When used in the normal way, MuPDF requires the entirety of a file to be present before it can be opened. For some applications, this can be a significant restriction - for instance, when downloading a PDF file over a slow internet link, being able to view just the first page or two may be enough to know whether it is the correct file or not.
Normal PDF files require the end of the file to be present before file reading can begin, as this is where the ‘trailer’ lives (effectively the index for the entire file). In an effort to allow early display of the first page, Adobe (the originators of the PDF format) introduced the concept of a ‘linearized’ PDF file. This is a PDF file that, while constructed in accordance with the original specification, also has some extra information contained within the file to allow fast access to the first page. This information is known as the ‘hint stream’. In addition, extra constraints are placed upon the ordering of data within the file in an effort to ensure that the first page will download quickly.
Unfortunately, Linearized PDF files are far from a panacea. The specification is overly-complex, unclear and consequently poorly supported in both readers and writers of the format. Even when implemented correctly, it is of limited use for pages other than the first one.
MuPDF therefore attempts to solve the problem using a combination of mechanisms, known together as “progressive mode”. When run in this mode, MuPDF can not only take advantage of the linearization information (if present) in a file, but is also capable of directing the actual download mechanism used by a file. By controlling the order in which sections of a file are fetched, any page required can be viewed before the whole fetch is complete.
For optimum performance a file should be both linearized and be available over a byte-range supporting link, but benefits can still be had with either one of these alone.
Coupled with the ability to render pages ignoring (and detecting) errors, this means that ‘rough renderings’ of pages can be given even before all the content (such as images and fonts) for a page have been downloaded.