NAME

SVG::Rasterize - rasterize SVG content to pixel graphics

VERSION

Version 0.003004

SYNOPSIS

    use SVG;
    use SVG::Rasterize;

    my $svg = SVG->new(width => 300, height => 200);
    $svg->line(x1 => 10, y1 => 20, x2 => 220, y2 => 150,
               style => {stroke => 'black', stroke-width => '2pt' });

    # add more svg content
    # .
    # .
    # .

    my $rasterize = SVG::Rasterize->new();
    $rasterize->rasterize(svg => $svg);
    $rasterize->write(type => 'png', file_name => 'out.png');

DESCRIPTION

SVG::Rasterize can be used to rasterize SVG objects to pixel graphics (currently png only) building on the Cairo library (by default, other underlying rasterization engines could be added). The direct rasterization of SVG files might be implemented in the future, right now you should have a look at SVG::Parser which can generate an SVG object from an svg file. See also SVG Input in the ADVANCED TOPICS section.

Motivation

In the past, I have used several programs to rasterize SVG graphics including Inkscape, Konqueror, Adobe Illustrator, and rsvg. While Inkscape was my favourite none of them made me entirely happy. There were always parts of the standard that I would have liked to use, but were unsupported.

So finally, I set out to write my own rasterization engine. The ultimate goal is complete compliance with the requirements for a Conforming Static SVG Viewer as described in the SVG specification: http://www.w3.org/TR/SVG11/conform.html#ConformingSVGViewers. Obviously, this is a long way to go. I do not know if any support for the dynamic features of SVG will ever be added. Anyway, the priority for SVG::Rasterize is accuracy, not speed.

Status

The following elements are drawn at the moment:

path
all basic shapes: rect, circle, ellipse, line, polyline, polygon.
text/tspan in a very limited way: position and stroke/fill colors can be set, but no alignment, no font properties and so on.

The inheritance of styling properties is implemented. The following attributes are at least partly interpreted:

transform
viewBox
preserveAspectRatio
style
fill and all associated properties
stroke and all associated properties

I hope that the interface described here will be largely stable. However, this is not guaranteed. Some features are documented as likely to change, but everything is subject to change at this early stage.

Here is my current view of the next part of the roadmap:

Version 0.004

text basics
clipping paths

Version 0.005

symbol/use
tref and such
gradients and patterns
masks

INTERFACE

Constructors

new

  $rasterize = SVG::Rasterize->new(%args)

Creates a new SVG::Rasterize object and calls init(%args). If you subclass SVG::Rasterize overload init, not new.

init goes through the arguments given to new. If a method of the same name exists it is called with the respective value as argument. This can be used for attribute initialization. Some of the values can be also given to rasterize to temporarily override the attribute values. The values of these overridable attributes are only validated once they are used by rasterize.

The most commonly used arguments are:

svg (optional): A DOM object to render.
width (optional): width (in pixels) of the generated output image
height (optional): height (in pixels) of the generated output image.

Public Attributes

svg

Holds the DOM object to render. It does not have to be a SVG object, but it has to offer certain DOM methods (see SVG Input for details).

width

The width of the generated output in pixels.

height

The height of the generated output in pixels.

current_color

The color which is used if an SVG element's fill or stroke property is set to currentColor and the color property has not been set directly. Setting current_color has the same effect as if the root SVG element's color property was set to the same value. See http://www.w3.org/TR/SVG11/painting.html#SpecifyingPaint for the background of this.

state

Readonly attribute. Holds the current SVG::Rasterize::State object during tree traversal.

There are other attributes that influence unit conversions, white space handling, and the choice of the underlying rasterization engine. See ADVANCED TOPICS.

Methods for Users

rasterize

  $rasterize->rasterize(%args)

Traverses through the given SVG content and renders the output. Does not return anything.

Examples:

  $rasterize->rasterize(svg => $svg);
  $rasterize->rasterize(svg => $svg, width => 640, height => 480);
  $rasterize->rasterize(svg => $svg, engine_class => 'My::Class');

Supported parameters:

svg (optional): DOM object to rasterize. If not specified the value of the svg attribute is used. One of them has to be set. It does not have to be a SVG object, but it has to provide a certain set of DOM methods (see SVG Input.

