PDF.js

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PDF.js is an HTML5-based Portable Document Format renderer.

Project Manager: Pascal Finette

Developers: Andreas Gal (part-time), Chris Jones (part-time), Vivien Nicolas (part-time), Shaon Barman

Repository: https://github.com/andreasgal/pdf.js

IRC: #pdfjs on irc.mozilla.org

Milestone: Big-splash demo

Probably will be of pixel-perfect rendering of tracemonkey paper, with nontrivial UI (i.e. eye candy).

Pixel-perfect rendering

  • Type1 fonts
  • Bitmaps and SMask blending
  • canvas.setDash()
  • even-odd fills
  • axial shading
  • TTF fonts (pass the sanitizer)

Non-trivial UI

  • zooming
  • pre-rendering pages
  • "continuous" scrolling

Milestone: PDF.js Firefox extension 1.0

PDF.js/Planning/1.0

Minimum Feature Set

Schedule

Tracking in-progress work

We'll try to use github issues to track work. (If that proves too unwieldy, we can move to another system.)

Like bugzilla, "issues" are used to track both bugs filed by users and specific work items for developers. Try to file one issue per problem observed. For example, 

Text looks bad in PDFs

is an unspecific issue. A more specific and more helpful one is 

Glyph spacing is weird on page 10 of http://foo.com/bar.pdf

Similarly, 

Implement fonts

is an unspecific work item. Better is 

Add code to convert Type 1 into CFF

For big projects that span many issues, let's try to use issue "labels" to track the work. (In the same way one would use metabugs to track big projects in bugzilla.) So for the font example above, while Type1->CFF conversion is a specific work item, we might want to tag it "fonts" for easier searching.

Once a specific work item (issue) has been filed, please assign the issue to yourself if you're working on it. This avoids multiple people working on the same projects and thereby wasting time.

(TODO)

Backend

  • zooming
    • the general idea is that the UI will set a zoom factor, say 200%
    • we'll redraw the canvas, but with a scale transform to 2x, and a translation set to move the content we want to fill the screen to the top-left
  • draw subpage
  • SVG backend
  • linearization
    • byte range requests
  • hyperlinks (hash URLs, intra-doc links)
  • perf (use workers for some stuff?)
  • color spaces (big, pervasive)
  • build something like gecko's display list, for hit testing
    • click-on-link (easy)
    • text selection (hard)

UI

  • animations (page flip, etc.)
  • hyperlinks
  • page transitions
  • dual-page display
  • page-transition animations
  • pan/zoom/next/prev gestures (Edit: Felipe Gomes and cjones discussed a better way to support these, but it will require new web APIs)

Platform

  • TextMetrics.maxHeight (to compute more accurate bounding boxes; can approximate without this, though)
  • implement text selection in SVG documents
  • (determine extent of SVG a11y implementation, if any)

Testing

  • reftest-style harness, compare hand-written PDF commands to hand-written canvas (?)
  • compare to poppler output, keep list of differences

Analysis

  • dump stream info
  • dump font info
  • dump raster image info

Big project: Color spaces

Approach: map input color values (fillcolor, strokecolor etc.) to output color space. Map input bitmaps to output space with SVG color-matrix filter/WebGL shader program/hand-written JS as available. Problem: will this work correctly for interpolated color values, like intermediate colors in a gradient, and other computed values like the result of composition operators? Does canvas need color-space support? Do we care enough? (What do other PDF renderers do?)

Big project: Hyperlinks

  • Parse link data from PDF
  • Add UI to highlight/set cursor on link hover
  • Implement "go to point X in page Y" interface in backend
  • Figure out encoding scheme for absolute links, e.g. http://foo.com/bar.pdf#[encoded link]

Big project: SVG backend

Most of SVG maps well to PDF (was influenced by?). There are existing PDF->SVG translators. Perf is the biggest concern. We want to build the SVG document in the background, without affecting main-thread interactivity. The way to do that is by building the document with a Web Worker thread. The problem is, Workers don't have access to any DOM APIs. We'll probably need to build the document as a string in the background, then send it over to the main thread for parsing.

Big project: Text selection

  • Option 1: In SVG backend
    • Draw to canvas first. On first selection, switch to SVG-rendered content.
    • Let Gecko do all text selection in SVG document
  • Option 2: In canvas backend
    • Build data structure representing text drawn to screen (e.g., display list/BSP/etc.). For best results, collapse adjacent and same-height/width "text runs".
    • Walk data structure and compute textruns at a particular point and/or within a bounding box
    • Add UI for "highlighted" text above PDF and saving selected text to clipboard
      • Corner cases: clipped text, occluded text, non-white backgrounds, non-black text
    • Maybe: render without display-list building first, then on first selection re-interpret PDF to build display list. Or pre-build display list in the background.

Big project: Accessibility

Kind of like text selection, except there's no web-visible accessibility API we could hook with canvas. So

  • Somehow detect that a11y is enabled, permanently switch to SVG backend
  • Let Gecko implement a11y interfaces

(Possibly) Big project: Vertical text

Somewhat pervasive mode switch in text-drawing code. Is it just a matter of transform hackery to put glyphs in the right place, or do we need canvas support? Canvas support might be a big project.

Utils

To run tests

  • Read this
  • Do what it says
  • Then,
pdf.js$ python test.py

To uncompress a PDF

Coding Style

  • add a "use strict"; statement (exactly that!) to the top of your JS files
  • 2 spaces for indentation. (sbarman: it seems like its 4 currently in pdf.js) (cjones: we're going to fix pdf.js after Type1 fonts merge)
  • Line break are free (I promise) don't hesitate to use them to separate logical block inside your functions.
  • Be sure to declare a variable with 'var' before using it you don't want to be hurt by random variables living on the global scope.
  • Files are named like_this.js.

Useful resources:

Also some particular points (sentence stolen from https://developer.mozilla.org/en/JavaScript_style_guide)

  • Don't use object methods and properties more than you have to. It is often faster to store the result in a temporary variable.

If you have to do DOM manipulations (hopefully not!):

  • Don't call getAttribute to see if an attribute exists, call hasAttribute instead.
  • Prefer to loop through childNodes rather than using first/lastChild with next/previousSibling. But prefer hasChildNodes() to childNodes.length > 0. Similarly prefer document.getElementsByTagName(aTag).item(0) != null to document.getElementsByTagName(aTag).length > 0.

Review (aka pull-request) policy

NBB: this isn't being enforced yet

  • New code has to pass all tests (FORTHCOMING)
  • New code can't regress performance on (TBD) as measured by (TBD). Unless the new code implements a new feature major enough to suffer a temporary perf regression. This is up to common sense.
  • Major new features should have architectural review from (TBD). Less major patches can be reviewed by (TBD).
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