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For all media, theSVG canvasdescribes "the space where the SVG content is rendered." The canvas is infinite for each dimension of the space, but rendering occurs relative to a finite rectangular region of the canvas. This finite rectangular region is called theSVG viewport. For visual media ([CSS2], section 7.3.1) the SVG viewport is the viewing area where the user sees the SVG content.
The size of the SVG viewport (i.e., its width and height) is determined by a negotiation process (see Establishing the size of the initial viewport) between the SVG document fragment and its parent (real or implicit). Once that negotiation process is completed, the SVG user agent is provided the following information:
Using the above information, the SVG user agent determines theviewport, an initialviewport coordinate systemand an initialuser coordinate systemsuch that the two coordinates systems are identical. Both coordinates systems are established such that the origin matches the origin of the viewport (for the root viewport, the viewport origin is at the top/left corner), and one unit in the initial coordinate system equals one "pixel" in the viewport. (See Initial coordinate system.) The viewport coordinate system is also calledviewport spaceand the user coordinate system is also calleduser space.
Lengths in SVG can be specified as:
The supported length unit identifiers are: em, ex, px, pt, pc, cm, mm, in, and percentages.
A new user space (i.e., a new current coordinate system) can be established at any place within an SVG document fragment by specifyingtransformationsin the form oftransformation matricesor simple transformation operations such as rotation, skewing, scaling and translation. Establishing new user spaces via coordinate system transformations are fundamental operations to 2D graphics and represent the usual method of controlling the size, position, rotation and skew of graphic objects.
New viewports also can be established. By establishing a new viewport, you can redefine the meaning of percentages units and provide a new reference rectangle for "fitting" a graphic into a particular rectangular area. ("Fit" means that a given graphic is transformed in such a way that its bounding box in user space aligns exactly with the edges of a given viewport.)
The SVG user agent negotiates with its parent user agent to determine the viewport into which the SVG user agent can render the document. In some circumstances, SVG content will be embedded (by reference or inline) within a containing document. This containing document might include attributes, properties and/or other parameters (explicit or implicit) which specify or provide hints about the dimensions of the viewport for the SVG content. SVG content itself optionally can provide information about the appropriate viewport region for the content via the‘width’and‘height’XML attributes on theoutermost svg element. The negotiation process uses any information provided by the containing document and the SVG content itself to choose the viewport location and size.
The‘width’attribute on theoutermost svg elementestablishes the viewport's width, unless the following conditions are met:
Under these conditions, the positioning properties establish the viewport's width.
Similarly, if there are positioning properties specified on the referencing element or on theoutermost svg elementthat are sufficient to establish the height of the viewport, then these positioning properties establish the viewport's height; otherwise, the‘height’attribute on theoutermost svg elementestablishes the viewport's height.
If the‘width’or‘height’attributes on theoutermost svg elementare in user units (i.e., no unit identifier has been provided), then the value is assumed to be equivalent to the same number of "px" units (see Units).
In the following example, an SVG graphic is embedded inline within a parent XML document which is formatted using CSS layout rules. Since CSS positioning properties are not provided on theoutermost svg element, thewidth="100px"andheight="200px"attributes determine the size of the initial viewport:
The initial clipping path for the SVG document fragment is established according to the rules described in The initial clipping path.
For theoutermost svg element, the SVG user agent determines an initialviewport coordinate systemand an initialuser coordinate systemsuch that the two coordinates systems are identical. The origin of both coordinate systems is at the origin of the viewport, and one unit in the initial coordinate system equals one "pixel" (i.e., apxunit as defined in CSS2 ([CSS2], section 4.3.2) in the viewport. In most cases, such as stand-alone SVG documents or SVG document fragments embedded (by reference or inline) within XML parent documents where the parent's layout is determined by CSS [CSS2] or XSL [XSL], the initial viewport coordinate system (and therefore the initial user coordinate system) has its origin at the top/left of the viewport, with the positive x-axis pointing towards the right, the positive y-axis pointing down, and text rendered with an "upright" orientation, which means glyphs are oriented such that Roman characters and full-size ideographic characters for Asian scripts have the top edge of the corresponding glyphs oriented upwards and the right edge of the corresponding glyphs oriented to the right.
If the SVG implementation is part of a user agent which supports styling XML documents using CSS2 compatiblepxunits, then the SVG user agent should get its initial value for the size of apxunit in real world units to match the value used for other XML styling operations; otherwise, if the user agent can determine the size of apxunit from its environment, it should use that value; otherwise, it should choose an appropriate size for onepxunit. In all cases, the size of apxmust be in conformance with the rules described in CSS2 ([CSS2], section 4.3.2).