The element can be any valid SVG element, e.g. <svg>, <g>, or even just a basic shape element or so.
width (optional): width of the target image in pixels, temporarily overrides the width attribute.
height (optional): height of the target image in pixels, temporarily overrides the height attribute.
current_color (optional): default color for stroke and fill properties specified as currentColor, temporarily overrides the current_color attribute.
engine_class (optional): alternative engine class to SVG::Rasterize::Cairo, temporarily overrides the engine_class attribute. See SVG::Rasterize::Cairo for details on the interface. The value has to match the regular p_PACKAGE_NAME.
engine_args (optional): arguments for the constructor of the rasterization engine, temporarily overriding the engine_args attribute (NB: in the future, this behaviour might be changed such that the two hashes are merged and only the values given here override the values in the attribute engine_args; however, at the moment, the whole hash is temporarily replaced if the parameter exists). The width and height of the output image can be set in several ways. The following values for the width are used with decreasing precedence (the same hierarchy applies to the height):

1. engine_args->{width}, given to rasterize

2. $rasterize->engine_args->{width}

3. width, given to rasterize

4. $rasterize->width

5. the width attribute of the root SVG object.
normalize_attributes (optional): Influences White Space Handling, temporarily overrides the normalize_attributes attribute. Defaults to 1.

If width (the same applies to height) is 0 it is treated as not set. If you encounter any scenario where you would wish an explicit size of 0 to be treated in some other way let me know.

If width and/or height are not specified they have to have absolute values in the root SVG element. If both the root SVG element and the rasterize method have width and/or height settings then the rasterize parameters determine the size of the output image and the specified SVG viewport is mapped to this image taking the viewBox and preserveAspectRatio attributes into account if they are present. See http://www.w3.org/TR/SVG11/coords.html#ViewportSpace for details.

The user can influence the rasterization process via hooks. See the Hooks section below.

write

  $rasterize->write(%args)

Writes the rendered image to a file.

Example:

  $rasterize->write(type => 'png', file_name => 'foo.png');

The supported parameters depend on the rasterization backend. The write method hands all parameters over to the backend. See write in SVG::Rasterize::Cairo for an example.

Methods for Developers

init

  $rasterize->init(%args)

If you overload init, your method should also call this one.

For each given argument, init calls the accessor with the same name to initialize the attribute. If such an accessor (or in fact, any method of that name) does not exist a warning is printed and the argument is ignored. Readonly attributes that are allowed to be set at initialization time are set separately at the beginning.

in_error

Expects an exception object or error message. Creates a fresh SVG::Rasterize::State object (without any transfrom etc.) and calls in_error_hook (which by default draws a translucent checkerboard across the image). After that, it dies with the given message.

Before you call in_error directly, check out SVG::Rasterize::Exception.

Class Methods

multiply_matrices

2D affine transformation can be represented by 3 x 3 matrices of the form:

  ( a  c  e )
  ( b  d  f )
  ( 0  0  1 )

In this case, the concatenation of such transformations is represented by canonical matrix multiplication. This method takes two ARRAY references of the form [a, b, c, d, e, f] whose entries correspond to the matrix entries above and returns an ARRAY reference with 6 entries representing the product matrix.

The method can be called either as subroutine or as class method or as object method:

  $product = multiply_matrices($m, $n)
  $product = SVG::Rasterize->multiply_matrices($m, $n)
  $product = $rasterize->multiply_matrices($m, $n)

Note that multiply_matrices does not perform any input check. It expects that you provide (at least) two ARRAY references with (at least) 6 numbers each. If you pass more parameters then the last two are used. If they contain more than 6 entries then the first 6 are used.

endpoint_to_center

  @result = endpoint_to_center(@input)
  @result = SVG::Rasterize->endpoint_to_center(@input)
  @result = $rasterize->endpoint_to_center(@input)

Rasterization engines like SVG::Rasterize::Cairo might use center parameterization instead of endpoint parametrization of an elliptical arc (see http://www.w3.org/TR/SVG11/implnote.html#ArcImplementationNotes). This method calculates the center parameters from the endpoint parameters given in a SVG path data string. As indicated above, it can be called as a subroutine or a class method or an object method. The required parameters are:

x coordinate of the starting point
y coordinate of the starting point
radius in x direction
radius in y direction
angle by which the ellipse is rotated with respect to the positive x axis (in radiant, not degrees)
large arc flag
sweep flag
x coordinate of the end point
y coordinate of the end point.

If the reparameterization cannot be computed an empty list is returned. This can have to possible reasons. Either one of the radii is equal (with respect to machine precision) to 0 or the start and end point of the arc are equal (with respect to machine precision). The first case should have been checked before (note that no rounding problems can occur here because no arithmetics is done with the passed values) because in this case the arc should be turned into a line. In the second case, the arc should just not be drawn. Be aware that this latter case includes a full ellipse. This means that a full ellipse cannot be drawn as one arc. The SVG specification is very clear on that point. However, an ellipse can be drawn as two arcs.

