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In the previous video we set up a complex non-linear retime.
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We were concentrating on getting the timing correct,
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but we weren't paying attention to the quality of the retime results,
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but in this module I'm going to show you some of the various approaches we can
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use to remove the artifacts caused by retiming algorithms.
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Guessing the perfect frames for my retime can take a lot of work,
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so it's always important to make sure we have the correct timing
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before we start worrying about cleaning up the frames.
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Show your retime to the director, the VFX supervisor,
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editorial department or anyone else who might have any feedback on
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the timing before you start the cleanup process.
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In this class we are going back to the clip of the
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skateboarders from module two.
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We're going to slow down the clip to half speed.
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This version of the clip runs from frame 210 to 245.
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When we double the length of the clip it's going to run
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to frame 280 of the Nuke timeline,
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so I'm going to adjust our project settings to match that range.
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Based on what we learned in module three,
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I'm going to set up the retime with a TimeWarp node.
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By default,
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the TimeWarp node will add a key frame at the first and last frame of the clip.
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The first frame doesn't need changing,
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but we need to move the second key frame along the
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timeline from frame 245 to frame 280.
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This gives us the correct timing for the shot,
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but we know we can get smoother results by using optical flow,
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so I'm going to use an expression to connect a Kronos node to this timewarp.
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I'm going to connect the source connection of the Kronos node to the Read node,
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and click the reset button next to the Input range of the clip.
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Then I'm going to change the timing dropdown from Output Speed to Frame.
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To add the expression that links this to the TimeWarp node I need to
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right-click on the property and choose Add expression.
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The expression I need is simply TimeWarp1.
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lookup, making sure that I get the upper case T and W in the word TimeWarp.
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Now I have a Kronos node referencing the retime from the TimeWarp node.
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In module two we found that setting the vector detail to one in the
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Kronos node gave us a pretty good result all around,
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so let's do that.
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In Nuke X the foundry has updated the optical flow
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algorithms that are used in Kronos,
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and the quality of results are massively improved from previous versions,
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but because Nuke is trying to create frames that never existed
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the results are never going to be perfect,
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and sometimes the errors can be very ugly and distracting.
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You usually see issues at the edges between two layers moving in different
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directions or when objects appear or disappear completely.
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In this clip the most obvious issues are the road that
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appears at the bottom edge of frame, the skater's arm that is moving too quickly,
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the detail in the background behind the main hero skater,
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and the car that drives into the shot from behind the building.
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Cleaning up retimes can be a slow and endless task.
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Sometimes it's impossible to make everything perfect in the time you have,
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so you have to pick the most obvious issues to fix first,
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and then keep working until you run out of time.
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In this example the first thing we need to fix is the road.
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One of the simplest ways of improving retime results is to use
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different settings for different areas of your shot.
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In module two I showed you how the Vector Detail slider and the
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Strength slider can make a big difference in the results.
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If I duplicate this Kronos node and in the new node
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reduce the Strength slider to 0.
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1 you can see that we get a much less distracting result for the road,
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but overall the results are much less smooth.
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If I draw some loose shapes around the corners of the frame
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we can use a Keymix node to combine the different results
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from the different Kronos nodes.
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I'm going to connect the Kronos with the good
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results for the road to the A input, and the other Kronos to the B input.
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Then I'm going to connect my roto shape to the mask input.
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This isn't perfect,
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but it's definitely a big improvement in a short amount of time.
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In module two I had roto for the foreground skater.
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That made a big difference to the retime results.
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The roto was done on the original plate before the retime had been applied.
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Let's connect the roto to the main Kronos node,
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and set the Matte channel to Matte Alpha.
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Again, this seems to be a big improvement,
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but if you look closely it actually introduces
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artifacts that weren't there before, like these sharp edges,
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and this roto took a significant amount of time to create.
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We're going to use this roto to fix some of the other issues,
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but if you didn't already have full body roto available it
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might not be worth the time or effort to create it just to
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plug into a Kronos node, but as I have it I'm going to use it.
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Sometimes combining different Retime nodes together
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can get you a good enough result,
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but when you start using multiple optical flow nodes your
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script can really start to slow down.
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If you're using this approach you might want to
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render out each different retime,
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and then read them back in and combine the rendered results.
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I still have some cleanup to do,
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so I'm going to render out these two Kronos nodes and use
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the renders for the rest of my work.
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Now that I have rendered out the Kronos nodes,
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and I've done some broad stroke fixes, it's time to get into the little details.
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There are two main techniques that I'm going to show you
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here; both are basic rig removal techniques,
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painting, and patching.
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Both use different parts of different frames or different areas of
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a frame to recreate the areas that need fixing.
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Using the Paint node to fix each frame at a time is relatively simple,
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but it is tricky to create consistent work over many frames.
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It's good for fixing individual frames or areas that are moving
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fast and are quite different on every frame,
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but if the movement is slow and steady you can quickly see popping or
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boiling in your paint work when the results are played back.
