Introduction to SVG Filters

SVG Filter Effects expand the graphic capabilities of the Web. An SVG Filter defines an operation on a graphical input. Just like other HTML elements, filters are declarative in nature and have a supporting DOM for dynamic manipulation. A filter is applied to an SVG element via the filter attribute, in the form of filter="url(#filterId)", or it can be applied as a CSS property filter:url(#filterId). Each filter is composed of one or more filter primitives, which are the basic building blocks for creating nifty effects. Each applies a fundamental effect to a graphic. To form more complex effects, filter primitives can be chained together, feeding the output of one to the input of another. When a filter is applied to an SVG element, that SVG element is used as the source graphic for the filter or the first filter in a chain.

   
An image of an SVG pig before (left) applying an SVG filter and after (right) applying an SVG filter

There are 16 different filter primitives. They enable effects ranging from providing light sources to applying matrix transformations to adding a Gaussian blur and much more. SVG Filters make it easy to manipulate and apply Photoshop-like effects to SVG elements. Akin to the rest of SVG, the results are scalable, retaining high quality at any resolution. The filter definition is completely reflected in the DOM as is the original SVG element. Effects can easily be removed by removing the filter attribute. The original, unfiltered image can be attained in this manner. ilter primitives vary widely to cover a large scope of possibilities but there are commonalities between them. Most filter primitives take one or two input parameters. These inputs typically reference the source element, the source element’s alpha channel, or the output of another filter primitive. Having a choice of inputs increases the range of possible effects.
All filter primitives let you specify an identifier for its output so that output can be referenced later. Although filter primitives use the output of the previous filter primitive by default, filter chains don’t have to be completely linear. In fact, more complex filter chains, especially ones that use filter primitives with multiple inputs, often aren’t.
Here’s a simple filter primitive in action:
The feColorMatrix filter primitive applies a matrix transform on the RGBA values of every pixel on the input. A custom matrix can be defined, or a keyword can be used. With it, SVG elements can easily be given a greyscale look or otherwise have their colors altered. Below is an example of the hueRotate keyword being used, and shifting the pixel hues 180 degrees to the opposite side of the color wheel.

<filter id="myHueRotate">

<feColorMatrix type="hueRotate" values="180"/>

filter>

<g id="myPig" filter="url(#myHueRotate)">

g>

   
An image of an SVG pig before (left) applying an feColorMatrix filter and after (right) applying the feColorMatrix filter