Example InitialCoordsbelow shows that the initial coordinate system has the origin at the top/left with the x-axis pointing to the right and the y-axis pointing down. The initial user coordinate system has one user unit equal to the parent (implicit or explicit) user agent's "pixel".
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A new user space (i.e., a new current coordinate system) can be established by specifyingtransformationsin the form of a‘transform’attribute on a container element or graphics element or a‘viewBox’attribute on an‘svg’,‘symbol’,‘marker’,‘pattern’and the‘view’element. The‘transform’and‘viewBox’attributes transform user space coordinates and lengths on sibling attributes on the given element (see effect of the‘transform’attribute on sibling attributes and effect of the‘viewBox’attribute on sibling attributes) and all of its descendants. Transformations can be nested, in which case the effect of the transformations are cumulative.
Example OrigCoordSysbelow shows a document without transformations. The text string is specified in the initial coordinate system.
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Example NewCoordSysestablishes a new user coordinate system by specifyingtransform="translate(50,50)"on the third‘g’element below. The new user coordinate system has its origin at location (50,50) in the original coordinate system. The result of this transformation is that the coordinate (30,30) in the new user coordinate system gets mapped to coordinate (80,80) in the original coordinate system (i.e., the coordinates have been translated by 50 units in X and 50 units in Y).
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Example RotateScaleillustrates simplerotateandscaletransformations. The example defines two new coordinate systems:
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Example Skewdefines two coordinate systems which areskewedrelative to the origin coordinate system.
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Mathematically, all transformations can be represented as 3x3transformation matricesof the following form:
Since only six values are used in the above 3x3 matrix, a transformation matrix is also expressed as a vector:[a b c d e f].
Transformations map coordinates and lengths from a new coordinate system into a previous coordinate system:
Simple transformations are represented in matrix form as follows:
Transformations can be nested to any level. The effect of nested transformations is to post-multiply (i.e., concatenate) the subsequent transformation matrices onto previously defined transformations:
For each given element, the accumulation of all transformations that have been defined on the given element and all of its ancestors up to and including the element that established the current viewport (usually, the‘svg’element which is the most immediate ancestor to the given element) is called thecurrent transformation matrixorCTM. The CTM thus represents the mapping of current user coordinates to viewport coordinates:
Example Nestedillustrates nested transformations.
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In the example above, the CTM within the third nested transformation (i.e., thetransform="translate(130,160)") consists of the concatenation of the three transformations, as follows:
The value of the‘transform’attribute is a
All numeric values are
If a list of transforms is provided, then the net effect is as if each transform had been specified separately in the order provided. For example,
is functionally equivalent to:
The‘transform’attribute is applied to an element before processing any other coordinate or length values supplied for that element. In the element
the x, y, width and height values are processed after the current coordinate system has been scaled uniformly by a factor of 2 by the‘transform’attribute. Attributes x, y, width and height (and any other attributes or properties) are treated as values in the new user coordinate system, not the previous user coordinate system. Thus, the above‘rect’element is functionally equivalent to:
The following is the Backus-Naur Form (BNF) for values for the‘transform’attribute. The following notation is used:
transform-list: wsp* transforms? wsp* transforms: transform | transform comma-wsp+ transforms transform: matrix | translate | scale | rotate | skewX | skewY matrix: "matrix" wsp* "(" wsp* number comma-wsp number comma-wsp number comma-wsp number comma-wsp number comma-wsp number wsp* ")" translate: "translate" wsp* "(" wsp* number ( comma-wsp number )? wsp* ")" scale: "scale" wsp* "(" wsp* number ( comma-wsp number )? wsp* ")" rotate: "rotate" wsp* "(" wsp* number ( comma-wsp number comma-wsp number )? wsp* ")" skewX: "skewX" wsp* "(" wsp* number wsp* ")" skewY: "skewY" wsp* "(" wsp* number wsp* ")" number: sign? integer-constant | sign? floating-point-constant comma-wsp: (wsp+ comma? wsp*) | (comma wsp*) comma: "," integer-constant: digit-sequence floating-point-constant: fractional-constant exponent? | digit-sequence exponent fractional-constant: digit-sequence? "." digit-sequence | digit-sequence "." exponent: ( "e" | "E" ) sign? digit-sequence sign: "+" | "-" digit-sequence: digit | digit digit-sequence digit: "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" wsp: (#x20 | #x9 | #xD | #xA)
For the‘transform’attribute:
Animatable: yes.
