Friday, 3 April 2009

Adobe After Effects Tips #6

Working with time, Motion Blur

Extracts from the book Adobe After Effects 7 Studio Techniques

- In the Time Stretch tab (Right-click on a layer -> Time -> Time Stretch) you can make the layer run slower (more than 100% in the Stretch Factor dialog) or faster (less than 100%).

- You can reverse a layers time by hiting Ctrl+Alt+R or typing -100% in Time Stretch tab. You can enable Time Remaping by hiting Ctrl+Alt+T. You can also freeze a layer (showing only the freezed frame all the time) by choosing Layer > Time > Freeze Frame command.

- When you make a video slower, the motion appears jerky, it's because some frame are duplicated (e.g. when you make it 50%, every second frame is duplicated - it's just not there, for the camera shot on a certain frame rate and it doesn't have more frames to show). A basic solution is to activate Frame Blending (Right-click -> Frame blanding). You have to options there, Frame Mix and Pixel Magic. Frame Mix basically blurs two frames together. However, the option you want to choose is Pixel Magic - it creates unique frames scanning its pixels and warping them together. You can also find the Frame Blending icon in your timeline Columns.

- You can also make it faster or slower (= stretch it) to a specific point. Just go to a frame you want to stretch it around and then in the Time Stretch tab choose Current Frame.

- You can set an exact amount of motion blur in your composition. Go to Composition Settings -> Advanced and choose your numbers in Shutter Angle (how long the shutter is open, higher number will make more blur) and Shutter Phase (when the blur starts, 0 means it starts when the object starts moving, if you add bigger number, the blur occurs some time after the start of animation).

- A formula for calculating Shutter speed properly is following: shutter speed = 1 / frame rate * (360 / shutter angle).

- Setting Shutter Phase to 50% is useful when motion tracking - the image is less blured.

This was the last part of my After Effects tips focusing on workflow. I hope it helped at least a bit.

- Martin

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