Filter effects: Gaussian blur

This is part of my Filter effects tutorial series.

Gaussian blur makes objects appear softer and less clearly defined. Even by itself it is so useful that it was the first filter effect to be implemented in Inkscape. It also has received an own place in the Fill and Stroke editor:

When you use that slider, it creates a filter effect that contains a single Gaussian blur primitive and attaches that to selected object.

There are different kinds of blur. In Gaussian blur the colour from any pixel spreads out according to the Gaussian function (aka. bell curve). That means that the colour of any pixel stays mostly near itself, but some colour spreads out to a large area – in fact the colour spreads out to an infinitely large area, but in practise the contribution can't be seen quite far away. Another example of blur would be bokeh, the depth of field effect seen when some objects in a photo are out-of-focus. It is quite different from Gaussian blur: bokeh depends on the camera lens used, but usually spreads colour evenly to a circular area.

Gaussian blur is also useful as part of more complex filters. Many of the pre-defined filters in Inkscape distribution include blur. Here I'll show a couple examples. First up is drop shadow:

Here the Flood primitive is used to set the shadow colour, Gaussian blur to make the shadow softer and Offset to move the shadow.

Next up is filter that shows only object edges:

The Composite is in mode "out", meaning that we take parts of original image that are not in blurred image.

Last up is lighting filter, which requires a height map. Gaussian blur can be used to create an easy height map.


The SVG sources for the images are available from my filter tutorials repository at GitHub.

Filtering the image background

It's beginning to seem, that my planned timetable for this summer project is way off. Luckily it seems, that I've overestimated the time I need instead of underestimating. So what does this mean exactly? Well, now it's been a full two weeks since I started the project but I had planned five weeks for doing what I've done up to this point. Also, I've planned two weeks for creating renderer support for BackgroundImage and BackgroundAlpha input images. Well, it seems that support for BackgroundImage exists already, it just needed these changes I committed today to enable it.

Well then, about this commit today: support for in-parameter in filter primitives. This means, that now we aren't limited to filtering only the image itself and building linear filter chains. The background image can be taken into account when filtering and building complex filters should be possible too.

There are lots of possibilities that this enables. Next picture shows one of them: frost glass effect. Here I've got an ordinary gaussian blur, but instead of applying it to the object itself, I apply it to the background image. The object itself disappears, all we see is that a square area of the image is blurred.

This support for different input images isn't without problems, though. One really annoying one is with blend filter: it sometimes doesn't use the specified blending mode, but seems to apply normal alpha blending. I don't know yet, if this is a bug in input images support, in blend filter or even in some seemingly unrelated part of code.

To help debugging this, I think I'll reorder my timetable and implement feOffset filter next. This will allow me to use some example images W3C has published and compare renderings from Inkscape to them.