See the‘animateTransform’element for information on animating transformations.
It is often desirable to specify that a given set of graphics stretch to fit a particular container element. The‘viewBox’attribute provides this capability.
All elements that establish a new viewport (see elements that establish viewports), plus the‘marker’,‘pattern’and‘view’elements have attribute‘viewBox’. The value of the‘viewBox’attribute is a list of four numbers
A negative value for
Example ViewBoxillustrates the use of the‘viewBox’attribute on theoutermost svg elementto specify that the SVG content should stretch to fit bounds of the viewport.
| Rendered into viewport with width=300px, height=200px |
Rendered into viewport with width=150px, height=200px |
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The effect of the‘viewBox’attribute is that the user agent automatically supplies the appropriate transformation matrix to map the specified rectangle in user space to the bounds of a designated region (often, the viewport). To achieve the effect of the example on the left, with viewport dimensions of 300 by 200 pixels, the user agent needs to automatically insert a transformation which scales both X and Y by 0.2. The effect is equivalent to having a viewport of size 300px by 200px and the following supplemental transformation in the document, as follows:
To achieve the effect of the example on the right, with viewport dimensions of 150 by 200 pixels, the user agent needs to automatically insert a transformation which scales X by 0.1 and Y by 0.2. The effect is equivalent to having a viewport of size 150px by 200px and the following supplemental transformation in the document, as follows:
(Note: in some cases the user agent will need to supply atranslatetransformation in addition to ascaletransformation. For example, on anoutermost svg element, atranslatetransformation will be needed if the‘viewBox’attributes specifies values other than zero for
Unlike the‘transform’attribute (see effect of the‘transform’on sibling attributes), the automatic transformation that is created due to a‘viewBox’does not affect the‘x’,‘y’,‘width’and‘height’attributes (or in the case of the‘marker’element, the‘markerWidth’and‘markerHeight’attributes) on the element with the‘viewBox’attribute. Thus, in the example above which shows an‘svg’element which has attributes‘width’,‘height’and‘viewBox’, the‘width’and‘height’attributes represent values in the coordinate system that existsbeforethe‘viewBox’transformation is applied. On the other hand, like the‘transform’attribute, it does establish a new coordinate system for all other attributes and for descendant elements.
For the‘viewBox’attribute:
Animatable: yes.
In some cases, typically when using the‘viewBox’attribute, it is desirable that the graphics stretch to fit non-uniformly to take up the entire viewport. In other cases, it is desirable that uniform scaling be used for the purposes of preserving the aspect ratio of the graphics.
AttributepreserveAspectRatio="[defer]
For elements that establish a new viewport (see elements that establish viewports), plus the‘marker’,‘pattern’and‘view’elements,‘preserveAspectRatio’only applies when a value has been provided for‘viewBox’on the same element. For these elements, if attribute‘viewBox’is not provided, then‘preserveAspectRatio’is ignored.
For‘image’elements,‘preserveAspectRatio’indicates how referenced images should be fitted with respect to the reference rectangle and whether the aspect ratio of the referenced image should be preserved with respect to the current user coordinate system.
If the value of‘preserveAspectRatio’on an‘image’element starts with'defer'then the value of the‘preserveAspectRatio’attribute on the referenced content if present should be used. If the referenced content lacks a value for‘preserveAspectRatio’then the‘preserveAspectRatio’attribute should be processed as normal (ignoring'defer'). For‘preserveAspectRatio’on all other elements the'defer'portion of the attribute is ignored.
The
The
Example PreserveAspectRatioillustrates the various options on‘preserveAspectRatio’. To save space, XML entities have been defined for the three repeated graphic objects, the rectangle with the smile inside and the outlines of the two rectangles which have the same dimensions as the target viewports. The example creates several new viewports by including‘svg’sub-elements embedded inside theoutermost svg element(see Establishing a new viewport).
"> "> "> ]>
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For the‘preserveAspectRatio’attribute:
Animatable: yes.
At any point in an SVG drawing, you can establish a new viewport into which all contained graphics is drawn by including an‘svg’element inside SVG content. By establishing a new viewport, you also implicitly establish a new viewport coordinate system, a new user coordinate system, and, potentially, a new clipping path (see the definition of the‘overflow’property). Additionally, there is a new meaning for percentage units defined to be relative to the current viewport since a new viewport has been established (see Units).
