What is the glow light effect in SVG?

The glow light effect in SVG (Scalable Vector Graphics) is a visual technique that creates the appearance of light emanating from or surrounding an element, giving it a soft, luminous aura. This effect is commonly used to highlight elements, create depth, or add visual appeal to graphics. It is achieved using SVG filters, specifically the <feGaussianBlur> and <feMerge> filter primitives.

Here's a breakdown of how it works:

1. <filter> Element: The effect is contained within a <filter> element, which encapsulates one or more filter primitives.

2. <feGaussianBlur> Primitive: This is the core component for creating the blur effect. It blurs the input graphic (typically the original shape) using a Gaussian blur algorithm. The stdDeviation attribute controls the amount of blur. A higher value means a larger and more diffused glow.

3. <feMerge> Primitive: This primitive is used to combine the blurred output with the original graphic or other outputs. This step is essential because, without it, only the blurred effect would be visible, not the original element.

4. <feMergeNode> Elements: These elements are used inside the <feMerge> primitive to specify the order in which the filter outputs are to be merged. The first <feMergeNode> usually contains the blurred image, and the second contains the original graphic to combine them.

Here's an example of a basic SVG glow effect using the explained elements and primitives:

<svg width="200" height="200">
  <defs>
    <filter id="glow">
      <feGaussianBlur in="SourceGraphic" stdDeviation="5" result="blur" />
      <feMerge>
        <feMergeNode in="blur" />
        <feMergeNode in="SourceGraphic" />
      </feMerge>
    </filter>
  </defs>
  <circle cx="100" cy="100" r="50" fill="red" filter="url(#glow)" />
</svg>

In this example:

- We define a filter named "glow" inside the <defs> section.

- The <feGaussianBlur> applies a blur to the circle with a standard deviation of 5. The input is 'SourceGraphic' which means it will take the circle's original graphic as input.

- The <feMerge> combines the blurred output and the original shape using <feMergeNode> primitives.

- The <circle> uses this filter through the filter="url(#glow)" attribute, applying the glow effect.

By adjusting the stdDeviation value, the color of the shape, and adding more filters and merge nodes, the glow effect can be further customized.

The glow effect is a powerful tool in SVG to make your graphics more visually engaging and dynamic. It can enhance user interfaces, icons, and more complex illustrations.

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