13 Gradients and Patterns

Contents

• 13.1 Introduction
• 13.2 Gradients
  • 13.2.1 Introduction
  • 13.2.2 Linear gradients
  • 13.2.3 Radial gradients
  • 13.2.4 Gradient stops
• 13.3 Patterns
• 13.4 DOM interfaces
  • 13.4.1 Interface SVGGradientElement
  • 13.4.2 Interface SVGLinearGradientElement
  • 13.4.3 Interface SVGRadialGradientElement
  • 13.4.4 Interface SVGStopElement
  • 13.4.5 Interface SVGPatternElement

13.1 Introduction

With SVG, you can fill (i.e., paint the interior) or stroke (i.e., paint the outline) of shapes and text using one of the following:

• color (using <color>)
• gradients (linear or radial)
• patterns (vector or image, possibly tiled)

SVG uses the general notion of a paint server. Gradients and patterns are just specific types of built-in paint servers.

Paint servers are referenced using an IRI reference on a ‘fill’ or ‘stroke’ property.

13.2 Gradients

13.2.1 Introduction

Gradients consist of continuously smooth color transitions along a vector from one color to another, possibly followed by additional transitions along the same vector to other colors. SVG provides for two types of gradients: linear gradients and radial gradients.

Once defined, gradients are then referenced using ‘fill’ or ‘stroke’ properties on a given graphics element to indicate that the given element shall be filled or stroked with the referenced gradient.

The angle of the color transitions along the gradient vector is defined by the gradient normal. Before any transforms are applied to the gradient or its referencing graphics element, the gradient normal is perpendicular with the gradient vector. If a graphics element references a gradient, conceptually the graphics element should take a copy of the gradient vector and gradient normal and treat it as part of its own geometry. Any transformations applied to the graphics element geometry also apply to the copied gradient vector and gradient normal. Any gradient transforms that are specified on the reference gradient are applied before any graphics element transformations are applied to the gradient.

13.2.2 Linear gradients

Linear gradients are defined by a ‘linearGradient’ element.

Percentages are allowed for ‘x1’, ‘y1’, ‘x2’ and ‘y2’. For gradientUnits="userSpaceOnUse", percentages represent values relative to the current viewport. For gradientUnits="objectBoundingBox", percentages represent values relative to the bounding box for the object.

If ‘x1’ = ‘x2’ and ‘y1’ = ‘y2’, then the area to be painted will be painted as a single color using the color and opacity of the last gradient stop.

Properties inherit into the ‘linearGradient’ element from its ancestors; properties do not inherit from the element referencing the ‘linearGradient’ element.

‘linearGradient’ elements are never rendered directly; their only usage is as something that can be referenced using the ‘fill’ and ‘stroke’ properties. The ‘display’ property does not apply to the ‘linearGradient’ element; thus, ‘linearGradient’ elements are not directly rendered even if the ‘display’ property is set to a value other than none, and ‘linearGradient’ elements are available for referencing even when the ‘display’ property on the ‘linearGradient’ element or any of its ancestors is set to none.

Example lingrad01 shows how to fill a rectangle by referencing a linear gradient paint server.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="8cm" height="4cm" viewBox="0 0 800 400" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example lingrad01 - fill a rectangle using a 
           linear gradient paint server</desc>
  <g>
    <defs>
      <linearGradient id="MyGradient">
        <stop offset="5%" stop-color="#F60" />
        <stop offset="95%" stop-color="#FF6" />
      </linearGradient>
    </defs>

    <!-- Outline the drawing area in blue -->
    <rect fill="none" stroke="blue" 
          x="1" y="1" width="798" height="398"/>

    <!-- The rectangle is filled using a linear gradient paint server -->
    <rect fill="url(#MyGradient)" stroke="black" stroke-width="5"  
          x="100" y="100" width="600" height="200"/>
  </g>
</svg>

Example lingrad01

View this example as SVG (SVG-enabled browsers only)

13.2.3 Radial gradients

Radial gradients are defined by a ‘radialGradient’ element.

Percentages are allowed for attributes ‘cx’, ‘cy’, ‘r’, ‘fx’ and ‘fy’. For gradientUnits="userSpaceOnUse", percentages represent values relative to the current viewport. For gradientUnits="objectBoundingBox", percentages represent values relative to the bounding box for the object.

If the point defined by ‘fx’ and ‘fy’ lies outside the circle defined by ‘cx’, ‘cy’ and ‘r’, then the user agent shall set the focal point to the intersection of the line from (‘cx’, ‘cy’) to (‘fx’, ‘fy’) with the circle defined by ‘cx’, ‘cy’ and ‘r’.

