What is an embedded alpha channel?

An embedded alpha channel can be though of as a selection mask that is associated with each frame of a movie file. Each movie frame consists of 3 RGB image channels and 1 alpha channel for the mask associated with the frame. Each pixel in the alpha channel is used to define percent coverage for the associated RGB pixels values.

An alpha value of 0 means 0% coverage, so the pixel RGB values are totally masked. An alpha value of 255 means 100% coverage, so the pixel RGB values are fully displayed. In between alpha values will define partial transparency for the pixel.

The ARGB frame and it’s associated image channels will probably be additionally compressed by whatever compression type (referred to as a codec) you have chosen for your movie file output. Keep in mind that not all compression types support embedded alpha channels. So if you are interested in including one in your generated movie file you need to choose a codec type that supports embedded alpha. This is discussed in more detail below.

How to generate an embedded alpha channel

Previous versions of Studio Artist had separate movie settings dialogs for movie stream and for action processing movie output. In version 4 all of the various movie output preferences are located in the movie tab of the main preferences dialog.

The Embedded Alpha preference option determines what the generated movie file’s alpha channel will be set to. If you don’t care about embedded alpha channels set this option to full on. If you want the movie alpha to match the current layer alpha channel you would set it to layer alpha as shown in the screen snapshot below.

The movie compression codec settings are accessed via the File : Movie Codec Settings menu dialog shown below.

Note that not all compression codec types provide support for embedded alpha channels. If you don’t choose a compression type that supports embedded alpha then the embedded alpha movie preference option is ignored.

For example, the Animation codec with ‘millions of colors’ depth does not support alpha. But the Animation codec with ‘millions of colors +’ depth does support embedded alpha. So this compression type is a good one to use for testing when working with generating embedded alpha channels if you get confused.

Movie Stream vs Action Processing Movie Output

The embedded alpha setting is used for all of the action menu movie processing output options. However, if you are outputting a movie stream as opposed to running an action movie generation menu there’s an additional menu flag you need to set if you want the embedded alpha movie preference setting to be used for your generated movie stream. The menu flag is the File : Stream : Flags : Use Alpha Gen Movie Pref menu flag shown below.

Using the embedded alpha preference options for movie stream generation can slow down a fast auto write movie stream, so this is the reason why there’s a menu flag to turn it on or off for movie stream output.

For more information

Here’s a tip that discusses wortking with alpha channels in layers.

Here’s a tip that discusses processing a movie file with a PASeq.

Here’s a tip that discusses working with Movie and Image Streams.

Movie Streams provide an alternative to the Action generate movie command menus for generating movie files. They are a little more difficult to use than the batch processing oriented Action menu commands, but also give you more flexibility over how individual movie frames are generated. An Image Stream is essentially the same as a movie stream, except the individual frames of the generated movie are output as individual numbered frame images into a folder. You can think of a Steam as an open movie file that frame images can be written to either manually or at specific times you specify.

To open a new movie or image stream use the appropriate File : Stream menu as shown above. The other stream menu options will be grayed out until you have a stream open. The Write Frame menu lets you manually write the current canvas view as a new frame in the open stream. The Flags menu options let you turn on automatic frame writing for different situations.

AutoWrite is an option that when turned on will automatically write frames based on a timer. You can set how often frames are written out when AutoWrite is enabled in the Movie Preferences. The screen snap of the movie preference dialog below shows that AutoWrite is setup to write frames at 15 frames per second when AutoWrite is enabled.

Enable Always AutoWrite will start writing frames immediately as soon as it’s turned on and will continue to do so whether you are doing anything in Studio Artist or not until it is turned off.

Enable Gated AutoWrite will only write out frames when you are doing something (like painting manually or automatically, or after a processing operation like an image operation is complete). The Gated AutoWrite option is useful because otherwise you may generate movie files with extended periods of inactivity.

Enable Write on Op End will automatically write out a frame whenever an operation (manual or automatic) in Studio Artist is completed. So if you had draw a paint stroke for example, you would get one frame written out when the stroke was finished drawing. The difference between this and AutoWrite is that AutoWrite could write out frame continuously as you did the drawing so the resulting movie would see the stroke being drawn as opposed to just popping into a new frame fully drawn.

Enable Write on PASeq Cycle will automatically write out a frame at the end of a complete Paint Action Sequence (PASeq) cycle (when the PASeq has finished all of it’s steps).

Enable Write on Gallery Show Cycle will automatically write out a frame at the end of a complete Gallery Show performance cycle. For more information on the new gallery show feature check out this tip.

When you are finished writing frames into your open movie stream you need to close it before you can view the generated Quicktime Movie File. Depending on your Movie Preference settings the finished movie file may open in the Quicktime Player for viewing when the open movie stream is closed if that preference option is turned on,

Generated Movie Timing

The actual playback rate of the generated Movie Stream is whatever you have specified in the File : Movie Codec Settings dialog. So if you have the Movie Codec Settings configured to generate a 10 fps movie then the generated movie stream will play back at 10 frames a second.

Be aware that the AutoWrite FPS preference setting is the timing for how often frames are written out in real time. Not the playback rate for the generated movie file. So if you AutoWrite frames at 20 frames a second and have setup the Movie Codec Settings to generate a movie that plays back at 10 frame per second, then the screen drawing you captured in the movie stream will appear to playback at half speed.

Loop Action

Loop Action is a new feature in version 4. When loop action is turned on using the Action : Run Loop Action menu command the current Studio Artist operation is run repeatably until loop action is stopped by pressing the space bar. You can import new presets when loop action is running and they will seamlessly be executed when the loop cycles. Using the favorites Preset Toolbar is a great way to quickly move between different presets when in loop action.

