Some file formats (such as PDF) have the concept of ‘tagged content’. Graphical objects within the page stream can be tagged with information regarding how they correspond to each other on the page.
PDF files are rarely authored directly, but are typically ‘distilled’ from documents created in other programs (such as illustrator or similar packages). These other programs frequently have the concept of ‘layers’, and this layer information is generally carried over into the created PDF file using this tagging mechanism.
In fact, layers are such a ubiquitous source of tag information, that the PDF processing community (and some applications) frequently refers to such information as ‘layer’ information, even though this is an abuse of the terminology. MuPDF follows this convention.
Other interpreters may use layer information to explicitly encode genuine layers of course!
There is no render-time special treatment of layered content, so content is simply composited blindly onto the same output image. Indeed, layers do not necessarily even nest nicely with other content, and can cross groups etc in unpredictable ways.
As a document is interpreted, when a layer (or a tag) start or end is found, the device is signalled using the following calls:
Not all content on a given layer may necessarily be sent at once - there may be several start/end pairs with the same tag.
As an example, consider a newspaper laid out in a desktop processor. Multiple ‘stories’ on the same page may be laid out each in multiple columns of content. The PDF file produced from this may tag each column with an identifier for the story from which it came, and the stories may be interleaved with one another.
By collating only the content tagged with a given identifier (layer name), individual stories may be able to be extracted from a PDF page.
The use of tagged content will vary from application to application, following convention, but no hard and fast rules.