pikepdf.Stream object works like a PDF dictionary with some encoded
bytes attached. The dictionary is metadata that describes how the stream is
encoded. PDF can, and regularly does, use a variety of encoding filters. A
stream can be encoded with one or more filters. Images are a type of stream
Most of the interesting content in a PDF (images and content streams) are inside stream objects.
Because the PDF specification unfortunately defines several terms involve the word stream, let’s attempt to clarify:
- stream object
- A PDF object that contains binary data and a metadata dictionary to describes
it, represented as
pikepdf.Stream. In HTML this is equivalent to a
<object>tag with attributes and data.
- object stream
- A stream object (not a typo, an object stream really is a type of stream object) in a PDF that contains a number of other objects in a PDF, grouped together for better compression. In pikepdf there is an option to save PDFs with this feature enabled to improve compression. Otherwise, this is just a detail about how PDF files are encoded.
- content stream
- A stream object that contains some instructions to draw graphics and text on a page, or inside a Form XObject. In HTML this is equivalent to the HTML file itself. Content streams do not cross pages.
- Form XObject
- A group of images, text and drawing commands that can be rendered elsewhere
in a PDF as a group. This is often used when a group of objects are needed
at different scales or multiple pages. In HTML this is like an
<svg>. It is not a fillable PDF form (although a fillable PDF form could involve Form XObjects).
Reading stream objects¶
pikepdf.Stream.read_bytes() will apply all filters
and decode the uncompressed bytes, or throw an error if this is not possible.
pikepdf.Stream.read_raw_bytes() provides access to the compressed bytes.
Three types of stream object are particularly noteworthy: content streams, which describe the order of drawing operators; images; and XMP metadata. pikepdf provides helper functions for working with these types of streams.