If you have an open movie stream and the Stream: Enable Write on Op End menu flag turned on then at the end of each loop in a loop action cycle a frame image will be written out into the open movie stream. There are also menu options under File : Source Settings that can be turned on to enable the source movie to frame advance while loop action is running.

The combination of loop action and movie streams provides a great way to use Studio Artist like a live video synthesizer where you can dynamically edit parameters in real time and see the effect on the continuously generated screen imagery while recording this generated imagery into a  movie file.  Using AutoWrite will record frames in real time. Using Enable on Op End will record frames after they are fully generated, and may be more effective if the operations you are running are slow.

This is because the final timing of the generated movie stream is a function of the FPS setting in the File : Movie Codec Settings dialog. The AutoWrite rate is just the rate the frames are written out at, it does not specify a playback rate for the generated movie file. The perceived rate of motion events occurring in a movie stream is a function of the interaction between the AutoWrite frame dump rate and the FPS playback rate of the generated movie file.

You don’t necessarily have to open the movie you want to process as the Studio Artist source. However it is useful if you want to preview the processing prior to starting a complete render run or if you are building the PASeq from scratch. To open a source movie with a new canvas you just run the File : New Source and Canvas menu command.

A standard file dialog will appear. Use this file dialog to choose the source movie you want to process.

After you choose the source movie you want to process a sizing dialog will appear. This dialog shows you the size of the source movie you have chosen and lets you specify the canvas size you will be working with. The canvas size does not have to match the source size unless you want it to. This is useful because you can work with small source movies and generate much larger processed output movie frame sizes with full resolution detail if you want to. So you could take standard def digital video and created processed footage at HD resolution.  

Choose the size of the canvas and resulting processed movie frame size you want to work with.

Now you need to either record a new Paint Action Sequence (PASeq) from scratch. Or import an existing PASeq preset.  We will use an existing PASeq for this example. A good place to start is the Process Movie Tutorial presets in the Default PASeq preset collection. Use the Operation : Preset : Paint Action Sequence preset menu to bring up the Preset Browser palette in Paint Action Sequence operation mode.

Choose the Default collection and the Process Movie Tutorial category. Now choose the particular preset you wish to work with. For this example we’ll choose the #4 preset example.

If the Paint Action Sequence palette is not currently in the workspace use the Windows : Paint Action Sequence check menu to bring it into the workspace.  

Each action step in the PASeq is a specific Studio Artist operation. The example PASeq starts by setting the canvas to white in the first action step. There are then 4 additional paint synthesizer AutoPaint steps. Each of these paint steps intelligently adds paint strokes to the white canvas to build a painted frame. You can press the Play button to run the PASeq on the current source movie frame to preview the processing effect for a single source frame.

When you are ready to process the entire movie, run the Action : Process a Movie with a Paint Action Sequence : Source to Movie menu.

If you don’t want to process your existing source movie there are other options like the Movie to Movie one that lets you specify which movie you want to process as well as resize the canvas.  Choosing the Source to Movie option will use the existing source movie and canvas size.  You will notice that there are other options for processing a folder of images to a movie or a movie to a folder of processed frame images, as well as Batch Process options that let you process a folder of source movies with a folder of PASeq presets.

You will be asked to specify your output movie file name and location using a standard file dialog. Name your output movie with this standard file dialog. If you press cancel rather than ok an output movie will not be created but the actual processing will continue and be previewed on the screen until you manually stop it. This is useful if you want to preview processing several frames before committing to a complete render run.

The movie processing will occur frame by frame until finished. The left side of the status bar at the bottom of the workspace will give you an indication of which frames are being processed.

If you need to stop the movie processing you can do so by pressing the spacebar. You will get a dialog asking you if you wish to stop processing or continue. Press the appropriate button.

Depending on your Studio Artist preference settings when the movie processing is finished it may be opened in Quicktime Player to preview.

There is also a preference setting to pass through the source movie audio to the processed output movie which can be turned on if you want this to happen. Be aware if you do this that the output movie frame timing needs to match the source movie frame timing if you are doing this. Your Movie preference settings give you the option of passing through the source movie timing or using the movie compression dialog fps settings.  Passing through the source timing is essential if you are passing through the source audio and will generate an output movie with exactly the same frame timing as the source movie.  

Using the MovieCodec FPS setting for Process Frame Timing will retime the individual movie frames to the FPS setting specified in the Movie Codec Settings dialog.  This option is useful if you want to change the movie playbackrate in the processed output.  

The Movie preferences also include controls to skip or repeat source fames as well as repeat output frames.  These can also be useful if you are changing the playback rate or number of frames in the processed output movie.  Hand drawn animation is typically rendered at 10-15 frames per second (fps), so slowing down or skipping frames may be useful if you are creating auto-rotoscoped paint animation and your source footage was shot at 30 fps as an approach to cutting down on flicker in the painted animation.

You can set your movie compression codec used to encode the processed movie frames using the File : Movie Codec Settings menu with brings up a standard Quicktime codec dialog. Depending on which movie compression codec you are using the amount of memory the processed output movie will use and the quality of the rendered frames can vary wildly. Many compression codecs are lossy, which means they throw away information in the rendered frame to use less memory when compressed. If you want the highest quality for your processed output movie you should use a lossless compression codec like animation codec at the highest quality.  Be aware that doing so will create large memory footprint movie files.

Many people use the same Movie Preference and Movie Codec Settings for all their work so they set the dialogs up initially and then forget about it since the settings are saved and restored for you.