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In the previous video I showed you how to set up a
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non-linear retime with the TimeWarp node.
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In this video we're going to use that node as a master node
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to control other retimes in your script.
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I will also show you how you can export this data in a way
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that can be used in other applications.
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It's common to end up with several Retime nodes in your
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script that need the same retime data.
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Perhaps you want to use several Kronos nodes,
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each with different settings for better results in
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different areas of the frame or perhaps you have two layers
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that need the same retime applied,
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but you get a better result if you retime each layer
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separately before you merge them.
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Let's connect a Kronos node to this clip, and set the method to Frame.
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We want to connect the Frame property and the Kronos node to the Frame
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variable in the TimeWarp node that we set up before.
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We can do this by opening the TimeWarp node and,
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while holding down the Command key, Ctrl if you aren't on a Mac,
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dragging the graph icon next to the frame variable to
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the frame variable in the Kronos node.
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You will see a little green circle up here in the
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bottom right corner of the Kronos node,
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and if you press Alt or Option and E you can see a green line
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from the TimeWarp node to the Kronos node.
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This signifies that the Kronos node has an expression
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linking it to the TimeWarp node.
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If you copy and paste the Kronos node,
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the new node will also be connected to the TimeWarp node.
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You can connect as many other Retime nodes to the
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TimeWarp like this as you like.
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This way the TimeWarp node becomes the master retime control.
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Any changes there will be applied to all the other Retime nodes in your script.
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You can also apply this retime data to nodes other than Retime nodes.
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In this example,
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I had a Multiply node that I have animated to create a flash
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every time the guy's foot hits the ground.
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I did this animation before I knew the footage was going to be retimed,
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and now that we have the retime I don't want to
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have to reanimate the whole thing.
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I could connect the multiply before the retime,
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but the flashing might mess with the results of the optical flow,
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so I want to apply the effect after the retime,
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but now the flashes don't match the action.
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I can use an expression to apply the retime from the
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TimeWarp to the animation of the Multiply node.
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If we open up the TimeWarp node and hover our cursor over
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the input frame control the tool hint will show us that the
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control is also called lookup.
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The name that shows whenever you hover over a property is the name that
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you need to use in expression to refer to that control.
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If we open up the Kronos node and right-click on the frame control,
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and choose Edit expression, we can see that the expression says,
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parent.
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TimeWarp3.
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lookup.
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This is telling Kronos to use the lookup value from the node
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TimeWarp3 at the parent or top level of our script.
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Because the Kronos node is also at the top level of the
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script we could delete the parent part, and the expression will still work.
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Parent is only relevant when we start working with gizmos and groups.
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Let's copy the rest of the expression.
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Now let's open the Multiply node and choose Edit
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expression for the multiplier value here.
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Because this property has already been animated the
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expression will already have the word curve.
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This means that the node is getting the multiply value from
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the animation curve in the Curve Editor.
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This value changes over time,
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so the node will read the value of the curve on each frame,
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but if we add a set of brackets after the word curve,
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and paste in the expression from the Kronos node we will now
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be telling the Multiply node to read the animation value from
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a different point on the curve.
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Rather than reading the value from the current frame,
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it will now read the value from the frame specified by the retime.
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Now when we play it back you can see that the flashes match the action again.
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As compositors we're usually the first department in the
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VFX pipeline to deal with the retime.
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Once we have it set up, and have got it approved by the editorial department,
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we will probably need to send the retime information to other departments.
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We usually do this by exporting an ASCII text file that
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can be read by other pieces of software.
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To do this open up your TimeWarp node,
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and then right-click on the input frame value,
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and choose File, Export Ascii.
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With the default settings Nuke will create a simple text file with the
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value of the retime curve for every frame on a new line.
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Now if we open the text file we can see the numbers.
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If we create a new empty TimeWarp node and choose File,
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Import Ascii, and select the file that we just created,
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we can see that the new node now has the same values as
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the node we exported the text file from.
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We can now send this ASCII file to other departments.
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So to summarize this module,
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working with speed values is often the most logical approach
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when we're setting up a nonlinear read time,
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but using frame numbers is more compatible with other software.
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Often,
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the editorial department has already decided what the retime should look like,
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and we can use expressions to link retime information between nodes.
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In the next module I'm going to show you some techniques
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for cleaning up the retime artifacts, including using a 3D projection setup,
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and some more uses for expressions.
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