Read the pages
+reader_content = PdfReader("content.pdf")
+reader_image = PdfReader("image.pdf")
+
+# Modify it
+modified = stamp(reader_content.pages[0], reader_image.pages[0])
+
+# Create the new document
+writer = PdfWriter()
+writer.add_page(modified)
+with open("out-watermark.pdf", "wb") as fp:
     writer.write(fp)
 ```
+
+![watermark.png](watermark.png)