2.3 Fixing Film Warp
In our previous extended study material for the Level 2 PFclean training (2.2 Fixing a Film Tear) we were looking at CHANTI_SHOT_003 where we tackled a tear running across the frame using the Fix Tear effect in the Workbench. In this extended study tutorial we will be looking at the second Chanti shot and tackling another common issue, film warping.
NOTE: To follow along in this tutorial, you will need access to the clips provided for the class. If you don’t have them to hand, below you will find the link to download the footage again.
01. How De-Warp Works
Let’s start where we left off on the previous tutorial and click back to CHANTI_SHOT_001 either by clicking on the clip in the Worklist or the back arrow at the top left of the GUI. Let’s play through the clip so we can see the warping. There is a lot of squash and stretch distortions, something that wouldn’t be fixed if we used a Stabilise effect. Instead we are going to use the De-Warp tool to fix problem. In the effects stack add in the De-Warp tool to the clip.
The De-Warp tool works works using two grids that control the warping of the clip:
The Source Grid is placed on the original warped clip and edited so that its vertices are in useful positions that can subsequently be tracked or identified easily from frame-to-frame.
The Destination Grid specifies where the vertices will move to, and hence control the way in which the original clip is de-warped. The Source Grid and Destination Grid can be copied and edited individually. The Source Grid can also be tracked throughout the clip to reduce the burden of placing grid vertices individually. Put simply Source and Destination are a bit like before and after. In most cases you will be dealing with vertical stretch and squash.
At the bottom of the GUI we have the De-Warp settings in the edit panel. Let’s familiarise ourselves with some of the settings before we start to De-Warp the clip.
02. Grid Controls
The first panel contains the Grid Control . In here we have the Source button which will display a grid overlaying the original footage and this is what we will be using to track the footage. The second button is the Destination button and that will show us the results of our De-Warp as well as allow us to make additional refinements to the clip.
Underneath these we have the Copy Src->Dest and Copy Dest->Src which are used to copy the Source Grid to the Destination Grid, or vice-versa. When the Source Grid and Destination Grid vertices are in the same position, no de-warp will occur.
The X Vertices and Y Vertices entry boxes specify the number of vertices that are in the Source and Destination Grids. A higher density of vertices will mean that the grid will track and render more slowly, but will be able to represent complex warps more accurately.
The Warp Channels controls which of the red/green/blue channels will be De-Warped. Any combination of channels can be used and this can be of great benefit when working with tri-strip material in which warping can occur in only a single channel.
03. Grid Editing
At the moment the grid has been drawn over the entire area of our footage including the soundtrack area on the left hand side. We are going to redraw the grid just over the image area using Draw Grid button inside of the Grid Editing. Go ahead and press the Draw Grid button and starting at the top left of the image area, draw a new grid.
From here we could copy our newly drawn grid to the destination grid using the Copy Src->Dest button and begin tracking, but we are going to make some more adjustments to really refine the De-Warp. If we imagine each one of the intersecting vertices as a tracking marker we are going to want to align them to the best possible position in the frame to give the best results. We are going to be looking for points in the frame that shouldn’t be moving, things like the pots and pans and the wooden shutters in the background the goal is to get a good spread of points across the frame. Once we have found some good points by playing through the clip a few times we are going to use the Move Vertices button to line up our trackers.
NOTE: Moving vertices over a number of frames will animate them.
Press the Move Vertices and begin to reposition each of your vertices. Continue to do this until you have a good spread across the frame in positions that will track well.
When we have finished moving the vertices we are going to deselect the Move Vertices button by clicking on it. This will prevent us from accidently moving vertices. Now our grid is setup we are going to copy the changes in the grid to the Destination grid by pressing the Copy Src->Dest button and confirming.
Let’s just check both Source and Destination are indeed the same. We can do this by pressing the Destination button and then back to the Source button. Both source and destination at this point should look exactly the same.
The next stage is to select the vertices we would like to track, which we can do by using the marquee tool. Click on the Marquee button and select by drawing a square around the vertices. This will now be highlighted in blue. You can draw multiple squares by holding the shift key when you are making a selection. Let’s go ahead and select all the vertices we moved.
04. Grid Motion
nce we have selected the points we want to track we are going to move onto the next panel called grid motion. The first two drop down menus Horizontal and Vertical specify how grid vertices can move in the horizontal and vertical directions. In our case we are dealing with squashing and stretching of the image so we will mostly be concerned with vertical motion. Let’s change the drop down next to Horizontal from Move Vertices to No Motion.
We are then going to change Vertical to Move Row to indicate that all of the vertices in a single row of the grid will move together in the vertical direction. The other options in the drop down menu are Move Vertices for moving each vertex individually and move grid which will force all vertices in the grid to move in the same way.
The Sensitivity parameter controls the size of the area around each grid vertex that is used to track Source Grid points. The sensitivity areas can be displayed by pressing the Show Sensitivity button in the Display part of the De-Warp interface. We are going to leave this at it’s default position.
The final thing we are going to look at in Grid Motion is the Lock Unselected Vertices and Ignore Global Motion buttons. Lock Unselected Vertices is used to control how anything we haven’t selected with the Marquee tool behaves when tracking. When the button is not pressed, unselected vertices will have their motion estimated using nearby vertices that are being tracked. This is useful when the Source Grid extends up to the edge of the frame, which means it becomes harder to track the edge vertices accurately
The Ignore Global Motion button is there if we were to De-Warp a shot with a pan or zoom in it. For this example we are going to leave both unselected.
Starting from the left we have a button for tracking backwards. In the middle there are buttons for tracking forwards or backwards one frame at a time. Finally on the right we have the button for tracking forwards which is what we will be using.The tracking process will take a bit of time depending on the speed of your system.
Let’s make sure we are on the first frame of the shot. In the Grid Motion panel press the button with the two forward arrows on to track our grid forwards. Once complete we can have a look at the results by pressing the Destination button in the Grid Control panel.
button to cache the clip. As we can see from the results it’s done a great job and the only thing that would be left to do is tidy up the edges either with a Paint tool or using a Pan And Scan.
NOTE: You can only track from the Source not from the Destination.
06. Further Refinement
Work on the clip doesn’t have to stop there we can continue to make adjustments to the clip after we have finished tracking. Making sure we are on the Destination grid we can use either the Move Vertices or Move Region buttons to tweak areas that are still distorting. These changes will be auto keyed and can be animated over the duration of the clip to correct for any minor distortions not fixed by the tracking.
Warper points can be used as additional tools for warping the clip. Warper points can be positioned anywhere in the frame, and act after the Source/Destination grid warp has been applied.
To create a new warper point, switch off both the Source and Destination Grids, and use the Add Local or Add Global button to draw a warper point that has either local or global effect. To draw a warper point, click and drag with the left mouse button to place the point centre and the area of influence. Once placed, dragging the centre point will start to warp the image.
NOTE: when adding a new point, the image will temporarily revert back to its original state so the warper point can be placed accurately. Once placed, the warper points can be edited using the left mouse button.
In the next tutorial we will be looking at the Colour Balance effect to tackle another two common film artefacts fade and colour shifts.