Note that the input values are not validated (e.g. if the values are numbers, if the flags are either 0 or 1 and so on). It is assumed that this has been checked before. Furthermore, it is not checked if the radii are very close to 0 or start and end point are nearly equal.

A list of the following parameters is returned (unless an empty list is returned due to the reasons mentioned above):

x coordinate of the center
y coordinate of the center
radius in x direction

This value might have been increased to make the ellipse big enough to connect start and end point. If it was negative the absolute value has been used (so the return value is always positive).
radius in y direction

This value might have been increased to make the ellipse big enough to connect start and end point. If it was negative the absolute value has been used (so the return value is always positive).
start angle in radiant
sweep angle (positive or negative) in radiant.

adjust_arc_radii

  @result = adjust_arc_radii(@input)
  @result = SVG::Rasterize->adjust_arc_radii(@input)
  @result = $rasterize->adjust_arc_radii(@input)

The SVG specification requires that the radii of an elliptic arc are increased automatically if the given values are too small to connect the given endpoints (see http://www.w3.org/TR/SVG11/implnote.html#ArcImplementationNotes). This situation can arise from rounding errors, but also for example during an animation. Moreover, if a given radius is negative then the absolute value is to be used. This method takes care of these adjustments and returns the new values plus some intermediate values that might be useful for callers, namely endpoint_to_center.

In detail, it requires the following parameters:

x coordinate of the starting point
y coordinate of the starting point
radius in x direction
radius in y direction
angle phi by which the ellipse is rotated with respect to the positive x axis (in radiant)
x coordinate of the end point
y coordinate of the end point.

Note that the input values are not validated (e.g. if the values are numbers etc.). It is assumed that this has been checked before. Furthermore, it is not checked if the radii are very close to 0 or start and end point are nearly equal.

The following values are guaranteed to be returned:

adjusted absolute value of the radius in x direction
adjusted absolute value of the radius in y direction

This is all if one of the radii is equal to 0. Otherwise, the following additional values are returned:

sin(phi)
cos(phi)
x_1' (see SVG specification link above)
y_1' (see SVG specification link above)
1 / Lambda - 1

This value is only returned if Lambda is greater than 0 which is equivalent (assuming exact arithmetics) to the end point of the arc being different to the starting point. Lambda is the value calculated in equation (F.6.6.2) of the specification (see link above). 1 / Lambda - 1 is equal to the radicand in equation (F.6.5.2).

ADVANCED TOPICS

`SVG` Input

In principle, SVG input could be present as a kind of XML tree object or as stringified XML document. Therefore SVG::Rasterize might eventually offer the following options:

1. The input data are provided in form of a SVG object tree generated by the user.
2. The input data are a SVG object tree generated from a file by SVG::Parser or a similar piece of software.
3. The input data are an object tree generated by a generic XML parser and offer a DOM interface.
4. The input data are stringified XML data in a file.
5. The input data are stringified XML data read from a file handle. This case is different from the previous one because a file can be read multiple times in order to get collect referenced SVG fragments.

Currently, the first three options are at least partly implemented. I will not work on the other ones before a substantial subset of SVG is supported. If the last two options will ever get implemented they will be designed to enable the rendering of files which are too large for the first options. Because it is harder to deal with cross-references in these cases, chances are that it will always be faster to use option 2. or 3. if this is possible.

Option 1. is the best tested one by far. However, option 2. should be very similar. To use option 3., the node objects have to provide at least the following DOM methods:

getType
getNodeName
getAttributes
getData
getChildNodes

Unfortunately, option 3. cannot be treated completely in the same way as options 1. and 2. due to the peculiarity of SVG to treat CDATA sections in a special way and not as child nodes of the element. SVG::Rasterize tries to support both SVG object trees and generic DOM trees, but this is neither well tested nor a main priority at the moment. Please report if you find SVG::Rasterize not cooperating with your favourite DOM parser.

Units

SVG supports the absolute units px, pt, pc, cm, mm, in, and the relative units em, ex, and %. Lengths can also be given as numbers without unit which is then interpreted as px. See http://www.w3.org/TR/SVG11/coords.html#Units.

SVG::Rasterize stores default values for unit conversion rates as class variables. You can either change these values or the corresponding object variables. If you have only one SVG::Rasterize object both approaches have the same effect.

