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Studio Artist Workflow: Autorotoscoping, Part 2

Working with source movie alphas By Dave Nagel
In our last jaunt through Synthetik Studio Artist, we took a look at a basic workflow for automatic rotoscoping. The goal was to create a hand-painted effect over a live-action QuickTime movie. This time around we'll look at a method for working with source footage that contains an alpha channel, including how to set up Studio Artist to deal with alphas, how to use alpha-based masks and incorporate them into a Paint Action Sequence and how to output a final movie with a new alpha channel. 

Before we get started, if you have not read the previous installment in this series, I highly recommend that you do. It gives you a basic outline for creating paint Action Sequences and then processing movies with them. This is key to understanding the information we'll discuss today. You can read the previous tutorial by clicking here.


Alpha preparation
Studio Artist's base configuration does not take alpha channels into account. For this reason, we need to do a little preparatory work by setting a few preferences and display options. After you've set these options once, you won't need to go back and set them again. But you must set these options in order to use alpha channels in Studio Artist properly for any type of project.

The first setting is at File > Preferences > Layer Preferences. Change the Default Save Output Alpha to "Layer Alpha," and set the Paint Alpha Fill Status to "On."



The second option you need to set is at File > Preferences > Movie Compression. This setting, of course, determines the final movie output format. In order to create an animation with an alpha channel, you'll need to choose a codec that supports alpha channels and set the colors to "Millions of Colors+." (The "+" is the alpha channel. Any codec that offers this color depth option supports alpha channels. Common ones include "None" and "Animation.")



The third is at File > Preferences > Movie Stream. Set the Out Alpha to "Layer Alpha" or "Source Alpha." (Source Alpha uses the alpha channel of your source image or movie. Layer Alpha uses the transparency of the current layer as an alpha.)



The fourth setting is in File > Preferences > Paint Action Sequence Configuration. This is an absolutely critical setting to change if you intend to use an alpha channel as the basis for a mask. To do this, change the Mask Track option to "On." You must do this before recording anything in your Paint Action Sequence.



Finally, you should go to the Layer window (Canvas > Layer Window) and turn the Alpha Status to "On."



Incorporating alpha-based masks into Paint Action Sequences
Now you're ready to start creating your Paint Action Sequence (PAS). This process will be almost identical to the one we discussed last time around, with the exception that, as part of the PAS, we need to tell Studio Artist to use the source movie's alpha channel as a mask on our canvas.

Step 1. Start recording your PAS, as described previously.

Step 2. Making sure you have the Mask option above the canvas window set to "None," click on the hollow circle next to the background fill pull-down menu in order to erase the canvas completely. This will add four steps to your PAS, all having to do with erasing all of the contents of the layer.



Step 3. Next, set the background to whatever color (or type) you wish by clicking the arrow next tot he background pull-down menu or by selecting an option from that pull-down menu.

Step 4. Choose Canvas > Set Region Selection To > Source Alpha Image. This sets Studio Artist's selection of the canvas to the source alpha.

Step 5. Then select one of the mask options from the Mask pull-down menu above the canvas. "Paint" constrains everything to the boundaries defined by the alpha channel. "Path" constrains the path of a stroke to the alpha channel boundaries but also allows paint from that stroke to spill out of the alpha region. And "Litho" is used to create complex wet/dry masks for specific types of paint brushes. (We'll get into that another time.) For now, choose between Paint and Path, and you'll see your alpha appear as a mask on the canvas.

From this point on, anything you do to the canvas will happen only within the boundaries defined by the mask.

So now simply go ahead and create the rest of your PAS using combinations of paint strokes and filters. In my case, I'll be creating a sort of ink and wash effect, which I'll later composite onto a layer of paper in a separate motion graphics application. (Apple Motion 2 for this project, in case you're wondering.)

Alpha output
Finally, when you're done creating your Paint Action Sequence, you have one more thing you need to do before you can process your movie,. You need to tell Studio Artist how you want the final alpha channel to be generated. To do this, go to Action > Process File Settings. There you can choose one of several options, only two of which are really relevant here. The first is "Source Alpha," which outputs an alpha channel identical to the one contained in your source movie. The second is "Layer Alpha," which will create a new alpha channel based on the contents of your layer. This is useful for those times when you use a "Path" mask type and want to allow the paint spill to affect the shape of your alpha. This isn't always a good option, as some brushes will create a mushy edge on the alpha channel, but there are other times when it can be effective.

When you've chosen your alpha output type, click OK, and you're ready to process a movie with your PAS. The example below shows my alpha output composited in Motion after processing in Studio Artist.



Next time around, we'll look further into rotoscoping workflows in Studio Artist, addressing the process of manually rotoscoping movies stroke by stroke. In the meantime, if you have any further questions, be sure to visit me in the Synthetik Studio Artist forum by clicking here.


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Related Keywords:synthetik studio artist, rotoscoping, paint effects, alpha channel

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