Getting Started with Adobe After Effects - Part 6: Motion Blur


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Getting Started with Adobe After Effects

Getting Started with Adobe After Effects - Part 6: Motion Blur

Apr 30 2013 12:00AM by jamesel   

Motion blur is incredibly important in both motion graphics and visual effects. This lesson will be broken into two sections, the first covering mograph and the second covering VFX.

Section 1 – Motion blur in Mograph

Often in motion graphics we have a need to show objects, graphics and text moving around the screen very quickly. It not only has to look good on a still frame, but it has to move fluidly as to not be jarring and distracting to the viewer. The problem is that most of our work is going to be presented at 25 frames per second and if we have text flying around the screen with no motion blur, it will be very jarring.

As an example, let’s create a new camera and text layer. Type some example text and under the text options, Enable Per-character 3D. This allows each individual character to be in a different position in 3D space. Now choose to animate the position from that same menu, and set the position to a value like -500 in Z (this will make the text come so close to camera it will be off screen).

Enable Per-character 3D

Go to frame 0 and keyframe the Offset (under the range selector) to -100%. Now go to frame 20 and set it to 100%. Under the advanced tab, change the shape to ramp up and set the ease low to 100%. Preview your work.

It looks very jarring because the text fly’s by so quickly. Depending on your composition settings you’re probably viewing this at around 24, 25 or 30fps. This is the reason the animation looks jarring. Press control K to get to the composition settings and change the frame rate to 60fps (or higher if your monitor supports it). You’ll notice that the animation looks a lot smoother.

Unfortunately, this won’t fix the problem because our work has to be viewed at 24, 25 or 30fps. So the solution is to use motion blur.

Change the composition settings back to 25fps. To enable motion blur we need to turn it on for each layer we’d like to have motion blur, and then turn it on inside the composition.

composition setting

This is because motion blur can be expensive to compute and often you’ll only want it turned on for your final render. Now that motion blur is on and working, preview your work. It will be much smoother, even at 25fps.

After Effects is able to generate this motion blur because it knows where the letters are in 3D space. It calculates where they came from, where they’re going and how fast.

Notice how in the above image the O looks odd. This is because we don’t have enough samples for our motion blur to generate smoothly. To increase the quality press control + K to get to the composition settings, and go to the advanced tab. Once these settings have been changed they will only affect that composition, so each composition in your project can have its own settings.

configuration settings

The samples per frame is what controls the quality of the motion blur. It defaults to 16 and the adaptive defaults to 128. This means that after effects chooses how many samples it thinks the motion blur needs (a number between 16 and 128 in our case). From the second image back we can still see our motion blur doesn’t look smooth. Go ahead and increase the samples to 64 and you’ll see near perfect motion blur.

sample per frames

Sometimes you’ll need to control how much or how little motion blur should be applied. Whilst this is affected by the compositions frame rate, we can also influence the amount of motion blur using the shutter angle. By default the value is set to 180 degrees. Lower shutter angles create less motion blur. If we set the angle to 0 degrees there would be no motion blur. Go ahead and try it.

Now experiment with increasing the shutter angle to something like 1,000 degrees. It creates a very distinct and ghost like aesthetic.

shutter angles

Motion blur in motion graphics is very important and should always be turned on unless it detracts from the style. For example, motion blur in a sketchy, handrawn styled piece may look out of place.

As mentioned before, motion blur can take a while to compute so I often have it activated on each layer, but turned off in the composition. Whenever you go to render that composition (which we’ll be discussing in lesson 10) after effects will automatically turn on motion blur for you so you don’t have to worry about remembering each time.

Section 2 – Motion blur in VFX

Footage shot of cameras often have motion blur. If it’s your job to composite elements into that live action scene, you’re going to need to add motion blur to your elements. The trick is how much motion blur to add.

Remember that we control the amount of motion blur inside the advanced tab in the composition settings using a shutter angle. Most modern day cameras measure this using shutter speed rather than angle, but there is a fairly easy way to convert to two. For this to work you’ll have needed to make a record of what shutter speed was used when filming the shot. Then simply convert it to a shutter angle using this formula:

(frame rate x 360) / shutter speed

For example: a frame rate of 25fps filmed at a shutter speed of 1/100th of a second.

(25 x 360) / 100 9000 / 100 = 90 degree shutter angle.

If you didn’t make a note of what shutter speed you used, you can probably make an educated guess and then eyeball the motion blur until it looks right.

Note that this only works for 2D elements moving inside of an after effects scene. Sometimes we need to composite stock footage which appears to be moving, but is static inside the after effects composition. An example of this might be blood. If I were to import some footage of blood and have it fly around the composition, then motion blur would be applied if I turned it on.

motion blur

But what if I want to have the blood static but still be blurred. We can use an effect called CC Force Motion Blur, which will analyse the footage and make a judgement on which way different elements are moving.

CC Force Motion Blur

Here we have options for the number of samples (or quality), options for shutter angle and whether or not we want to override the compositions shutter angle.

Be warned that footage can often trick this effect into producing inaccurate results, but when it does work, the results can be quite pleasing.

CC Force Motion Blur

In the next lesson we will be covering masks and rotoscoping.


jamesel
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