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Lesson 7. RotoShape > RotoShape Exercise

RotoShape Exercise

Your mission, if you choose to accept it, is to create multiple, animated rotoshapes so that the garbage surrounding the spaceship in the next example can be removed.The shapes that you create on a frame-by-frame basis will be used to place the ship on a uniform green background so that the ship can be eventually extracted and placed on a different background using a keyer. You'll learn about keying in Lesson 10.

Choose File > New Script and answer No when prompted to save the script.

FileIn the ship clip from the Lesson07 folder.

Take a look at the ship clip in a flipbook.

In the Globals tab, click the Auto button to the right of the timeRange button.

Click on the Home icon at the bottom-right corner of the interface.

The Time Bar is automatically set to the Globals timeRange.

Click the Flipbook icon.

As you can see, the ship is suspended by a wire from above and a stand from below. The green screen is also too small to cover the entire area traveled by the ship. The first step is to create a loose rotoshape for the ship.

From the Image tab, select a RotoShape node.

Click on the left side of the ship clip so that you can view it while drawing the shape.

It will be helpful to set the resolution of the RotoShape1 node to be the same size as the ship clip.

In the Rotoshape1 node, set the width to 549 and the height to 467.

I have no idea how this clip ended up at this bizarre resolution. But hey, did I mention that Shake is resolution independent?

Go to frame 1 and turn on the AutoKey button under the Viewer.


There's nothing worse than rotoing a shape and forgetting to turn on the keyframe button. Turning the keyframe button on will ensure that any changes you make to your shape will be animated over time.

Draw a loose shape around all of the unnecessary rigging.

Close the shape by clicking on the first knot.

Your shape should look something like this:

You will need to adjust the shape every five frames or so, keeping the rotoshape line outside the spaceship but on the green screen. Move to frame 6 and adjust the shape.

Continue to adjust the shape at frames 11 and 16.

You will need to refine the rotoshape starting at frame 18 because the ship moves off of the green screen.

Refine the rotoshape starting at frame 18 so that it exactly matches the edge of the ship where it extends beyond the green screen. You will need to change the shape on each frame from 18 through 30 to match the edge of the ship as it moves off of the green screen. See the following image.

You want to make sure that all nongreen areas are masked out by the shape.

Finish keyframing the first shape.

For your reference, or if you feel like cheating, I have prepared three scripts of this exercise at various stages of completion.

If you would like to see what I have done up to this point, select File > Add Script and select ship_roto1.shk from the Lesson07/scripts folder.

Add Script adds to your current script. In this case, one node called RotoShape_Pt1 is added to the Node workspace. Feel free to compare it to what you have done so far. Next, proceed with the more detailed roto work. You will create two new shapes that will cover the rigging above and below the ship.

Go to frame 1 and click the Add Shapes button.

Create a shape for the rigging on the bottom of the screen and close the shape by clicking on the first knot.


If at any time the transform controls of a shape interfere with the editing of the points of another shape, move the offending transform controls out of the way by Ctrl-clicking and dragging them out of the way.

Click the Add Shapes button again.

Create a shape for the rigging on the top of the screen and close the shape by clicking on the first knot.

Continue to adjust the top and bottom shapes at various frames until all necessary frames have been adjusted.

Compare your roto to my version by selecting File > Add Script and selecting ship_roto2.shk from the Lesson07/scripts folder.

When you get to around frame 18 and later, you will notice that the top and bottom shapes intersect with some exhaust ports. In the following image, the white cursor is hovering over the lower one.

Rather than trying to roto around the exhaust ports, it will be easier to create a new, black shape that will knock out a hole in the rigging shape.

Go to frame 18 and click on Add Shape.

Draw a shape around the exhaust ports at the bottom of the ship and close the shape by clicking on the first knot.


When you have multiple shapes that overlap each other, you may need to zoom in on the knots you want to move, especially if they are near other shapes or knots.

Fine tune the points so that the shape matches the exhaust ports.

Click on the left side of the RotoShape1 node to view it.

Right-click on one of the knots of the exhaust port shape and select Black.

The exhaust port shape now knocks a hole in all other white shapes that intersect with it.

Go back to viewing the ship clip.

Animate the exhaust port shape during the time that it intersects with the rigging shape.

Create another black shape for the upper exhaust port and animate where necessary as you did with the lower exhaust port.

Okay, it is hours later and you are finished rotoing the ship using multiple shapes. To see how well you have done, layer the color green over the ship clip.

Highlight the ship clip and choose KeyMix from the Layer tab.

The KeyMix node mixes two images together through the specified channel (usually a mask) of a third image. You can control the mix percentage and also invert the mask. The ordering of the images in a KeyMix node are background, foreground, and matte.

From the Image tab, click on the Color node.

Connect Color1 to the middle input of KeyMix1 and hook RotoShape1 into the far-right input of KeyMix1.

Your tree should look like this:

View and edit KeyMix1.

The composite is not working yet because black shapes in the rotoshape are not properly represented in the alpha channel. By default, KeyMix uses the alpha channel to control the composite, so it needs to be changed to one of the color channels.

Select G for the channel parameter.

Edit the parameters for Color1 by clicking on the right side of the node.

Click on Color1's Color Picker and then click on the green screen in the Viewer.

Your image in the Viewer should look like this:

Compare your final version with mine by selecting File > Add Script and select ship_roto3.shk from the Lesson07/scripts folder.

So, what do you think? Whose version is better? Well, thank you. I appreciate that. You are so kind.

Make a flipbook of KeyMix1 to see the result of your rotoshape animation.

Well, there you have it. The garbage around the ship is replaced with the green color selected from the Color Picker, and the ship is ready to be placed over a different background using a keyer. You'll learn more about keying in Lesson 10.

As a roto artist in training, you have gained valuable knowledge. You might even be able to make some money. RotoShape animation is an essential skill for every compositor; most large post-production companies have dedicated artists to create animated mattes.

Quit Shake.



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