The bounds of the new viewport are defined by the‘x’,‘y’,‘width’and‘height’attributes on the element establishing the new viewport, such as an‘svg’element. Both the new viewport coordinate system and the new user coordinate system have their origins at (‘x’,‘y’), where‘x’and‘y’represent the value of the corresponding attributes on the element establishing the viewport. The orientation of the new viewport coordinate system and the new user coordinate system correspond to the orientation of the current user coordinate system for the element establishing the viewport. A single unit in the new viewport coordinate system and the new user coordinate system are the same size as a single unit in the current user coordinate system for the element establishing the viewport.
Here is an example:
For an extensive example of creating new viewports, see Example PreserveAspectRatio.
The following elements establish new viewports:
Whether a new viewport also establishes a new additional clipping path is determined by the value of the‘overflow’property on the element that establishes the new viewport. If a clipping path is created to correspond to the new viewport, the clipping path's geometry is determined by the value of the‘clip’property. Also, see Clip to viewport vs. clip to‘viewBox’.
All coordinates and lengths in SVG can be specified with or without aunit identifier.
When a coordinate or length value is a number without a unit identifier (e.g., "25"), then the given coordinate or length is assumed to be in user units (i.e., a value in the current user coordinate system). For example:
Text size is 50 user units
Alternatively, a coordinate or length value can be expressed as a number followed by a unit identifier (e.g., "25cm" or "15em"). (Note that CSS definedpropertiesused in a CSS style sheet or the‘style’attribute require units for non-zero lengths, see SVG's styling properties.) The list of unit identifiers in SVG matches the list of unit identifiers in CSS: em, ex, px, pt, pc, cm, mm and in. The
As in CSS, theemandexunit identifiers are relative to the current font'sfont-sizeandx-height, respectively.
Onepxunit is defined to be equal to one user unit. Thus, a length of "5px" is the same as a length of "5".
Note that at initialization, a user unit in the the initial coordinate system is equivalenced to the parent environment's notion of apxunit. Thus, in the the initial coordinate system, because the user coordinate system aligns exactly with the parent's coordinate system, and because often the parent's coordinate system aligns with the device pixel grid, "5px" might actually map to 5 devices pixels. However, if there are any coordinate system transformation due to the use of‘transform’or‘viewBox’attributes, because "5px" maps to 5 user units and because the coordinate system transformations have resulted in a revised user coordinate system, "5px" likely will not map to 5 device pixels. As a result, in most circumstances, "px" units will not map to the device pixel grid.
The other absolute unit identifiers from CSS (i.e., pt, pc, cm, mm, in) are all defined as an appropriate multiple of onepxunit (which, according to the previous item, is defined to be equal to one user unit), based on what the SVG user agent determines is the size of apxunit (possibly passed from the parent processor or environment at initialization time). For example, suppose that the user agent can determine from its environment that "1px" corresponds to "0.2822222mm" (i.e., 90dpi). Then, for all processing of SVG content:
Note that use ofpxunits or any other absolute unit identifiers can cause inconsistent visual results on different viewing environments since the size of "1px" may map to a different number of user units on different systems; thus, absolute units identifiers are only recommended for the‘width’and the‘height’on and situations where the content contains no transformations and it is desirable to specify values relative to the device pixel grid or to a particular real world unit size.
For percentage values that are defined to be relative to the size of viewport:
sqrt((actual-width)**2 + (actual-height)**2))/sqrt(2).Example Unitsbelow illustrates some of the processing rules for different types of units.
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The three rectangles on the left demonstrate the use of one of the absolute unit identifiers, the "in" unit (inch). The reference image above was generated on a 96dpi system (i.e., 1 inch = 96 pixels). Therefore, the topmost rectangle, which is specified in inches, is exactly the same size as the middle rectangle, which is specified in user units such that there are 96 user units for each corresponding inch in the topmost rectangle. (Note: on systems with different screen resolutions, the top and middle rectangles will likely be rendered at different sizes.) The bottom rectangle of the group illustrates what happens when values specified in inches are scaled.
The three rectangles in the middle demonstrate the use of one of the relative unit identifiers, the "em" unit. Because the‘font-size’property has been set to150on the outermost‘g’element, each "em" unit is equal to 150 user units. The topmost rectangle, which is specified in "em" units, is exactly the same size as the middle rectangle, which is specified in user units such that there are 150 user units for each corresponding "em" unit in the topmost rectangle. The bottom rectangle of the group illustrates what happens when values specified in "em" units are scaled.