Properties inherit into the ‘radialGradient’ element from its ancestors; properties do not inherit from the element referencing the ‘radialGradient’ element.

‘radialGradient’ elements are never rendered directly; their only usage is as something that can be referenced using the ‘fill’ and ‘stroke’ properties. The ‘display’ property does not apply to the ‘radialGradient’ element; thus, ‘radialGradient’ elements are not directly rendered even if the ‘display’ property is set to a value other than none, and ‘radialGradient’ elements are available for referencing even when the ‘display’ property on the ‘radialGradient’ element or any of its ancestors is set to none.

Example radgrad01 shows how to fill a rectangle by referencing a radial gradient paint server.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="8cm" height="4cm" viewBox="0 0 800 400" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example radgrad01 - fill a rectangle by referencing a 
           radial gradient paint server</desc>
  <g>
    <defs>
      <radialGradient id="MyGradient" gradientUnits="userSpaceOnUse"
                      cx="400" cy="200" r="300" fx="400" fy="200">
        <stop offset="0%" stop-color="red" />
        <stop offset="50%" stop-color="blue" />
        <stop offset="100%" stop-color="red" />
      </radialGradient>
    </defs>

    <!-- Outline the drawing area in blue -->
    <rect fill="none" stroke="blue" 
          x="1" y="1" width="798" height="398"/>

    <!-- The rectangle is filled using a radial gradient paint server -->
    <rect fill="url(#MyGradient)" stroke="black" stroke-width="5"  
          x="100" y="100" width="600" height="200"/>
  </g>
</svg>

Example radgrad01

View this example as SVG (SVG-enabled browsers only)

13.2.4 Gradient stops

The ramp of colors to use on a gradient is defined by the ‘stop’ elements that are child elements to either the ‘linearGradient’ element or the ‘radialGradient’ element.

The ‘stop-color’ property indicates what color to use at that gradient stop. The keyword currentColor and ICC colors can be specified in the same manner as within a <paint> specification for the ‘fill’ and ‘stroke’ properties.

The ‘stop-opacity’ property defines the opacity of a given gradient stop.

Some notes on gradients:

• Gradient offset values less than 0 (or less than 0%) are rounded up to 0%. Gradient offset values greater than 1 (or greater than 100%) are rounded down to 100%.
• It is necessary that at least two stops defined to have a gradient effect. If no stops are defined, then painting shall occur as if 'none' were specified as the paint style. If one stop is defined, then paint with the solid color fill using the color defined for that gradient stop.
• Each gradient offset value is required to be equal to or greater than the previous gradient stop's offset value. If a given gradient stop's offset value is not equal to or greater than all previous offset values, then the offset value is adjusted to be equal to the largest of all previous offset values.
• If two gradient stops have the same offset value, then the latter gradient stop controls the color value at the overlap point. In particular:

  <stop offset="0" stop-color="white"/>
  <stop offset=".2" stop-color="red"/>
  <stop offset=".2" stop-color="blue"/>
  <stop offset="1" stop-color="black"/>
  

  will have approximately the same effect as:

  <stop offset="0" stop-color="white"/>
  <stop offset=".1999999999" stop-color="red"/>
  <stop offset=".2" stop-color="blue"/>
  <stop offset="1" stop-color="black"/>
  

  which is a gradient that goes smoothly from white to red, then abruptly shifts from red to blue, and then goes smoothly from blue to black.

13.3 Patterns

A pattern is used to fill or stroke an object using a pre-defined graphic object which can be replicated ("tiled") at fixed intervals in x and y to cover the areas to be painted. Patterns are defined using a ‘pattern’ element and then referenced by properties ‘fill’ and ‘stroke’ on a given graphics element to indicate that the given element shall be filled or stroked with the referenced pattern.

Attributes ‘x’, ‘y’, ‘width’, ‘height’ and ‘patternUnits’ define a reference rectangle somewhere on the infinite canvas. The reference rectangle has its top/left at (x, y) and its bottom/right at (x + width, y + height). The tiling theoretically extends a series of such rectangles to infinity in X and Y (positive and negative), with rectangles starting at (x + m*width, y + n* height) for each possible integer value for m and n.

