Filter effects

There are situations, where basic shapes do not provide the flexibility you need to achieve a certain effect. Drop shadows, to provide a popular example, cannot be created reasonably with a combination of gradients. Filters are SVG's mechanism to create sophisticated effects.

A basic example is to add a blur effect to SVG content. While basic blurs can be achieved with the help of gradients, the blur filter is needed to do anything beyond.

Example

Filters are defined by <filter> element, which should be put in the <defs> section of your SVG file. Between the filter tags, goes a list of primitives, basic operations that build on top of the previous operations (like blurring, adding a lighting effect, etc). To apply your created filter on a graphic element, you set the filter attribute.

<svg width="250" viewBox="0 0 200 85"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <defs>
    <!-- Filter declaration -->
    <filter id="MyFilter" filterUnits="userSpaceOnUse"
            x="0" y="0"
            width="200" height="120">

      <!-- offsetBlur -->
      <feGaussianBlur in="SourceAlpha" stdDeviation="4" result="blur"/>
      <feOffset in="blur" dx="4" dy="4" result="offsetBlur"/>

      <!-- litPaint -->
      <feSpecularLighting in="blur" surfaceScale="5" specularConstant=".75" 
                          specularExponent="20" lighting-color="#bbbbbb"  
                          result="specOut">
        <fePointLight x="-5000" y="-10000" z="20000"/>
      </feSpecularLighting>
      <feComposite in="specOut" in2="SourceAlpha" operator="in" result="specOut"/>
      <feComposite in="SourceGraphic" in2="specOut" operator="arithmetic" 
                   k1="0" k2="1" k3="1" k4="0" result="litPaint"/>

      <!-- merge offsetBlur + litPaint -->
      <feMerge>
        <feMergeNode in="offsetBlur"/>
        <feMergeNode in="litPaint"/>
      </feMerge>
    </filter>
  </defs>

  <!-- Graphic elements -->
  <g filter="url(#MyFilter)">
      <path fill="none" stroke="#D90000" stroke-width="10" 
            d="M50,66 c-50,0 -50,-60 0,-60 h100 c50,0 50,60 0,60z" />
      <path fill="#D90000" 
            d="M60,56 c-30,0 -30,-40 0,-40 h80 c30,0 30,40 0,40z" />
      <g fill="#FFFFFF" stroke="black" font-size="45" font-family="Verdana" >
        <text x="52" y="52">SVG</text>
      </g>
  </g>
</svg>

Step 1

<feGaussianBlur in="SourceAlpha"
                stdDeviation="4"
                result="blur"/>

<feGaussianBlur> takes in "SourceAlpha", which is the alpha channel of the source graphic, applies a blur of 4, and stores the result in a temporary buffer named "blur".

Step 2

<feOffset in="blur"
          dx="4" dy="4"
          result="offsetBlur"/>

<feOffset> takes in "blur", which we previously created, shifts the result 4 to the right and 4 to the bottom, and stores the result in the buffer "blur". The two first primitives just created a drop shadow.

Step 3

<feSpecularLighting in="blur"
                    surfaceScale="5" specularConstant=".75" 
                    specularExponent="20" lighting-color="#bbbbbb"
                    result="specOut">
  <fePointLight x="-5000" y="-10000" z="20000"/>
</feSpecularLighting>

<feSpecularLighting> takes in "offsetBlur", generates a lighting effect, and stores the result in the buffer "specOut".

Step 4

<feComposite in="specOut" in2="SourceAlpha"
             operator="in"
             result="specOut"/>

The first <feComposite> takes in "specOut" and "SourceAlpha", masks out the result of "specOut" so that the result is not bigger than "SourceAlpha" (the original source graphic), and overrides the result "specOut".

Step 5

<feComposite in="SourceGraphic" in2="specOut"
             operator="arithmetic" 
             k1="0" k2="1" k3="1" k4="0"
             result="litPaint"/>

The second <feComposite> takes in "SourceAlpha" and "specOut", adds the result of "specOut" on top of "SourceAlpha", and stores the result in "litPaint".

Step 6

<feMerge>
  <feMergeNode in="offsetBlur"/>
  <feMergeNode in="litPaint"/>
</feMerge>

Finallly, <feMerge> merges together "offsetBlur", which is the drop shadow, and "litPaint", which is the original source graphic with a lighting effect.

Source graphic

Primitive 1

Primitive 2

Primitive 3

Primitive 4

Primitive 5

Primitive 6