The default values are listed below. Except px_per_in, they are taken from the CSS specification. See http://www.w3.org/TR/2008/REC-CSS2-20080411/syndata.html#length-units. The default for px_per_in is arbitrarily set to 90.

Currently, the relative units listed above are not supported by SVG::Rasterize.

Unit conversions:

px_per_in

Pixels per inch. Defaults to 90.
dpi

Alias for px_per_in. This is realized via a typeglob copy:
```
  *dpi = \&px_per_in
```
in_per_cm

Inches per centimeter. Defaults to 1/2.54. This is the internationally defined value. I do not see why I should prohibit a change, but it would hardly make sense.
in_per_mm

Inches per millimeter. Defaults to 1/25.4. This is the internationally defined value. I do not see why I should prohibit a change, but it would hardly make sense.
in_per_pt

Inches per point. Defaults to 1/72. According to [1], this default was introduced by the Postscript language. There are other definitions. However, the CSS specification is quite firm about it.
in_per_pc

Inches per pica. Defaults to 1/6. According to the CSS specification, 12pc equal 1pt.
map_abs_length
```
  $number = $rasterize->map_abs_length($length)
  $number = $rasterize->map_abs_length($number, $unit)
```
This method takes a length and returns the corresponding value in px according to the conversion rates above. Surrounding white space is not allowed.

Examples:
```
  $x = $rasterize->map_abs_length('5.08cm');  # returns 180
  $x = $rasterize->map_abs_length(10);        # returns 10
  $x = $rasterize->map_abs_length(10, 'pt')   # returns 12.5
  $x = $rasterize->map_abs_length('  1in ');  # error
  $x = $rasterize->map_abs_length('50%')      # error
```
The unit has to be absolute, em, ex, and % trigger an exception. See map_length in SVG::Rasterize::State.

There are two different interfaces. You can either pass one string or the number and unit separately. NB: In the second case, the input is not validated. This interface is meant for situations where the length string has already been parsed (namely in map_length in SVG::Rasterize::State) to avoid duplicate validation. The number is expected to be an A_NUMBER and the unit to be a UNIT. However, it is still checked if the unit is absolute.

The corresponding class attributes are listed below. Note that these values are not validated. Take care that you only set them to numbers.

PX_PER_IN

Defaults to 90.
DPI

Alias for PX_PER_IN. This is realized via a typeglob copy
```
  *DPI = \$PX_PER_IN
```
IN_PER_CM

Defaults to 1/2.54.
IN_PER_MM

Defaults to 1/25.4.
IN_PER_PT

Defaults to 1/72.
IN_PER_PC

Defaults to 1/6.

Hooks

The rasterize method traverses through the SVG tree and creates an SVG::Rasterize::State object for each node (node means here element or text node if relevant, attributes are not treated as nodes). Hooks allow you to execute your own subroutines at given steps of this traversal. However, the whole hook business is experimental at the moment and likely to change. Right now, to set your own hooks you can set one of the following attributes to a code reference of your choice.

Currently, there are four hooks:

before_node_hook

Executed at encounter of a new node right before the new SVG::Rasterize::State object is created. It is called as an object method and receives the hash that would be passed to the SVG::Rasterize::State constructor. It contains the elements listed below. A custom before_node_hook is expected to return a hash of the same form which will then be handed over to the SVG::Rasterize::State constructor. The default before_node_hook just returns the input hash.

The argument hash always contains the following elements:
- rasterize: the SVG::Rasterize object
- node: the node object
- node_name: the DOM node name
- node_attributes: the attributes as HASH reference, already normalized
- cdata: string or undef
- child_nodes: ARRAY reference with node objects or undef.
In addition, it may contain the following elements:
- matrix: ARRAY reference with six numbers (see multiply_matrices); this element is present when processing the root node
- parent: the parent SVG::Rasterize::State object; this element is present when not processing the root node.
start_node_hook

Executed right after creation of the SVG::Rasterize::State object. The attributes have been parsed, properties and matrix have been set etc. The method receives the SVG::Rasterize object and the SVG::Rasterize::State object as parameters.
end_node_hook

Executed right before a SVG::Rasterize::State object runs out of scope because the respective node is done with. The method receives the SVG::Rasterize object and the SVG::Rasterize::State object as parameters.
in_error_hook

Executed right before die when the document is in error (see In error below. Receives the SVG::Rasterize object and a newly created SVG::Rasterize::State object as parameters.