SVG's user agent style sheet sets the ‘overflow’ property for ‘pattern’ elements to hidden, which causes a rectangular clipping path to be created at the bounds of the pattern tile. Unless the ‘overflow’ property is overridden, any graphics within the pattern which goes outside of the pattern rectangle will be clipped. Note that if the ‘overflow’ property is set to visible the rendering behavior for the pattern is undefined. Example pattern01 below shows the effect of clipping to the pattern tile.

The contents of the ‘pattern’ are relative to a new coordinate system. If there is a ‘viewBox’ attribute, then the new coordinate system is fitted into the region defined by the ‘x’, ‘y’, ‘width’, ‘height’ and ‘patternUnits’ attributes on the ‘pattern’ element using the standard rules for ‘viewBox’ and ‘preserveAspectRatio’. If there is no ‘viewBox’ attribute, then the new coordinate system has its origin at (x, y), where x is established by the ‘x’ attribute on the ‘pattern’ element, and y is established by the ‘y’ attribute on the ‘pattern’ element. Thus, in the following example:

<pattern x="10" y="10" width="20" height="20">
   <rect x="5" y="5" width="10" height="10"/>
</pattern>

the rectangle has its top/left located 5 units to the right and 5 units down from the origin of the pattern tile.

The ‘viewBox’ attribute introduces a supplemental transformation which is applied on top of any transformations necessary to create a new pattern coordinate system due to attributes ‘x’, ‘y’, ‘width’, ‘height’ and ‘patternUnits’.

Properties inherit into the ‘pattern’ element from its ancestors; properties do not inherit from the element referencing the ‘pattern’ element.

‘pattern’ elements are never rendered directly; their only usage is as something that can be referenced using the ‘fill’ and ‘stroke’ properties. The ‘display’ property does not apply to the ‘pattern’ element; thus, ‘pattern’ elements are not directly rendered even if the ‘display’ property is set to a value other than none, and ‘pattern’ elements are available for referencing even when the ‘display’ property on the ‘pattern’ element or any of its ancestors is set to none.

Event attributes and event listeners attached to the contents of a ‘pattern’ element are not processed; only the rendering aspects of ‘pattern’ elements are processed.

Example pattern01 shows how to fill a rectangle by referencing a pattern paint server. Note how the blue stroke of each triangle has been clipped at the top and the left. This is due to SVG's user agent style sheet setting the ‘overflow’ property for ‘pattern’ elements to hidden, which causes the pattern to be clipped to the bounds of the pattern tile.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="8cm" height="4cm" viewBox="0 0 800 400" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <defs>
    <pattern id="TrianglePattern" patternUnits="userSpaceOnUse"
             x="0" y="0" width="100" height="100"
             viewBox="0 0 10 10" >
      <path d="M 0 0 L 7 0 L 3.5 7 z" fill="red" stroke="blue" />
    </pattern> 
  </defs>

  <!-- Outline the drawing area in blue -->
  <rect fill="none" stroke="blue" 
        x="1" y="1" width="798" height="398"/>

  <!-- The ellipse is filled using a triangle pattern paint server
       and stroked with black -->
  <ellipse fill="url(#TrianglePattern)" stroke="black" stroke-width="5"  
           cx="400" cy="200" rx="350" ry="150" />
</svg>

Example pattern01

View this example as SVG (SVG-enabled browsers only)

13.4 DOM interfaces

13.4.1 Interface SVGGradientElement

The SVGGradientElement interface is a base interface used by SVGLinearGradientElement and SVGRadialGradientElement.

interface SVGGradientElement : SVGElement,
                               SVGURIReference,
                               SVGExternalResourcesRequired,
                               SVGStylable,
                               SVGUnitTypes {

  // Spread Method Types
  const unsigned short SVG_SPREADMETHOD_UNKNOWN = 0;
  const unsigned short SVG_SPREADMETHOD_PAD = 1;
  const unsigned short SVG_SPREADMETHOD_REFLECT = 2;
  const unsigned short SVG_SPREADMETHOD_REPEAT = 3;

  readonly attribute SVGAnimatedEnumeration gradientUnits;
  readonly attribute SVGAnimatedTransformList gradientTransform;
  readonly attribute SVGAnimatedEnumeration spreadMethod;
};

Constants in group “Spread Method Types”:

SVG_SPREADMETHOD_UNKNOWN (unsigned short)

The type is not one of predefined types. It is invalid to attempt to define a new value of this type or to attempt to switch an existing value to this type.

SVG_SPREADMETHOD_PAD (unsigned short)

Corresponds to value 'pad'.

SVG_SPREADMETHOD_REFLECT (unsigned short)

Corresponds to value 'reflect'.

