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Svg Elements Parsing

Project description

svg.elements

SVG Parsing for Elements, Paths, and other SVG Objects.

The Path element of this project is based in part on the regebro/svg.path ( https://github.com/regebro/svg.path ) project. It is also may be based, in part, on some elements of mathandy/svgpathtools ( https://github.com/mathandy/svgpathtools ).

The Zingl-Bresenham plotting algorithms are from Alois Zingl's "The Beauty of Bresenham's Algorithm" ( http://members.chello.at/easyfilter/bresenham.html ). They are all MIT Licensed and this library is also MIT licensed. In the case of Zingl's work this isn't explicit from his website, however from personal correspondence "'Free and open source' means you can do anything with it like the MIT licence."

Goals/Philsophy

The goal of this project is to provide svg spec-like objects and structures. The svg standard 1.1 and elements of 2.0 will be used to provide much of the decisions making for implementation objects. If there is a question on implementation and the SVG documentation has a methodology, that is the preferred methodology.

The primary goal is to make a more robust version of svg.path including other elements like Point and Matrix with clear emphasis on conforming to the SVG spec in all ways that realworld uses for SVG demands.

svg.elements should conform to the SVG Conforming Interpretor class (2.5.4. Conforming SVG Interpreters):

An SVG interpreter is a program which can parse and process SVG document fragments. Examples of SVG interpreters are server-side transcoding tools or optimizers (e.g., a tool which converts SVG content into modified SVG content) or analysis tools (e.g., a tool which extracts the text content from SVG content, or a validity checker).

For real world functionality we must correctly and reasonably provide the ability to do transcoding of svg as well as accessing and modifying content.

This project began as part of meerK40t which does svg loading of files for laser cutting. It attempts to more fully map out the svg spec, objects, and paths, while remaining easy to use and highly backwards compatable.

Elements

The core functionality of this class are the elements. These are svg-based objects which interact work in various reasonable methods.

Path

The base code for this is regebro's code and methods from the svg.path class. The primary methods is to use different PathSegment classes for the various elements within a pathd code. These should always have a high degree of backwards compatability. And for most purposes importing the relevant classes from svg_elements should be highly compatable with any existing code.

There is a Path object that acts as a collection of the path segment objects.

While svg.path objects used complex values for coordinate data. We use Point objects which are backwards compatible with other point objects, including complex numbers. Because of this, there should be a high degree of compatibility between this project and ones that used svg.path. You can use complex numbers as points, and they should seemlessly convert.

Segments

There are 6 path segment objects: Line, Arc, CubicBezier, QuadraticBezier, Move and Close. These have a 1:1 correspondance to the commands in a pathd.

>>> from svg_elements import Path, Line, Arc, CubicBezier, QuadraticBezier, Close

All of these objects have a .point() function which will return the coordinates of a point on the path, where the point is given as a floating point value where 0.0 is the start of the path and 1.0 is end.

You can calculate the length of a Path or its segments with the .length() function. For CubicBezier and Arc segments this is done by geometric approximation and for this reason may be very slow. You can make it faster by passing in an error option to the method. If you don't pass in error, it defaults to 1e-12::

>>> CubicBezier(300+100j, 100+100j, 200+200j, 200+300j).length(error=1e-5)
297.2208145656899

CubicBezier and Arc also has a min_depth option that specifies the minimum recursion depth. This is set to 5 by default, resulting in using a minimum of 32 segments for the calculation. Setting it to 0 is a bad idea for CubicBeziers, as they may become approximated to a straight line.

Line.length() and QuadraticBezier.length() also takes these parameters, but they unneeded as direct values rather than approximations are returned.

CubicBezier and QuadraticBezier also have is_smooth_from(previous) methods, that check if the segment is a "smooth" segment compared to the given segment.

Unlike svg.path the preferred method of getting an Path from a pathd string is as an argument:

>>> from svg_elements import Path
>>> Path('M 100 100 L 300 100')
Path(Move(end=Point(100,100)), Line(start=Point(100,100), end=Point(300,100)))

Segment Classes

These are the SVG PathSegment classes. See the SVG specifications <http://www.w3.org/TR/SVG/paths.html>_ for more information on what each parameter means.

  • Move(start, end) The move object describes a move to the start of the next subpath. It may lack a start position but not en end position.

  • Close(start, end) The close object describes a close path element. It will have a length if and only if the end point is not equal to the subpath start point. Neither the start point or end point is required.

  • Line(start, end) The line object describes a line moving straight from one point to the next point.

  • Arc(start, radius, rotation, arc, sweep, end) The arc object describes an arc across a circular path. This supports multiple types of parameterizations. The given default there is compatable with svg.path and has a complex radius. It is also valid to divide radius into rx and ry or Arc(start, end, center, prx, pry, sweep) where start, end, center, prx, pry are points and sweep is the radians value of the arc distance travelled.

  • QuadraticBezier(start, control, end) the quadraticbezier object describes a single control point bezier curve.

  • CubicBezier(start, control1, control2, end) the cubic bezier curve object describes a two control point bezier curve.


In addition to that, there is the Path class, which is instantiated with a sequence of path segments:

  • Path(*segments)

The Path class is a mutable sequence, so it behaves like a list. You can add to it and replace path segments etc:

>>> path = Path(Line(100+100j,300+100j), Line(100+100j,300+100j))
>>> path.append(QuadraticBezier(300+100j, 200+200j, 200+300j))
>>> path[0] = Line(200+100j,300+100j)
>>> del path[1]

The path object also has a d() method that will return the SVG representation of the Path segments:

>>> path.d()
'M 200,100 L 300,100 Q 200,200 200,300'

In elements this is also the preferred result of str() typically str(path).

