next.js/packages/next/src/shared/lib/image-blur-svg.ts

/**
 * A shared function, used on both client and server, to generate a SVG blur placeholder.
 */
export function getImageBlurSvg({
  widthInt,
  heightInt,
  blurWidth,
  blurHeight,
  blurDataURL,
  objectFit,
}: {
  widthInt?: number
  heightInt?: number
  blurWidth?: number
  blurHeight?: number
  blurDataURL: string
  objectFit?: string
}): string {
  const std = 20
  const svgWidth = blurWidth ? blurWidth * 40 : widthInt
  const svgHeight = blurHeight ? blurHeight * 40 : heightInt

  const viewBox =
    svgWidth && svgHeight ? `viewBox='0 0 ${svgWidth} ${svgHeight}'` : ''
  const preserveAspectRatio = viewBox
    ? 'none'
    : objectFit === 'contain'
      ? 'xMidYMid'
      : objectFit === 'cover'
        ? 'xMidYMid slice'
        : 'none'

  return `%3Csvg xmlns='http://www.w3.org/2000/svg' ${viewBox}%3E%3Cfilter id='b' color-interpolation-filters='sRGB'%3E%3CfeGaussianBlur stdDeviation='${std}'/%3E%3CfeColorMatrix values='1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 100 -1' result='s'/%3E%3CfeFlood x='0' y='0' width='100%25' height='100%25'/%3E%3CfeComposite operator='out' in='s'/%3E%3CfeComposite in2='SourceGraphic'/%3E%3CfeGaussianBlur stdDeviation='${std}'/%3E%3C/filter%3E%3Cimage width='100%25' height='100%25' x='0' y='0' preserveAspectRatio='${preserveAspectRatio}' style='filter: url(%23b);' href='${blurDataURL}'/%3E%3C/svg%3E`
}