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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 the <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 "offsetBlur". The two first primitives just created a drop shadow.

Step 3

<feSpecularLighting
  in="offsetBlur"
  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 "SourceGraphic" and "specOut", adds the result of "specOut" on top of "SourceGraphic", and stores the result in "litPaint".

Step 6

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

Finally, <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

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https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/Filter_effects