Examples:

  $rasterize->start_node_hook(sub { ... })

Some hooks have non-trivial defaults. There SVG::Rasterize provides the following methods to restore the default behaviour:

restore_before_node_hook
restore_start_node_hook
restore_end_node_hook
restore_in_error_hook
restore_all_hooks

Calls all the other restore... methods. Takes an optional named parameter preserve. If this is set to a true value then only hooks are restored which are undefined. This is only documented for completeness, I do not see why you should need it. This option only exists such that the method can be used to initialize the hooks at construction time and preserve hooks that have been set by the user via an init parameter.

Rasterization Backend

SVG::Rasterize does not render pixel graphics itself. By default, it uses the cairo library through its Perl bindings. The interface is documented in SVG::Rasterize::Cairo. However, this interface could also be implemented by other backends. In the future, it will be documented in SVG::Rasterize::Cairo. Currently, it has to be considered unstable, though, and the documentation is sparse.

engine_class

This attribute defaults to SVG::Rasterize::Cairo. It can be set as an object attribute or temporarily as a parameter to the rasterize method.

engine_args

This attribute can hold a HASH reference. The corresponding hash is given to the constructor of the rasterization engine when it is called by rasterize. engine_args can be set as an object attribute or temporarily as a parameter to the rasterize method.

engine

  $rasterize->engine

This attribute holds the interface object to the rasterization backend, by default a SVG::Rasterize::Cairo object. The object is created by the rasterize method.

The attribute is readonly, but, of course, you are able to manipulate the object directly via its methods. However, this is not part of the normal workflow and you do this on your own risk ;-).

White Space Handling

The XML specification (http://www.w3.org/TR/2006/REC-xml11-20060816/#AVNormalize) states that an attribute value unless it is of the type CDATA shall be normalized such that leading and trailing white space is removed and internal white space is flattened to single space characters. XML entities can complicate this normalization, see the specification for details.

If the SVG tree to be rasterized by SVG::Rasterize comes out of an parsed XML document then the parser should have performed this normalization already. However, the tree might also be constructed directly using the SVG module. In order to prevent SVG::Rasterization from choking on an attribute like stroke-width="2pt " it performs by default an additional normalization run:

  $value =~ s/^$WSP*//;
  $value =~ s/$WSP*$//;
  $value =~ s/$WSP+/ /g;

where

  $WSP = qr/[\x{20}\x{9}\x{D}\x{A}]/;  # space, tab, CR, LF

To prevent this normalization, you can set the normalize_attributes attribute (as object attribute or as parameter to rasterize to a false value.

`SVG` Validation

SVG::Rasterize is not an SVG validator. It does check a lot of things including the validity of the element hierarchy, the required presence and absence of attributes and the values of all attributes in interpretes plus some that it does not interprete. However, it does not (and probably will never) claim to detect all errors in an SVG document.

EXAMPLES

There are a few example scripts in the examples directory of the tar ball. However, they rather illustrate the currently supported SVG subset than options of SVG::Rasterize. In order to run the example scripts, you need to have the SVG module installed which is formally only required for testing.

DIAGNOSTICS

Error processing

The SVG documentation specifies how SVG interpreters should react to certain incidents. The relevant section can be found here: http://www.w3.org/TR/SVG11/implnote.html#ErrorProcessing.

This section describes how some of these instructions are implemented by SVG::Rasterize and how it reacts in some other situations in which the specification does not give instructions.

In error

According to the SVG specification (see http://www.w3.org/TR/SVG11/implnote.html#ErrorProcessing, a document is "in error" if:

"the content does not conform to the XML 1.0 specification, such as the use of incorrect XML syntax"

SVG::Rasterize currently does not parse SVG files and will therefore not detect such an error.
"an element or attribute is encountered in the document which is not part of the SVG DTD and which is not properly identified as being part of another namespace"

Currently, SVG::Rasterize will also reject elements that are properly identified as being part of another namespace.
"an element has an attribute or property value which is not permissible according to this specification"

This is checked for those attributes and properties that are currently supported by SVG::Rasterize. Values that are currently ignored may or may not be checked.
"Other situations that are described as being in error in this specification"

In these cases, the rendering is supposed to stop before the incriminated element. Exceptions are path, polyline, and polygon elements which are supposed to be partially rendered up to the point where the error occurs.

Furthermore, a "highly perceivable indication of error shall occur. For visual rendering situations, an example of an indication of error would be to render a translucent colored pattern such as a checkerboard on top of the area where the SVG content is rendered."