Examples

This SVG path example draws a triangle:

>>> path1 = Path('M 100 100 L 300 100 L 200 300 z')

You can format SVG paths in many different ways, all valid paths should be accepted::

>>> path2 = Path('M100,100L300,100L200,300z')

And these paths should be equal:

>>> path1 == path2
True

You can also build a path from objects:

>>> path3 = Path(Move(100 + 100j), Line(100 + 100j, 300 + 100j), Line(300 + 100j, 200 + 300j), Close(200 + 300j, 100 + 100j))

And it should again be equal to the first path::

>>> path1 == path2
True

Paths are mutable sequences, you can slice and append::

>>> path1.append(QuadraticBezier(300+100j, 200+200j, 200+300j))
>>> len(path1[2:]) == 3
True

Note that there is no protection against you creating paths that are invalid. You can for example have a Close command that doesn't end at the path start:

>>> wrong = Path(Line(100+100j,200+100j), Close(200+300j, 0))

Matrix (Transformations)

A large goal of this project is to provide a more robust modifications of Path objects including matrix transformations. This is done by three major shifts from svg.paths methods.

  • Points are not stored as complex numbers. These are stored as Point objects, which have backwards compatability with complex numbers, without the data actually being backed by a complex.
  • A matrix is added which conforms to the SVGMatrix Element. The matrix contains valid versions of all the affline transformations elements required by the SVG Spec.
  • The Arc object is fundamentally backed by a different point-based parameterization.

The objects themselves have robust dunder methods. So if you have a path object you may simply multiply it by a matrix.

>>> Path(Line(0+0j, 100+100j)) * Matrix.scale(2)
>>> Path(Line(start=Point(0.000000000000,0.000000000000), end=Point(200.000000000000,200.000000000000)))

Or rotate a parsed path.

>>> Path("M0,0L100,100") * Matrix.rotate(30)
Path(Move(end=Point(0,0)), Line(start=Point(0,0), end=Point(114.228307398045,-83.378017420528)))

Or modify an svg path.

>>> str(Path("M0,0L100,100") * Matrix.rotate(30))
'M 0,0 L 114.228,-83.378'

Transform Parsing

Within the SVG node schema where objects svg nodes are dictonaries. The transform tags within objects are combined together. This means that if you get a the 'd' object from an end-node in the SVG you can choose to apply the transformations. This list of transformations complies with the SVG spec. They merely aren't applied until requested.

>>> node = { 'd': "M0,0 100,0, 0,100 z", 'transform': "scale(0.5)"}
>>> print(Path(node['d']) * Matrix(node['transform']))
M 0,0 L 50,0 L 0,50 Z

Real Size scaling

There is need in many applications to append a transformation for the viewbox, height, width. So as to prevent a variety of errors where the expected size is far vastly different from the actual size. If we have a viewbox of "0 0 100 100" but the height and width show that to be 50cm wide, then a path "M25,50L75,50" within that viewbox has a real size of length of 25cm which can be quite different from 50 (unitless value).

parse_viewbox_transform performs this operation. It uses the conversion of the width and height to real world units. With a variable setting of ppi or pixels_per_inch. The standard default value for this is 96. Though other values have been used in other places. And this property can be configured.

This can be easily invoked calling the nodes generator on the SVG object. If called with viewport_transform=True it will parse this viewport appending the required transformation to the SVG root object, which will be passed to all the child nodes. If you then apply the transform to the path object it will be scaled to the real size.

The parse_viewbox_transform code conforms to the algorithm given in SVG 1.1 7.2, SVG 2.0 8.2 'equivalent transform of an SVG viewport.' This will also fully implement the preserveAspectRatio, xMidYMid, and meetOrSlice values.

Distance

The conversion of to distances to utilizes another parser element Distance It's a minor element and is a backed by a float. As such you can call Distance.mm(25) and it will convert 25mm to pixels with the default 96 pixels per inch. It provides conversions for mm, cm, in, px, pt, pc. You can also parse an element like the string '25mm' calling Distance.parse('25mm') and get the expected results. You can also call Distance.parse('25mm').as_inch which will return 25mm in inches.

>>> Distance.parse('25mm').as_inch
0.9842524999999999

Color

Color is another important element it is back by int in the form of an ARGB 32-bit integer. It will parse all the SVG color functions.

If we get the fill or stroke of an object from a node be a text element. This needs to be converted to a consistent form. We could have a 3, 4, 6, or 8 digit hex. rgb(r,g,b) value, a static dictionary name or percent rgb(r,g,b). And must be properly parsed according to the spec.

>>> Color.parse("red").hex
'#ff0000'

Angle

Angle is backed by a 'float' and contains all the CSS angle values. 'deg', 'rad', 'grad', 'turn'.

>>> Angle.degrees(360).as_radians
Angle(6.283185307180)

Point

Point is used in all the SVG path segment objects. With regard to svg.path it is not back by, but implements all the same functionality as a complex and will take a complex as an input. So older svg.path code will remain valid. While also allowing for additional functionality like finding a distance.

>>> Point(0+100j).distance_to([0,0])
100.0

The class supports complex subscriptable elements, .x and .y methods, and .imag and .real. As well as providing several of these indexing methods.

Interactions

Most of the usefulness is in the simplicity of the implemented values.

>>> Line((20,20), (40,40)) * Matrix("Rotate(45)")
Line(start=Point(0,28.284271247462), end=Point(0,56.568542494924))

Points, PathSegments, Paths can be multiplied by a matrix.

License

This module is under a MIT License.

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