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Creating patches takes a lot more work to set up,
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but once you have them working they're smooth and consistent.
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We could spend hours or days cleaning up everything in this shot,
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so I'm just going to show you a few examples of how
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I would use these approaches.
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I'm going to start by using the Paint node to clean up the
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section of the shot where the skater's arm lifts up and
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crosses the skater behind him.
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We can see that the version of Kronos that uses the roto gives us a nice,
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consistent, solid shape for the arm for most of the action,
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but it does have some ugly, sharp edges that need cleaning up.
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Because the arm is moving fast in this section of the shot we
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can use the frames before and after to source clean areas of
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the background to fix these edges.
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Let's fix up frame 243.
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Connect a Paint node to the Read node,
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and choose the Clone tool from the menu on the left of the viewer.
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By default, the Clone tool clones from different areas of the same frame,
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but if we set the source to bg using this dropdown at the top of the
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Viewer we reveal the time offset option labeled delta t.
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This lets us choose to clone from a different frame.
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The arm is moving upwards at this point,
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so if we want to clone a clean area of the face we
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need to source the previous frame.
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So let's set the time offset to -1.
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Everything in this shot is moving between each frame,
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so we need to line up the source frame to the destination frame.
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We can do this using the onion skin feature.
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Press T to overlay the source frame over the current frame.
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Now hold down the Command or Ctrl key and drag on the viewer to line the two up.
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Use the hard edge of the white sunglasses to get
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the lineup as close as possible.
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Hit T again to turn off the overlay.
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Start painting over the edge of the arm.
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This will clone the clean image of the face from the
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previous frame over the areas that need fixing.
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If we tried to paint on the lower edge of the arm we will
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clone in more arm from the previous frame,
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so we need to change the clone source.
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Change the time offset from -1 to positive 1 to clone from the
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next frame rather than the previous frame.
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Hit T again to show the overlay, and line up the new source frame.
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Then paint in the lower edge of the arm to fix the hard edge.
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We can see that the arm on frame 242 looks okay,
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but if you move backwards through the sequence from here the
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arm breaks or doubles up between 232 and 242.
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The truck behind it also breaks up a lot.
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We're going to use a couple of patches to fix these areas.
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We're going to work from the back layer first,
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then restore the foreground layers on top of the clean background,
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so let's start with the truck.
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We're going to create a patch for the truck between frames 232 and 242.
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Frame 232 shows more of the front of the truck than
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any other frame in the sequence, so we can use this frame to create the patch.
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Let's add a FrameHold node, and set it to 232.
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Then I'm going to add a Roto node below the FrameHold node,
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and draw around the top half of the truck,
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and then use a premult to apply this roto shape to the image.
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We need to track this patch over the moving image.
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I would usually try to use a Tracker node to track the movement,
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and then apply that to the patch,
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but because of how the skater covers the area there
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isn't much that we can track.
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We only need to use this patch for 10 frames, so we can track it in by hand.
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I'm going to add a Transform node below the Premult node,
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and on frame 232 add a key frame to the translate,
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rotate, and scale properties.
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We can also hold down the Command key or Ctrl if you aren't on a Mac,
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and drag the control handles to the middle of the patch.
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Now we can merge this over the main image.
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On this frame we can set the mix value and the merge
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node to 0 and add a key frame.
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Then go to the next frame and set the mix to one.
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If we look before and after this Merge node we can now see that
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the patch doesn't line up with the truck behind.
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We're going to need to manually line that up on each frame,
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but because the camera was pulling backwards at a reasonably
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steady pace the scale is going to change,
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as well as the position.
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Because this is a steady change we should set a key frame on the last frame,
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and then hopefully we won't need to adjust the scale on every frame.
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Let's go to frame 242 and line up the patch on this frame (clicking).
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Once the patch is lined up we can set the mix property in
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the Merge node to 0 for this frame, and go back 1 frame,
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and set it back to 1.
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Now we have the patch onscreen from 233 to 241,
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and we have lined up the patch with the frames before and after that.
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The next step is to go through the eight frames in that range,
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and manually line up the patch on each frame.
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If we constantly compared the before and after we'll make sure that
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we get the lineup as close as we can (clicking).
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Once it's done it should look something like this.
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Now we need to restore the skater back over the top,
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and fix some of the issues with the arm as we do it.
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Because we have the full body roto of the skater we can mask
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the skater and run a premultiplied version of the skater
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through another copy of the Kronos setup.
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Because we have removed all the surrounding details this actually
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produces a much cleaner result of the skater.
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There are still a few bad edges,
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and on frame 239 there is a weird artifact in the alpha channel,
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but we can clean these up with a few paint nodes,
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and place this layer over everything else.
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If we compare the chart before and after we can see
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that we have a much better result.
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There are still lots of areas that need fixing,
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but we can use similar techniques to these to fix them all up.
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In the next video I'm going to show you how we can use camera
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tracking and 3D projections to help with the process.
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