In SVG::Rasterize this is done by the in_error_hook. By default, it indeed draws a translucent (rgb(45, 45, 45) with opacity 0.6) checkerboard with 8 fields along the width or height (whichever is shorter). This behaviour can be changed by setting the in_error_hook. Setting the hook to undef or sub {} will disable the process.

`SVG::Rasterize` exceptions

When SVG::Rasterize encounters a problem it usually throws an exception. The cases where only a warning is issued a rare. This behaviour has several reasons:

If the document is in error (see section above) the SVG specification requires that the rendering stops.
In case of failed parameter validation, Params::Validate expects the code execution to stop. One could work around this in an onerous and fragile way, but I will not do this.
Often there is no good fallback without knowing what the user inteded to do. In these cases, it is better to just bail out and let the user fix the problem himself.
A too forgiving user agent deludes the user into bad behaviour. I think that if the rules are clear, a program should enforce them rather strictly.

The exceptions are thrown in form of objects. See Exception::Class for a detailed description. See below for a description of the classes used in this distribution. All error messages are described in SVG::Rasterize::Exception.

Invalid and numerically unstable values

There are situations where certain values cannot be dealt with, e.g. denominators of 0 or negative radicands. Examples are skews of 90 degrees or elliptical arcs where one radius is 0. In these situations, SVG::Rasterize checks for these cases and acts accordingly. Great care is taken to check directly those values which are used as denominator, radicand etc. and not some mathematically equivalent expression which might evaluate to a slightly different value due to rounding errors. However, it is not checked if such an expression is very close to a critical value which might render the processing numerically unstable. I do not want to impose a certain notion of "too close" on SVG authors. Instead it is left to them to check for these border cases. However, the underlying rasterization engine might still impose boundaries.

Exceptions

SVG::Rasterize currently uses the following exception classes. This framework is experimental and might change considerably in future versions. See Exception::Class on how you can make use of this framework. See SVG::Rasterize::Exception for a detailed list of error messages.

SVG::Rasterize::Exception::Base

Base class for the others. Defines the state attribute which holds the current SVG::Rasterize::State object at the time the exception is thrown.
SVG::Rasterize::Exception::InError

The processing encountered an error in the SVG content.
SVG::Rasterize::Exception::Setting

The exception was triggered by an error during the general preparation of the processing, e.g. an error during initialization of the rasterization backend.
SVG::Rasterize::Exception::Parse

An error occured during parsing (usually of an attribute value). An exception of this class always indicates an inconsistency between validation and parsing of this value and should be reported as a bug.
SVG::Rasterize::Exception::Unsupported

The document (or user) tried to use a feature that is currently unsupported.
SVG::Rasterize::Exception::Attribute

Attribute means class attribute here, not SVG attribute. An example for such an exception is the attempt to change a readonly attribute.
SVG::Rasterize::Exception::ParamsValidate

A method parameter did not pass a Params::Validate check.
SVG::Rasterize::Exception::Param

A method parameter passed the Params::Validate check, but is still invalid (an example is that the Params::Validate check only included that the value must be a number, but it also has to be in a certain range which is checked individually later).
SVG::Rasterize::Exception::Return

A method returned an invalid value (example: the before_node_hook did not return a hash).

Warnings

"Unrecognized init parameter %s."

You have given a parameter to the new method which does not have a corresponding method. The parameter is ignored in that case.
"Unable to load %s: %s. Falling back to SVG::Rasterize::Cairo."

The engine_class you were trying to use for rasterization could not be loaded. SVG::Rasterize then tries to use its default backend SVG::Rasterize::Cairo. If that also fails, it gives up.
"Surface width is 0, nothing to do."

The width of output image evaluates to 0. This value is rounded to an integer number of pixels, therefore this warning does not mean that you have provided an explicit number of 0 (it could also have been e.g. 0.005in at a resolution of 90dpi). In this case, nothing is drawn.
"Surface height is 0, nothing to do."

Like above.

DEPENDENCIES

Class::Accessor, version 0.30 or higher
SVG, version 2.37 or higher
Cairo, version 1.061 or higher

The version of the underlying C library has to be at least 1.8.8. This is not automatically fulfilled by installing a sufficiently high version of the Perl module because the release cycles are completely decoupled. Regarding the functionality that is used directly, version 1.2 might actually be sufficient, but version 1.8 was the smallest I got pango (see below) compiled with.

