1. Introduction

The main thing to understand is that the object to which the Motion Blur should be applied should have been animated beforehand. This means that the object must follow a Path, or be animated using F-Curves. In fact, the rendered picture will be turned into a 3D model with depth and a speed vector will be calculated for each pixel of the picture. As a post-process, the Composite Nodes editor will add the motions to the final pic in a blurred way. This is why defining the motion of the object (through any mean of your choice, but the use of F-Curves would certainly be the most common) is absolutely mandatory.

Capture-00.pngExample of IPO curves effecting a moving object

A very brief example file of such an animation, based on a path animation, is attached to this post. Please see Attachments below.

Here's how would look a regular animation rendering, without vector based motion blur. The background stays in focus all the time.

2. A step by step of the Vector Blur process

At first, you will need to display the Node Editor view. It can be done either by turning the default 3D view into a Node Editor view, or by changing the type of Screen from the Default prototype to the Compositing prototype. The later choice offers a Screen that is divided optimally for typical work to setup nodes.

Capture-01.png

In the header of the Node Editor, you will find a menu for the view (View, Select, Add, Node) and then three icons: the first one enables Material Nodes , the second one the Textures Nodes, and then the third the Composite Nodes. Please activate the third one, and then check the Use Nodes option.

Capture-02.pngClick on the Compositing nodes mode icon and then activate the Use Nodes button.

  • The default Composite nodes network should be displayed on the screen. Detach the link between the two default nodes by clicking using the LeftMouse button on the Image input (yellow dot) of the Composite node. Make sure all nodes are unselected.
  • Now, use the Add menu to insert a Vector Blur node (Add > Filter > Vector Blur). A new node should appear, titled Vector Blur.
  • Move it between the two previous default nodes, and then start the magic:
    • connect the output Image, Z and Speed from the Render Layer node to the input Image, Z and Speed from the Vector Blur node, using the LeftMouse button to draw the links
    • connect the output Image from the Vector Blur node to the Image input of the Composite node

Capture-03.pngThe Composite nodes are set to deliver a Vector Blur based rendering

Where the Hell is my Speed output???

You don't have a Speed output to your Render Layers node? That's quite the normal default behavior. Please go into the Render menu, and in the Layers panel, in the Passes area, check the Vector option. That's it. Your Render Layers node is now updated with a Speed output.

Capture-04.pngIn the Passes area, check the Vector option

While still in the Render menu, make sure that the Compositing option is checked in the Post Processing panel.

Capture-05.pngThe Compositing option should be "on" by default

You know just have to render your image using F12-Key or your animation using Ctrl+F12-Key.

Here's how would look the same animation, with vector based motion blur enabled. The red sphere stays in focus almost all time, but the background gets blurred because of the camera movement.

3. Some explanations on the Vector Blur node

This node only has a few settings, most of them could work quite well with their default values. But sometimes (if not everytime!), you need to go beyond default settings to get real cool results. There are four settings to know about.

Capture-06.pngThe Vector Blur node

  • Samples: this setting controls the blur effect intensity: the higher the samples, the more blurred the object will look
  • Blur: this parameter will scale the vector speeds calculating from the movements of the object. The visual effect is close to the shutter speed of a regular camera.
  • Speed: Min: if everything is blurred but if some objects are a lot slower than others (or totally static, as the background of the pic, for example) then this parameter will help differenciating high velocity objects from null velocity or slow velocity of the objects. Really useful during camera movement or slight background movements.
  • Speed: Max: if you have extremely fast objects in the scene but if the blur does not render well enough, then you can make use of this parameter to better the vector blur. Note that a 0.0 value means that no maximum is used.
  • Curved: interpolation between two frames is made according a bezier curve instead of linearly

Concerning the use of Vector Blur, a good tip is to put fast moving objects on a Render Layer, slow or static objects on another, and to specify independant parameters for each, for a better control. Moreover, large objects not entirely seen from the camera should be subdivided so that at least some of the normals of the object are actually seen from the camera, or the blur effect could be drawn on the wrong side, compared to the movement.

But there also some limitations that you should know of, due to the fact the Vector Blur is a post-process effect:

  • if an object is vector blurred, its shadow is not!
  • if a vector blurred object is moving behing a transparent glass (Transparency option activated), it won't appear blurred through the glass
  • if a vector blurred object is moving in front of a reflective mirror (Mirror option activated), it won't appear blurred in the reflection

Article written on March the 8th, 2005.
Updated on July the 17th, 2010 for Blender 2.50 Alpha 2. Comments re-initialized.