Posts Tagged ‘SVG filterUnits’

SVG filters and invisible paths

The setup

Let’s just right into it and look at a few paths:

  • We have the arrow thingy (M100 100 L330 453 L349 349 L527 349 L100 100)
  • The horizontal line (M50 50 L200 50)
  • The vertical line (M125 10 L125 50)

There’s some CSS to style the paths:

path { fill: none; stroke-width: 3px; stroke: url('#gradient'); }

There’s also some code for the linear gradient, but that’s not relevant here.

A blur filter

A simple SVG gaussian blur filter can be done as follows:

<defs> <filter id="blur"> <feGaussianBlur in="SourceGraphic" stdDeviation="2" /> </filter> </defs>

Applying that filter to the paths (via filter:url(#blur) CSS rule), we get the following:

So, that kinda works, but the horizontal and vertical paths are now invisible!

A problem with filterUnits

The issue surfaces due to the value of the filterUnits attribute on the filter element is set to objectBoundingBox (which is also the default when a value is not specified). From the SVG spec:

Keyword objectBoundingBox should not be used when the geometry of the applicable element has no width or no height, such as the case of a horizontal or vertical line, even when the line has actual thickness when viewed due to having a non-zero stroke width since stroke width is ignored for bounding box calculations. When the geometry of the applicable element has no width or height and objectBoundingBox is specified, then the given effect (e.g., a gradient or a filter) will be ignored.

objectBoundingBox simply means that the x, y, width, height attributes on the filter are relative to the bounding box of the element referencing the filter, so it’s confusing why this should be an issue at all. In any case, it’s of course problematic for paths which have no width and height.

The solution

The solution is simply to change the filterUnits attribute to userSpaceOnUse. If you make use of the x, y, width, height attributes on the element, they will also need to be updated, as these attributes will now represent the coordinate system in which the element referencing the filter is (as opposed to the bounding box of that element).

<defs> <filter id="blur" filterUnits="userSpaceOnUse"> <feGaussianBlur in="SourceGraphic" stdDeviation="2" /> </filter> </defs>

A simple fix but this is an annoying issue and I see no clear reason as to why filterUnits="objectBoundingBox" should be problematic for elements without a defined width and height.