With respect to the module code, the dependency on Cairo is not strict. The code only requires Cairo in case no other rasterization engine is specified (see documentation for details). However, if you do not provide a different backend, which would probably at least require a wrapper written by you, then you cannot do anything without Cairo. Therefore I have included it as a strict dependency. You could take it out of the Makefile.PL if you know what you are doing. However, the distribution will not pass the test suite without Cairo.
Pango, version 1.220 or higher

The version of the underlying C library has to be at least 1.22.4. This is not automatically fulfilled by installing a sufficiently high version of the Perl module because the release cycles are completely decoupled.

The rest of what has been said about Cairo above is also true for Pango. Both are loaded by SVG::Rasterize::Cairo and that is only loaded if no other backend has been specified.
Params::Validate, version 0.91 or higher
Scalar::Util, version 1.19 or higher
Exception::Class, version 1.29 or higher
Test::More, version 0.86 or higher
Test::Exception, version 0.27 or higher

BUGS AND LIMITATIONS

Bugs

At time of release, there are no open bugs. Please report any bugs or feature requests to bug-svg-rasterize at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=SVG-Rasterize. I will be notified, and then you will automatically be notified of progress on your bug as I make changes.

Limitations

Relative units

The relative units em, ex, and % are currently not supported.

`ICC` colors

ICC color settings for the fill and stroke properties are understood, but ignored. I do not know enough about color profiles to foresee how support would look like. Unless requested, ICC color profiles will probably not be supported for a long time.

Caveats

`eval BLOCK` and `$SIG{DIE}`

Several methods in this distribution use eval BLOCK statements without setting a local $SIG{__DIE__}. Therefore, a $SIG{__DIE__} installed somewhere else can be triggered by these statements. See die and eval in perlfunc and $^S in perlvar.

Thread safety

I do not know much about threads and how to make a module thread safe. No specific measures have been taken to achieve thread safety of this distribution.

IMPLEMENTATION NOTES

This documentation is largely for myself. Read on if you are interested, but this section generally does not contain documentation on the usage of SVG::Rasterize.

Deferred Rasterization

Some elements (namely text and textPath elements) can only be rasterized once their entire content is known (e.g. for alignment issues). In these situations, the SVG::Rasterize::State objects representing the deferred nodes are kept (by preventing their destruction, see child_states in SVG::Rasterize::State). The content is then only rasterized once the root element of this subtree is about to run out of scope.

INTERNALS

Regular Expressions

All reused regular expressions are located in SVG::Rasterize::Regexes. In general, they should be considered as private variables and are documented there for inspection only. Anyway, most of them are compiled into other expressions, so changing them would probably not achieve what you might expect. The expressions listed here are exceptions to this rule. They are considered part of the interface and you can change them (at your own risk ;-) ).

$package_part (internal)
```
  qr/[a-zA-Z][a-zA-Z0-9\_]*/
```
$SVG::Rasterize::Regexes::RE_PACKAGE{p_PACKAGE_NAME}
```
  qr/^$package_part(?:\:\:$package_part)*$/
```
Package names given to methods in this distribution, namely the engine_class parameters have to match this regular expression. I am not sure which package names exactly are allowed. If you know where in the Perl manpages or the Camel book this is described, please point me to it. If this pattern is too strict for your favourite package name, you can change this variable.

Internal Methods

These methods are just documented for myself. You can read on to satisfy your voyeuristic desires, but be aware of that they might change or vanish without notice in a future version.

_create_engine

Expects a HASH reference as parameter. No validation is performed. The entries width, height, and engine_class are used and expected to be valid if present.
_initial_viewport

Expects two HASH references. The first one is expected to be defined and a HASH reference, but the content can be arbitrary. The second is expected to be validated. Does not return anything.
_traverse_object_tree

Called by rasterize. Expects a hash with the rasterization parameters after all substitutions and hierarchies of defaults have been applied. Handles the traversal of an SVG or generic DOM object tree for rasterization.
_process_node_object

Called by _traverse_object_tree. Expects a node object and a hash with the rasterization parameters. Performs
- extraction of the node name
  
  This uses the getNodeName DOM method. Takes whatever this method returns.
- extraction and normalization of attributes
  
  This uses the getAttributes DOM method. The return value is validated as being either undef or a HASH reference. The result is further processed by _process_normalize_attributes. The final result is guaranteed to be a HASH reference.
- extraction of child nodes
  
  This uses the getChildNodes DOM method. The return value is validated as being either undef or an ARRAY reference. A copy of the array is made to enable addition or removal of child nodes (by hooks) without affecting the node object.
- transformation of SVG::Element character data into a SVG::Rasterize::TextNode object (see there for background information).
Returns a list of the following values. The result is not further validated than listed below.
- the node name, whatever getNodeName on the object returned
- a HASH reference with the (potentially normalized) attributes
- an ARRAY reference or undef with the list of child node objects (as returned by getChildNodes).
_process_normalize_attributes

Expects a flag (to indicate if normalization is to be performed) and a HASH reference. The second parameter can be false, but if it is true it is expected (without validation) to be a HASH reference. Makes a copy of the hash and returns it after removing (if the flag is true) enclosing white space from each value.

