How can I create a " glow " around a rectangle with svg?

Creating a 'glow' effect around a rectangle in SVG is typically achieved using the <filter> element. Here is a step-by-step implementation and example:

Step 1: Define SVG Elements

First, we need to define an SVG element and include a rectangle within it.

xml
<svg width="200" height="200">
  <rect x="50" y="50" width="100" height="100" style="fill:blue" />
</svg>

Step 2: Add Filter Definition

Next, we add a <defs> element inside the SVG to define the filter. Within the filter, we use <feGaussianBlur> to create a blur effect, which is essential for generating the glow.

xml
<svg width="200" height="200">
  <defs>
    <filter id="glow">
      <feGaussianBlur stdDeviation="8" result="blurred"/>
      <feMerge>
        <feMergeNode in="blurred"/>
        <feMergeNode in="SourceGraphic"/>
      </feMerge>
    </filter>
  </defs>
  <rect x="50" y="50" width="100" height="100" style="fill:blue" filter="url(#glow)" />
</svg>

Explanation:

1. <filter id="glow">: Defines a filter with the ID glow, which can be referenced where needed.
2. <feGaussianBlur stdDeviation="8" result="blurred"/>: This element creates a Gaussian blur effect. The stdDeviation attribute controls the blur intensity, and result specifies the output of this blur operation.
3. <feMerge>: This element merges multiple image streams. By using <feMergeNode>, we combine the blurred image with the original graphic, achieving a glow effect while preserving visibility of the original shape.

Step 3: Apply the Filter

Finally, we apply the filter to the rectangle element by referencing the defined filter #glow using the filter attribute.

In this example, we create a blue rectangle with a glow effect. This method is ideal for creating visually appealing effects on SVG graphics and is commonly used to highlight specific elements or enhance graphical interfaces.