The three rectangles on the right demonstrate the use of percentages. Note that the width and height of the viewport in the user coordinate system for the viewport element (in this case, theoutermost svg element) are 4000 and 2000, respectively, because processing the‘viewBox’attribute results in a transformed user coordinate system. The topmost rectangle, which is specified in percentage units, is exactly the same size as the middle rectangle, which is specified in equivalent user units. In particular, note that the‘stroke-width’property in the middle rectangle is set to 1% of thesqrt((actual-width)**2 + (actual-height)**2)) / sqrt(2), which in this case is .01*sqrt(4000*4000+2000*2000)/sqrt(2), or 31.62. The bottom rectangle of the group illustrates what happens when values specified in percentage units are scaled.
The following elements offer the option of expressing coordinate values and lengths as fractions (and, in some cases, percentages) of thebounding box, by setting a specified attribute to'objectBoundingBox'on the given element:
| Element | Attribute | Effect |
|---|---|---|
| ‘linearGradient’ | ‘gradientUnits’ | Indicates that the attributes which specify the gradient vector (‘x1’,‘y1’,‘x2’,‘y2’) represent fractions or percentages of the bounding box of the element to which the gradient is applied. |
| ‘radialGradient’ | ‘gradientUnits’ | Indicates that the attributes which specify the center (‘cx’,‘cy’), the radius (‘r’) and focus (‘fx’,‘fy’) represent fractions or percentages of the bounding box of the element to which the gradient is applied. |
| ‘pattern’ | ‘patternUnits’ | Indicates that the attributes which define how to tile the pattern (‘x’,‘y’,‘width’,‘height’) are established using the bounding box of the element to which the pattern is applied. |
| ‘pattern’ | ‘patternContentUnits’ | Indicates that the user coordinate system for the contents of the pattern is established using the bounding box of the element to which the pattern is applied. |
| ‘clipPath’ | ‘clipPathUnits’ | Indicates that the user coordinate system for the contents of the‘clipPath’element is established using the bounding box of the element to which the clipping path is applied. |
| ‘mask’ | ‘maskUnits’ | Indicates that the attributes which define the masking region (‘x’,‘y’,‘width’,‘height’) is established using the bounding box of the element to which the mask is applied. |
| ‘mask’ | ‘maskContentUnits’ | Indicates that the user coordinate system for the contents of the‘mask’element are established using the bounding box of the element to which the mask is applied. |
| ‘filter’ | ‘filterUnits’ | Indicates that the attributes which define the filter effects region (‘x’,‘y’,‘width’,‘height’) represent fractions or percentages of the bounding box of the element to which the filter is applied. |
| ‘filter’ | ‘primitiveUnits’ | Indicates that the various length values within the filter primitives represent fractions or percentages of the bounding box of the element to which the filter is applied. |
In the discussion that follows, the termapplicable elementis the element to which the given effect applies. For gradients and patterns, the applicable element is the graphics element which has its‘fill’or‘stroke’property referencing the given gradient or pattern. (See Inheritance of Painting Properties. For special rules concerning text elements, see the discussion of object bounding box units and text elements.) For clipping paths, masks and filters, the applicable element can be either a container element or a graphics element.
When keywordobjectBoundingBoxis used, then the effect is as if a supplemental transformation matrix were inserted into the list of nested transformation matrices to create a new user coordinate system.
First, the (minx,miny) and (maxx,maxy) coordinates are determined for the applicable element and all of its descendants. The valuesminx,miny,maxxandmaxyare determined by computing the maximum extent of the shape of the element in X and Y with respect to the user coordinate system for the applicable element. The bounding box is the tightest fitting rectangle aligned with the axes of the applicable element's user coordinate system that entirely encloses the applicable element and its descendants. The bounding box is computed exclusive of any values for clipping, masking, filter effects, opacity and stroke-width. For curved shapes, the bounding box encloses all portions of the shape, not just end points. For‘text’elements, for the purposes of the bounding box calculation, each glyph is treated as a separate graphics element. The calculations assume that all glyphs occupy the full glyph cell. For example, for horizontal text, the calculations assume that each glyph extends vertically to the full ascent and descent values for the font.