Independently of the flag, it processes the style attribute. If this is a HASH reference it is turned into a string. This means double work, because it is split into a hash again later by State, but it is a design decision that State should not see if the input data came as an object tree or XML string. So this has to be done, and this seemed to be a good place although this method was not started for womething like that (maybe it should be renamed).
_rasterize_deferred_subtree

Called (possibly indirectly) by rasterize in order to rasterize a deferred subtree (see Deferred Rasterization).

Expects an SVG::Rasterize::State object which represents the root of the deferred subtree. Returns nothing.
_process_path

Expects a SVG::Rasterize::State object. Expects that $state->node_attributes have been validated. The d attribute is handed over to _split_path_data which returns a list of instructions to render the path. This is then handed over to the rasterization backend (which has its own expectations).
_angle
```
  $angle = _angle($x1, $y1, $x2, $y2)
  $angle = SVG::Rasterize->_angle($x1, $y1, $x2, $y2)
  $angle = $rasterize->_angle($x1, $y1, $x2, $y2)
```
Expects two vectors and returns the angle between them in rad. No parameter validation is performed. If one of the vectors is 0 (and only if it is exactly 0), undef is returned.
_split_path_data

Expects a path data string. This is expected (without validation) to be defined. Everything else is checked within the method.

Returns a list. The first entry is either 1 or 0 indicating if an error has occured (i.e. if the string is not fully valid). The rest is a list of ARRAY references containing the instructions to draw the path.
_process_rect

Expects a SVG::Rasterize::State object. Expects that $state->node_attributes have been validated.

The rest is handed over to the rasterization backend (which has its own expectations).
_process_circle

Same es _process_rect.
_process_ellipse

Same es _process_rect.
_process_line

Same es _process_rect.
_process_polyline

Same es _process_path.
_process_polygon

Same es _process_path.
_process_text

Expects a SVG::Rasterize::State object. Expects that $state->node_attributes have been validated. Establishes the initial current text position.
_process_tspan

Expects a SVG::Rasterize::State object. Expects that $state->node_attributes have been validated. Currently does not do anything. In the future, it will establish a new current text position if necessary.
_process_cdata

Expects a SVG::Rasterize::State object. Gets the character data and the current text position from the State object and hands them over to the rasterization backend. Receives the new current text position and updates it in the State object.
make_ro_accessor

This piece of documentation is mainly here to make the POD coverage test happy. SVG::Rasterize overloads make_ro_accessor to make the readonly accessors throw an exception object (of class SVG::Rasterize::Exception::Attribute) instead of just croaking.

FOOTNOTES

[1] Yannis Haralambous: Fonts & Encodings. O'Reilly, 2007.

Tons of information about what the author calls the "digital space for writing".

ACKNOWLEDGEMENTS

This distribution builds heavily on the cairo library and its Perl bindings.

AUTHOR

Lutz Gehlen, <perl at lutzgehlen.de>

LICENSE AND COPYRIGHT

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.

To install SVG::Rasterize, copy and paste the appropriate command in to your terminal.

cpanm

cpanm SVG::Rasterize

CPAN shell

perl -MCPAN -e shell
install SVG::Rasterize

For more information on module installation, please visit the detailed CPAN module installation guide.

	Global
`s`	Focus search bar
`?`	Bring up this help dialog

	GitHub
`g` `p`	Go to pull requests
`g` `i`	go to github issues (only if github is preferred repository)

	POD
`g` `a`	Go to author
`g` `c`	Go to changes
`g` `i`	Go to issues
`g` `d`	Go to dist
`g` `r`	Go to repository/SCM
`g` `s`	Go to source
`g` `b`	Go to file browse

	Search terms
module: (e.g. module:Plugin)
distribution: (e.g. distribution:Dancer auth)
author: (e.g. author:SONGMU Redis)
version: (e.g. version:1.00)