PDF Theory of Operation

Internally, SkPDFDocument and SkPDFDevice represents PDF documents and pages. This document describes how the backend operates, but these interfaces are not part of the public API and are subject to perpetual change.

See Using Skia’s PDF Backend to find out how to use SkPDF as a client calling Skia’s public API.


Contents

Typical usage of the PDF backend

SkPDFDevice is the main interface to the PDF backend. This child of SkDevice can be set on an SkCanvas and drawn to. Once drawing to the canvas is complete (SkDocument::onEndPage() is called), the device’s content and resouces are added to the SkPDFDocument that owns the device. A new SkPDFDevice should be created for each page or layer desired in the document. After all the pages have been added to the document, SkPDFDocument::onClose() is called to finish serializing the PDF file.

PDF Objects and Document Structure

PDF Logical Document Structure

Background: The PDF file format has a header, a set of objects and then a footer that contains a table of contents for all of the objects in the document (the cross-reference table). The table of contents lists the specific byte position for each object. The objects may have references to other objects and the ASCII size of those references is dependent on the object number assigned to the referenced object; therefore we can’t calculate the table of contents until the size of objects is known, which requires assignment of object numbers. The document uses SkWStream::bytesWritten() to query the offsets of each object and build the cross-reference table.

Furthermore, PDF files can support a linearized mode, where objects are in a specific order so that pdf-viewers can more easily retrieve just the objects they need to display a specific page, i.e. by byte-range requests over the web. Linearization also requires that all objects used or referenced on the first page of the PDF have object numbers before the rest of the objects. Consequently, before generating a linearized PDF, all objects, their sizes, and object references must be known. Skia has no plans to implement linearized PDFs.

%PDF-1.4
…objects...
xref
0 31  % Total number of entries in the table of contents.
0000000000 65535 f
0000210343 00000 n
…
0000117055 00000 n
trailer
<</Size 31 /Root 1 0 R>>
startxref
210399  % Byte offset to the start of the table of contents.
%%EOF

The class SkPDFObjNumMap and the virtual class SkPDFObject are used to manage the needs of the file format. Any object that will represent a PDF object must inherit from SkPDFObject and implement the methods to generate the binary representation and report any other SkPDFObjects used as resources. SkPDFTypes.h defines most of the basic PDF object types: bool, int, scalar, string, name, array, dictionary, and stream. (A stream is a dictionary containing at least a Length entry followed by the data of the stream.)

All of these PDF object types except the stream type can be used in both a direct and an indirect fashion, i.e. an array can have an int or a dictionary as an inline entry, which does not require an object number. The stream type, cannot be inlined and must be referred to with an object reference. Most of the time, other objects types can be referred to with an object reference, but there are specific rules in the PDF specification that requires an inline reference in some place or an indirect reference in other places. All indirect objects must have an object number assigned.

  • bools: true false
  • ints: 42 0 -1
  • scalars: 0.001
  • strings: (strings are in parentheses or byte encoded) <74657374>
  • name: /Name /Name#20with#20spaces
  • array: [/Foo 42 (arrays can contain multiple types)]
  • dictionary: <</Key1 (value1) /key2 42>>
  • indirect object:
    5 0 obj (An indirect string. Indirect objects have an object number and a generation number, Skia always uses generation 0 objects) endobj
  • object reference: 5 0 R
  • stream: <</Length 56>> stream ...stream contents can be arbitrary, including binary... endstream

The PDF backend requires all indirect objects used in a PDF to be added to the SkPDFObjNumMap of the SkPDFDocument. The catalog is responsible for assigning object numbers and generating the table of contents required at the end of PDF files. In some sense, generating a PDF is a three step process. In the first step all the objects and references among them are created (mostly done by SkPDFDevice). In the second step, SkPDFObjNumMap assigns and remembers object numbers. Finally, in the third step, the header is printed, each object is printed, and then the table of contents and trailer are printed. SkPDFDocument takes care of collecting all the objects from the various SkPDFDevice instances, adding them to an SkPDFObjNumMap, iterating through the objects once to set their file positions, and iterating again to generate the final PDF.

As an optimization, many leaf nodes in the direct graph of indirect objects can be assigned object numbers and serialized early.