Then, coordinate (0,0) in the new user coordinate system is mapped to the (minx,miny) corner of the tight bounding box within the user coordinate system of the applicable element and coordinate (1,1) in the new user coordinate system is mapped to the (maxx,maxy) corner of the tight bounding box of the applicable element. In most situations, the following transformation matrix produces the correct effect:
[ (maxx-minx) 0 0 (maxy-miny) minx miny ]
When percentages are used with attributes that define the gradient vector, the pattern tile, the filter region or the masking region, a percentage represents the same value as the corresponding decimal value (e.g., 50% means the same as 0.5). If percentages are used within the content of a‘pattern’,‘clipPath’,‘mask’or‘filter’element, these values are treated according to the processing rules for percentages as defined in Units.
Any numeric value can be specified for values expressed as a fraction or percentage of object bounding box units. In particular, fractions less are zero or greater than one and percentages less than 0% or greater than 100% can be specified.
KeywordobjectBoundingBoxshould not be used when the geometry of the applicable element has no width or no height, such as the case of a horizontal or vertical line, even when the line has actual thickness when viewed due to having a non-zero stroke width since stroke width is ignored for bounding box calculations. When the geometry of the applicable element has no width or height andobjectBoundingBoxis specified, then the given effect (e.g., a gradient or a filter) will be ignored.
SVG needs to specify how to calculate some intrinsic sizing properties to enable inclusion within other languages. The intrinsic width and height of theviewportof SVG content must be determined from the‘width’and‘height’attributes. If either of these are not specified, a value of'100%'must be assumed.Note:the‘width’and‘height’attributes arenot the sameas the CSS width and height properties. Specifically, percentage valuesdo not provide an intrinsic width or height, and do not indicate a percentage of the containing block. Rather, once the viewport is established, they indicate the portion of the viewport that is actually covered by image data.
The intrinsic aspect ratio of theviewportof SVG content is necessary for example, when including SVG from an‘object’element in HTML styled with CSS. It is possible (indeed, common) for an SVG graphic to have an intrinsic aspect ratio but not to have an intrinsic width or height. The intrinsic aspect ratio must be calculated based upon the following rules:
The aspect ratio is calculated by dividing a width by a height.
If the‘width’and‘height’of therootmost‘svg’elementare both specified with unit identifiers (in, mm, cm, pt, pc, px, em, ex) or inuser units, then the aspect ratio is calculated from the‘width’and‘height’attributes after resolving both values to user units.
If either/both of the‘width’and‘height’of therootmost‘svg’elementare in percentage units (or omitted), the aspect ratio is calculated from the width and height values of the‘viewBox’specified for thecurrent SVG document fragment. If the‘viewBox’is not correctly specified, or set to'none', the intrinsic aspect ratio cannot be calculated and is considered unspecified.
Examples:
In this example the intrinsic aspect ratio of theviewportis 2:1. The intrinsic width is 10cm and the intrinsic height is 5cm.
In this example the intrinsic aspect ratio of the rootmostviewportis 1:1. An aspect ratio calculation in this case allows embedding in an object within a containing block that is only constrained in one direction.
In this case the intrinsic aspect ratio is 1:1.
In this example, the intrinsic aspect ratio is 1:1.
In order to allow interoperability between SVG content generators and user agents dealing with maps encoded in SVG, the use of a common metadata definition for describing the coordinate system used to generate SVG documents is encouraged.
Such metadata must be added under the‘metadata’element of the topmost‘svg’element describing the map, consisting of an RDF description of the Coordinate Reference System definition used to generate the SVG map [RDF-PRIMER]. Note that the presence of this metadata does not affect the rendering of the SVG in any way; it merely provides added semantic value for applications that make use of combined maps.
The definition must be conformant to the XML grammar described inGML 3.2.1, an OpenGIS Standard for encoding common CRS data types in XML [GML]. In order to correctly map the 2-dimensional data used by SVG, the CRS must be of subtypeProjectedCRSorGeographic2dCRS. The first axis of the described CRS maps the SVGx-axis and the second axis maps the SVGy-axis.
The main purpose of such metadata is to indicate to the user agent that two or more SVG documents can be overlayed or merged into a single document. Obviously, if two maps reference the same Coordinate Reference System definition and have the same SVG‘transform’attribute value then they can be overlayed without reprojecting the data. If the maps reference different Coordinate Reference Systems and/or have different SVG‘transform’attribute values, then a specialized cartographic user agent may choose to transform the coordinate data to overlay the data. However, typical SVG user agents are not required to perform these types of transformations, or even recognize the metadata. It is described in this specification so that the connection between geographic coordinate systems and the SVG coordinate system is clear.
Attribute definition:
Specifies the affine transformation that has been applied to the map data. The syntax is identical to that described in The‘transform’attribute section.