SVG_SPREADMETHOD_REPEAT (unsigned short)

Corresponds to value 'repeat'.

Attributes:

gradientUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘gradientUnits’ on the given element. Takes one of the constants defined in SVGUnitTypes.

gradientTransform (readonly SVGAnimatedTransformList)

Corresponds to attribute ‘gradientTransform’ on the given element.

spreadMethod (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘spreadMethod’ on the given element. One of the Spread Method Types defined on this interface.

13.4.2 Interface SVGLinearGradientElement

The SVGLinearGradientElement interface corresponds to the ‘linearGradient’ element.

interface SVGLinearGradientElement : SVGGradientElement {
  readonly attribute SVGAnimatedLength x1;
  readonly attribute SVGAnimatedLength y1;
  readonly attribute SVGAnimatedLength x2;
  readonly attribute SVGAnimatedLength y2;
};

Attributes:

x1 (readonly SVGAnimatedLength)

Corresponds to attribute ‘x1’ on the given ‘linearGradient’ element.

y1 (readonly SVGAnimatedLength)

Corresponds to attribute ‘y1’ on the given ‘linearGradient’ element.

x2 (readonly SVGAnimatedLength)

Corresponds to attribute ‘x2’ on the given ‘linearGradient’ element.

y2 (readonly SVGAnimatedLength)

Corresponds to attribute ‘y2’ on the given ‘linearGradient’ element.

13.4.3 Interface SVGRadialGradientElement

The SVGRadialGradientElement interface corresponds to the ‘radialGradient’ element.

interface SVGRadialGradientElement : SVGGradientElement {
  readonly attribute SVGAnimatedLength cx;
  readonly attribute SVGAnimatedLength cy;
  readonly attribute SVGAnimatedLength r;
  readonly attribute SVGAnimatedLength fx;
  readonly attribute SVGAnimatedLength fy;
};

Attributes:

cx (readonly SVGAnimatedLength)

Corresponds to attribute ‘cx’ on the given ‘radialGradient’ element.

cy (readonly SVGAnimatedLength)

Corresponds to attribute ‘cy’ on the given ‘radialGradient’ element.

r (readonly SVGAnimatedLength)

Corresponds to attribute ‘r’ on the given ‘radialGradient’ element.

fx (readonly SVGAnimatedLength)

Corresponds to attribute ‘fx’ on the given ‘radialGradient’ element.

fy (readonly SVGAnimatedLength)

Corresponds to attribute ‘fy’ on the given ‘radialGradient’ element.

13.4.4 Interface SVGStopElement

The SVGStopElement interface corresponds to the ‘stop’ element.

interface SVGStopElement : SVGElement,
                           SVGStylable {
  readonly attribute SVGAnimatedNumber offset;
};

Attributes:

offset (readonly SVGAnimatedNumber)

Corresponds to attribute ‘offset’ on the given ‘stop’ element.

13.4.5 Interface SVGPatternElement

The SVGPatternElement interface corresponds to the ‘pattern’ element.

interface SVGPatternElement : SVGElement,
                              SVGURIReference,
                              SVGTests,
                              SVGLangSpace,
                              SVGExternalResourcesRequired,
                              SVGStylable,
                              SVGFitToViewBox,
                              SVGUnitTypes {
  readonly attribute SVGAnimatedEnumeration patternUnits;
  readonly attribute SVGAnimatedEnumeration patternContentUnits;
  readonly attribute SVGAnimatedTransformList patternTransform;
  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
  readonly attribute SVGAnimatedLength width;
  readonly attribute SVGAnimatedLength height;
};

Attributes:

patternUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘patternUnits’ on the given ‘pattern’ element. Takes one of the constants defined in SVGUnitTypes.

patternContentUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘patternContentUnits’ on the given ‘pattern’ element. Takes one of the constants defined in SVGUnitTypes.

patternTransform (readonly SVGAnimatedTransformList)

Corresponds to attribute ‘patternTransform’ on the given ‘pattern’ element.

x (readonly SVGAnimatedLength)

Corresponds to attribute ‘x’ on the given ‘pattern’ element.

y (readonly SVGAnimatedLength)

Corresponds to attribute ‘y’ on the given ‘pattern’ element.

width (readonly SVGAnimatedLength)

Corresponds to attribute ‘width’ on the given ‘pattern’ element.

height (readonly SVGAnimatedLength)

Corresponds to attribute ‘height’ on the given ‘pattern’ element.