%PDF-1.4
2 0 obj <<
  /Type /Catalog
  /Pages 1 0 R
>>
endobj
3 0 obj <<
  /Type /Page
  /Parent 1 0 R
  /Resources <>
  /MediaBox [0 0 612 792]
  /Contents 4 0 R
>>
endobj
4 0 obj <> stream
endstream
endobj
1 0 obj <<
  /Type /Pages
  /Kids [3 0 R]
  /Count 1
>>
endobj
xref
0 5
0000000000 65535 f
0000000236 00000 n
0000000009 00000 n
0000000062 00000 n
0000000190 00000 n
trailer
<</Size 5 /Root 2 0 R>>
startxref
299
%%EOF

PDF drawing

Most drawing in PDF is specified by the text of a stream, referred to as a content stream. The syntax of the content stream is different than the syntax of the file format described above and is much closer to PostScript in nature. The commands in the content stream tell the PDF interpreter to draw things, like a rectangle (x y w h re), an image, or text, or to do meta operations like set the drawing color, apply a transform to the drawing coordinates, or clip future drawing operations. The page object that references a content stream has a list of resources that can be used in the content stream using the dictionary name to reference the resources. Resources are things like font objects, images objects, graphic state objects (a set of meta operations like miter limit, line width, etc). Because of a mismatch between Skia and PDF’s support for transparency (which will be explained later), SkPDFDevice records each drawing operation into an internal structure (ContentEntry) and only when the content stream is needed does it flatten that list of structures into the final content stream.

4 0 obj <<
  /Type /Page
  /Resources <<
    /Font <</F1 9 0 R>>
    /XObject <</Image1 22 0 R /Image2 73 0 R>>
  >>
  /Content 5 0 R
>> endobj

5 0 obj <</Length 227>> stream
% In the font specified in object 9 and a height
% of 12 points, at (72, 96) draw ‘Hello World.’
BT
  /F1 12 Tf
  72 96 Td
  (Hello World) Tj
ET
% Draw a filled rectange.
200 96 72 72 re B
...
endstream
endobj

Interned objects

There are a number of high level PDF objects (like fonts, graphic states, etc) that are likely to be referenced multiple times in a single PDF. To ensure that there is only one copy of each object instance these objects an implemented with an interning pattern. As such, the classes representing these objects (like SkPDFGraphicState) have private constructors and static methods to retrieve an instance of the class.

The SkPDFCanon object owns the interned objects. For obvious reasons, the returned instance should not be modified. A mechanism to ensure that interned classes are immutable is needed. See issue 2683.

Graphic States

PDF has a number of parameters that affect how things are drawn. The ones that correspond to drawing options in Skia are: color, alpha, line cap, line join type, line width, miter limit, and xfer/blend mode (see later section for xfer modes). With the exception of color, these can all be specified in a single pdf object, represented by the SkPDFGraphicState class. A simple command in the content stream can then set the drawing parameters to the values specified in that graphic state object. PDF does not allow specifying color in the graphic state object, instead it must be specified directly in the content stream. Similarly the current font and font size are set directly in the content stream.

6 0 obj <<
  /Type /ExtGState
  /CA 1  % Opaque - alpha = 1
  /LC 0  % Butt linecap
  /LJ 0  % Miter line-join
  /LW 2  % Line width of 2
  /ML 6  % Miter limit of 6
  /BM /Normal  % Blend mode is normal i.e. source over
>>
endobj

Clip and Transform

Similar to Skia, PDF allows drawing to be clipped or transformed. However, there are a few caveats that affect the design of the PDF backend. PDF does not support perspective transforms (perspective transform are treated as identity transforms). Clips, however, have more issues to cotend with. PDF clips cannot be directly unapplied or expanded. i.e. once an area has been clipped off, there is no way to draw to it. However, PDF provides a limited depth stack for the PDF graphic state (which includes the drawing parameters mentioned above in the Graphic States section as well as the clip and transform). Therefore to undo a clip, the PDF graphic state must be pushed before the clip is applied, then popped to revert to the state of the graphic state before the clip was applied.

As the canvas makes drawing calls into SkPDFDevice, the active transform, clip region, and clip stack are stored in a ContentEntry structure. Later, when the ContentEntry structures are flattened into a valid PDF content stream, the transforms and clips are compared to decide on an efficient set of operations to transition between the states needed. Currently, a local optimization is used, to figure out the best transition from one state to the next. A global optimization could improve things by more effectively using the graphics state stack provided in the PDF format.

Generating a content stream

For each draw call on an SkPDFDevice, a new ContentEntry is created, which stores the matrix, clip region, and clip stack as well as the paint parameters. Most of the paint parameters are bundled into an SkPDFGraphicState (interned) with the rest (color, font size, etc) explicitly stored in the ContentEntry. After populating the ContentEntry with all the relevant context, it is compared to the the most recently used ContentEntry. If the context matches, then the previous one is appended to instead of using the new one. In either case, with the context populated into the ContentEntry, the appropriate draw call is allowed to append to the content stream snippet in the ContentEntry to affect the core of the drawing call, i.e. drawing a shape, an image, text, etc.