Specifies that no supplemental affine transformation has been applied to the map data. Using this value has the same meaning as specifying the identity matrix, which in turn is just the same as not specifying the‘svg:transform’the attribute at all.
Animatable: no.
This attribute describes an optional additional affine transformation that may have been applied during this mapping. This attribute may be added to the OpenGIS‘CoordinateReferenceSystem’element. Note that, unlike the‘transform’attribute, it does not indicate that a transformation is tobe appliedto the data within the file. Instead, it simply describes the transformation thatwas already appliedto the data when being encoded in SVG.
There are three typical uses for the‘svg:transform’global attribute. These are described below and used in the examples.
Most ProjectedCRS have the north direction represented by positive values of the second axis and conversely SVG has ay-down coordinate system. That's why, in order to follow the usual way to represent a map with the north at its top, it is recommended for that kind of ProjectedCRS to use the‘svg:transform’global attribute with a'scale(1, -1)'value as in the third example below.
Most Geographic2dCRS have the latitude as their first axis rather than the longitude, which means that the south-north axis would be represented by thex-axis in SVG instead of the usualy-axis. That's why, in order to follow the usual way to represent a map with the north at its top, it is recommended for that kind of Geographic2dCRS to use the‘svg:transform’global attribute with a'rotate(-90)'value as in the first example (while also adding the'scale(1, -1)'as for ProjectedCRS).
In addition, when converting for profiles which place restrictions on precision of real number values, it may be useful to add an additional scaling factor to retain good precision for a specific area. When generating an SVG document from WGS84 geographic coordinates (EPGS 4326), we recommend the use of an additional 100 times scaling factor corresponding to an‘svg:transform’global attribute with a'rotate(-90) scale(100)'value (shown in the second example). Different scaling values may be required depending on the particular CRS.
Below is a simple example of the coordinate metadata, which describes the coordinate system used by the document via a URI.
The second example uses a well-known identifier to describe the coordinate system. Note that the coordinates used in the document have had the supplied transform applied.
The third example defines the coordinate system completely within the SVG document.
Many of the SVG DOM interfaces refer to objects of class SVGPoint. An SVGPoint is an (x, y) coordinate pair. When used in matrix operations, an SVGPoint is treated as a vector of the form:
[x] [y] [1]
If an SVGRect object is designated asread only, then attempting to assign to one of its attributes will result in an exception being thrown.
interfaceSVGPoint{ attribute float x setraises(DOMException); attribute float y setraises(DOMException); SVGPoint matrixTransform(in SVGMatrix matrix); };
Applies a 2x3 matrix transformation on this SVGPoint object and returns a new, transformed SVGPoint object:
newpoint = matrix * thispoint
This interface defines a list of SVGPoint objects.
SVGPointList has the same attributes and methods as other SVGxxxList interfaces. Implementers may consider using a single base class to implement the various SVGxxxList interfaces.
interfaceSVGPointList{ readonly attribute unsigned long numberOfItems; void clear() raises(DOMException); SVGPoint initialize(in SVGPoint newItem) raises(DOMException); SVGPoint getItem(in unsigned long index) raises(DOMException); SVGPoint insertItemBefore(in SVGPoint newItem, in unsigned long index) raises(DOMException); SVGPoint replaceItem(in SVGPoint newItem, in unsigned long index) raises(DOMException); SVGPoint removeItem(in unsigned long index) raises(DOMException); SVGPoint appendItem(in SVGPoint newItem) raises(DOMException); };
Many of SVG's graphics operations utilize 2x3 matrices of the form:
[a c e] [b d f]
which, when expanded into a 3x3 matrix for the purposes of matrix arithmetic, become:
[a c e] [b d f] [0 0 1]
interfaceSVGMatrix{ attribute float a setraises(DOMException); attribute float b setraises(DOMException); attribute float c setraises(DOMException); attribute float d setraises(DOMException); attribute float e setraises(DOMException); attribute float f setraises(DOMException); SVGMatrix multiply(in SVGMatrix secondMatrix); SVGMatrix inverse() raises(SVGException); SVGMatrix translate(in float x, in float y); SVGMatrix scale(in float scaleFactor); SVGMatrix scaleNonUniform(in float scaleFactorX, in float scaleFactorY); SVGMatrix rotate(in float angle); SVGMatrix rotateFromVector(in float x, in float y) raises(SVGException); SVGMatrix flipX(); SVGMatrix flipY(); SVGMatrix skewX(in float angle); SVGMatrix skewY(in float angle); };
interfaceSVGTransform{ // Transform Types const unsigned short SVG_TRANSFORM_UNKNOWN = 0; const unsigned short SVG_TRANSFORM_MATRIX = 1; const unsigned short SVG_TRANSFORM_TRANSLATE = 2; const unsigned short SVG_TRANSFORM_SCALE = 3; const unsigned short SVG_TRANSFORM_ROTATE = 4; const unsigned short SVG_TRANSFORM_SKEWX = 5; const unsigned short SVG_TRANSFORM_SKEWY = 6; readonly attribute unsigned short type; readonly attribute SVGMatrix matrix; readonly attribute float angle; void setMatrix(in SVGMatrix matrix) raises(DOMException); void setTranslate(in float tx, in float ty) raises(DOMException); void setScale(in float sx, in float sy) raises(DOMException); void setRotate(in float angle, in float cx, in float cy) raises(DOMException); void setSkewX(in float angle) raises(DOMException); void setSkewY(in float angle) raises(DOMException); };
The matrix that represents this transformation. The matrix object is live, meaning that any changes made to the SVGTransform object are immediately reflected in the matrix object and vice versa. In case the matrix object is changed directly (i.e., without using the methods on the SVGTransform interface itself) then the type of the SVGTransform changes to SVG_TRANSFORM_MATRIX.