When all drawing is complete, SkPDFDocument::onEndPage() will call SkPDFDevice::content() to request the complete content stream for the page. The first thing done is to apply the initial transform specified in part in the constructor, this transform takes care of changing the coordinate space from an origin in the lower left (PDF default) to the upper left (Skia default) as well as any translation or scaling requested by the user (i.e. to achieve a margin or scale the canvas). Next (well almost next, see the next section), a clip is applied to restrict drawing to the content area (the part of the page inside the margins) of the page. Then, each ContentEntry is applied to the content stream with the help of a helper class, GraphicStackState, which tracks the state of the PDF graphics stack and optimizes the output. For each ContentEntry, commands are emitted to the final content entry to update the clip from its current state to the state specified in the ContentEntry, similarly the Matrix and drawing state (color, line joins, etc) are updated, then the content entry fragment (the actual drawing operation) is appended.

Drawing details

Certain objects have specific properties that need to be dealt with. Images, layers (see below), and fonts assume the standard PDF coordinate system, so we have to undo any flip to the Skia coordinate system before drawing these entities. We don’t currently support inverted paths, so filling an inverted path will give the wrong result (issue 241). PDF doesn’t draw zero length lines that have butt of square caps, so that is emulated.

Layers

PDF has a higher level object called a form x-object (form external object) that is basically a PDF page, with resources and a content stream, but can be transformed and drawn on an existing page. This is used to implement layers. SkPDFDevice has a method, makeFormXObjectFromDevice(), which uses the SkPDFDevice::content() method to construct a form x-object from the the device. SkPDFDevice::drawDevice() works by creating a form x-object of the passed device and then drawing that form x-object in the root device. There are a couple things to be aware of in this process. As noted previously, we have to be aware of any flip to the coordinate system - flipping it an even number of times will lead to the wrong result unless it is corrected for. The SkClipStack passed to drawing commands includes the entire clip stack, including the clipping operations done on the base layer. Since the form x-object will be drawn as a single operation onto the base layer, we can assume that all of those clips are in effect and need not apply them within the layer.

Fonts

There are many details for dealing with fonts, so this document will only talk about some of the more important ones. A couple short details:

  • We can’t assume that an arbitrary font will be available at PDF view time, so we embed all fonts in accordance with modern PDF guidelines.
  • Most fonts these days are TrueType fonts, so this is where most of the effort has been concentrated.
  • Because Skia may only be given a glyph-id encoding of the text to render and there is no perfect way to reverse the encoding, the PDF backend always uses the glyph-id encoding of the text.

Type1/Type3 fonts

Linux supports Type1 fonts, but Windows and Mac seem to lack the functionality required to extract the required information from the font without parsing the font file. When a non TrueType font is used any any platform (except for Type1 on Linux), it is encoded as a Type3 font. In this context, a Type3 font is an array of form x-objects (content streams) that draw each glyph of the font. No hinting or kerning information is included in a Type3 font, just the shape of each glyph. Any font that has the do-not embed copy protection bit set will also get embedded as a Type3 font. From what I understand, shapes are not copyrightable, but programs are, so by stripping all the programmatic information and only embedding the shape of the glyphs we are honoring the do-not embed bit as much as required by law.

PDF only supports an 8-bit encoding for Type1 or Type3 fonts. However, they can contain more than 256 glyphs. The PDF backend handles this by segmenting the glyphs into groups of 255 (glyph id 0 is always the unknown glyph) and presenting the font as multiple fonts, each with up to 255 glyphs.

Font subsetting

Many fonts, especially fonts with CJK support are fairly large, so it is desirable to subset them. Chrome uses the SFNTLY package to provide subsetting support to Skia for TrueType fonts.

Shaders

Skia has two types of predefined shaders, image shaders and gradient shaders. In both cases, shaders are effectively positioned absolutely, so the initial position and bounds of where they are visible is part of the immutable state of the shader object. Each of the Skia’s tile modes needs to be considered and handled explicitly. The image shader we generate will be tiled, so tiling is handled by default. To support mirroring, we draw the image, reversed, on the appropriate axis, or on both axes plus a fourth in the vacant quadrant. For clamp mode, we extract the pixels along the appropriate edge and stretch the single pixel wide/long image to fill the bounds. For both x and y in clamp mode, we fill the corners with a rectangle of the appropriate color. The composed shader is then rotated or scaled as appropriate for the request.

Gradient shaders are handled purely mathematically. First, the matrix is transformed so that specific points in the requested gradient are at pre-defined locations, for example, the linear distance of the gradient is always normalized to one. Then, a type 4 PDF function is created that achieves the desired gradient. A type 4 function is a function defined by a resticted postscript language. The generated functions clamp at the edges so if the desired tiling mode is tile or mirror, we hav to add a bit more postscript code to map any input parameter into the 0-1 range appropriately. The code to generate the postscript code is somewhat obtuse, since it is trying to generate optimized (for space) postscript code, but there is a significant number of comments to explain the intent.

Xfer modes

PDF supports some of the xfer modes used in Skia directly. For those, it is simply a matter of setting the blend mode in the graphic state to the appropriate value (Normal/SrcOver, Multiply, Screen, Overlay, Darken, Lighten, !ColorDOdge, ColorBurn, HardLight, SoftLight, Difference, Exclusion). Aside from the standard SrcOver mode, PDF does not directly support the porter-duff xfer modes though. Most of them (Clear, SrcMode, DstMode, DstOver, SrcIn, DstIn, SrcOut, DstOut) can be emulated by various means, mostly by creating form x-objects out of part of the content and drawing it with a another form x-object as a mask. I have not figured out how to emulate the following modes: SrcATop, DstATop, Xor, Plus.

At the time of writing [2012-06-25], I have a CL outstanding to fix a misunderstanding I had about the meaning of some of the emulated modes. I will describe the system with this change applied.

First, a bit of terminology and definition. When drawing something with an emulated xfer mode, what’s already drawn to the device is called the destination or Dst, and what’s about to be drawn is the source or Src. Src (and Dst) can have regions where it is transparent (alpha equals zero), but it also has an inherent shape. For most kinds of drawn objects, the shape is the same as where alpha is not zero. However, for things like images and layers, the shape is the bounds of the item, not where the alpha is non-zero. For example, a 10x10 image, that is transparent except for a 1x1 dot in the center has a shape that is 10x10. The xfermodes gm test demonstrates the interaction between shape and alpha in combination with the port-duff xfer modes.

The clear xfer mode removes any part of Dst that is within Src’s shape. This is accomplished by bundling the current content of the device (Dst) into a single entity and then drawing that with the inverse of Src’s shape used as a mask (we want Dst where Src isn’t). The implementation of that takes a couple more steps. You may have to refer back to the content stream section. For any draw call, a ContentEntry is created through a method called SkPDFDevice::setUpContentEntry(). This method examines the xfer modes in effect for that drawing operation and if it is an xfer mode that needs emulation, it creates a form x-object from the device, i.e. creates Dst, and stores it away for later use. This also clears all of that existing ContentEntry’s on that device. The drawing operation is then allowed to proceed as normal (in most cases, see note about shape below), but into the now empty device. Then, when the drawing operation in done, a complementary method is called,SkPDFDevice::finishContentEntry(), which takes action if the current xfer mode is emulated. In the case of Clear, it packages what was just drawn into another form x-object, and then uses the Src form x-object, an invert function, and the Dst form x-object to draw Dst with the inverse shape of Src as a mask. This works well when the shape of Src is the same as the opaque part of the drawing, since PDF uses the alpha channel of the mask form x-object to do masking. When shape doesn’t match the alpha channel, additional action is required. The drawing routines where shape and alpha don’t match, set state to indicate the shape (always rectangular), which finishContentEntry uses. The clear xfer mode is a special case; if shape is needed, then Src isn’t used, so there is code to not bother drawing Src if shape is required and the xfer mode is clear.

SrcMode is clear plus Src being drawn afterward. DstMode simply omits drawing Src. DstOver is the same as SrcOver with Src and Dst swapped - this is accomplished by inserting the new ContentEntry at the beginning of the list of ContentEntry’s in setUpContentEntry instead of at the end. SrcIn, SrcOut, DstIn, DstOut are similar to each, the difference being an inverted or non-inverted mask and swapping Src and Dst (or not). SrcIn is SrcMode with Src drawn with Dst as a mask. SrcOut is like SrcMode, but with Src drawn with an inverted Dst as a mask. DstIn is SrcMode with Dst drawn with Src as a mask. Finally, DstOut is SrcMode with Dst draw with an inverted Src as a mask.

Known issues

  • issue 237 SkMaskFilter is not supported.
  • issue 238 SkColorFilter is not supported.
  • issue 249 SrcAtop Xor, and Plus xfer modes are not supported.
  • issue 240 drawVerticies is not implemented.
  • issue 244 Mostly, only TTF fonts are directly supported. (User metrics show that almost all fonts are truetype.)
  • issue 260 Page rotation is accomplished by specifying a different size page instead of including the appropriate rotation annotation.