A convenience attribute for SVG_TRANSFORM_ROTATE, SVG_TRANSFORM_SKEWX and SVG_TRANSFORM_SKEWY. It holds the angle that was specified.
For SVG_TRANSFORM_MATRIX, SVG_TRANSFORM_TRANSLATE and SVG_TRANSFORM_SCALE, angle will be zero.
Sets the transform type to SVG_TRANSFORM_MATRIX, with parametermatrixdefining the new transformation. The values from the parametermatrixare copied, thematrixparameter does not replace SVGTransform::matrix.
This interface defines a list of SVGTransform objects.
The SVGTransformList and SVGTransform interfaces correspond to the various attributes which specify a set of transformations, such as the‘transform’attribute which is available for many of SVG's elements.
SVGTransformList has the same attributes and methods as other SVGxxxList interfaces. Implementers may consider using a single base class to implement the various SVGxxxList interfaces.
An SVGTransformList object can be designated asread only, which means that attempts to modify the object will result in an exception being thrown, as described below.
interfaceSVGTransformList{ readonly attribute unsigned long numberOfItems; void clear() raises(DOMException); SVGTransform initialize(in SVGTransform newItem) raises(DOMException); SVGTransform getItem(in unsigned long index) raises(DOMException); SVGTransform insertItemBefore(in SVGTransform newItem, in unsigned long index) raises(DOMException); SVGTransform replaceItem(in SVGTransform newItem, in unsigned long index) raises(DOMException); SVGTransform removeItem(in unsigned long index) raises(DOMException); SVGTransform appendItem(in SVGTransform newItem) raises(DOMException); SVGTransform createSVGTransformFromMatrix(in SVGMatrix matrix); SVGTransform consolidate() raises(DOMException); };
Creates an SVGTransform object which is initialized to transform of type SVG_TRANSFORM_MATRIX and whose values are the given matrix. The values from the parametermatrixare copied, thematrixparameter is not adopted as SVGTransform::matrix.
interfaceSVGAnimatedTransformList{ readonly attribute SVGTransformList baseVal; readonly attribute SVGTransformList animVal; };
An SVGPreserveAspectRatio object can be designated asread only, which means that attempts to modify the object will result in an exception being thrown, as described below.
interfaceSVGPreserveAspectRatio{ // Alignment Types const unsigned short SVG_PRESERVEASPECTRATIO_UNKNOWN = 0; const unsigned short SVG_PRESERVEASPECTRATIO_NONE = 1; const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMIN = 2; const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMIN = 3; const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMIN = 4; const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMID = 5; const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMID = 6; const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMID = 7; const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMAX = 8; const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMAX = 9; const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMAX = 10; // Meet-or-slice Types const unsigned short SVG_MEETORSLICE_UNKNOWN = 0; const unsigned short SVG_MEETORSLICE_MEET = 1; const unsigned short SVG_MEETORSLICE_SLICE = 2; attribute unsigned short align setraises(DOMException); attribute unsigned short meetOrSlice setraises(DOMException); };
interfaceSVGAnimatedPreserveAspectRatio{ readonly attribute SVGPreserveAspectRatio baseVal; readonly attribute SVGPreserveAspectRatio animVal; };