There are 2 different approaches to processing movies in Studio Artist. The most conventional approach is to work with a Quicktime movie file as a source, and output to a processed movie file. using the 'Action : Process with Paint Action Sequence : Source to Movie' menu command.
However, some people prefer to work with movies that are stored as a series of numbered frame images in a folder. And to output their processed movie as a second folder of numbered processed frame images. Using the 'Action : Process with Paint Action Sequence : Image to Image' menu command.
In either of these 2 movie processing scenarios, a user might want to work with movie frames that have embedded alpha channels. Both for the input movie to be processed, and to also generate embedded alpha channels for the output frames. When working with Quicktime movie files and embedded alpha, you need to make sure you select a Quicktime compression codec that supports alpha (animation and ProRes 4444 codecs both support embedded alpha channels with the millions of colors + option). When working with movie frame images for movie processing, you need to select an image format for the frame images that supports an alpha channel (tiff and photoshop formats both support alpha channels).
An alpha channel associated with a movie frame image can be thought of as a built in mask for the image. So for each pixel in the image, if the alpha channel is full on (255 or white) then the RGB pixel information at that pixel is displayed. If the alpha channel is full off ( 0 or black) at a particular pixel, then the RGB pixel information is totally masked out. Alpha pixels with intermediate gray-tone values will provide a partial mask of the RGB pixel information.
Some software programs will display fully alpha masked pixels using a gray background or gray-square patterning. Others (like Studio Artist) have a default background color that will show through when a pixel is alpha masked. The default background color for an alpha masked pixel in Studio Artist is black. For example, attached below is a Butterfly tiff image that has a built in alpha channel. The RGB pixels of the image are shown below.
Note that the butterfly is yellow on a white background in the RGB image. The alpha channel associated with the Butterfly tiff image is shown below.
Again, the white part of the alpha channel does not mask the RGB pixels, and the black part of the alpha channel does mask the RGB pixels.
When you open a source image with a built in alpha channel in Studio Artist, the alpha channel acts to mask the source image as displayed in the source preview area. The alpha channel masks to a default black color, as shown below.
Note how the un-alpha-masked yellow part of the butterfly shows through, while the white background of the source image is masked out and displays the default black colored background.
Studio Artist is very flexible about how it works with alpha channels in source images and in layers. You can choose to ignore the alpha channel for the purposes of display, of the alpha channel can be used to mask the view of the layers. It's all up to how you setup the 'Canvas : Alpha' menu flags.
The 'Canvas : Alpha : Enable for View' menu flag determines whether a layers alpha channel is used as an active alpha mask or not for the purposes of display. And the 'Canvas : Alpha : Paint Synth Alpha Enable' menu flag determines whether the paint synthesizer paints into a layer's alpha channel when painting. So if you want to work with an alpha enabled layer for the purposes of generating
alpha matted paint strokes, then you need to turn on both of these 2 menu flags as shown below.
The first 'Enable for View' menu flag turns of alpha matting for generating the layer view. The second 'Paint Synth Alpha Enable' menu flag insures that the paint synthesizer will paint into the layer alpha channel when painting.
When alpha view is enabled, the other thing you need to be concerned about is insuring that when you erase a canvas layer you also erase that layer's alpha channel. To do this, you can either run the 'Canvas : Alpha : Set to : Full Off' menu command. Or you can run the 'Canvas : Full Erase' as opposed to the 'Canvas ; Erase' menu command when erasing the canvas. Running Full Erase is the equivalent of shift clicking the erase button at the top of the Studio Artist interface. Full Erase means that the layer's alpha channel is erased as well as the contents of the layer's RGB channels.
Typically if you are generating alpha masked painting, you also want to restrict auto-generated paint strokes to only start painting in un-alpha masked parts of the source image. For example, the alpha masked painting below has paint strokes starting in the body of the butterfly as well as the masked white background.
In most cases, you would not want those white paint strokes to be drawn. One approach available to do that is to inhibit paint strokes from starting in masked areas of the source image. The screen shot below shows the path Start control panel in the paint synthesizer with the Inhibit parameter set to SrcAlpha Full On Only. Turning on this Inhibitor option insures that paint strokes will only start painting in areas of the source image that are not alpha masked,
The alpha matted painted canvas below shows the difference you get when you use the Inhibitor option discussed above to prevent paint strokes from starting in alpha masked parts of the source image. Note that only yellow paint strokes associated with the body of the butterfly are painted. But the individual alpha matted paint strokes are free to paint outside of the original source alpha matte boundaries, leading to a more natural painted appearance.
When using the Action : Process with paint Action Sequence : Image to Image menu command to process a folder of numbered frame images to a second folder of output processed frame images, you need to make sure you are using an output image format that supports embedded alpha channels.
You can choose the output image format used for Action commands that output to numbered frame images using the Image Frame Format movie preference option (movie tab in main preference dialog as shown above).
When processing movie output to numbered frame images, the frame image alpha channel is always generated from the layer view output. When outputting to a movie file, you have more options for how an embedded alpha channel is generated. You aren't restricted to only using the layer alpha channel output. The Embedded Alpha movie preference option allows you very flexible options for how an embedded alpha channel in a processed output movie file is generated. This is discussed in more detail in this tip on generating movie output with embedded alpha channels.
The PASeq that i used for this simple movie frame image processing example just had 2 action steps. The first full erases the canvas. I did this full erase to insure that the layer's alpha channel was erased to full off. The second auto-paints, where the auto-painting is restricted so that paint strokes only start in non-alpha-masked areas of the source image.
I've attached the simple paint preset i used for this simple example below, along with the alpha masked tiff source image discussed above.
Thanks John. You must have been reading my mind. I have been working on a project that uses alpha... tho mostly it's creating alpha channels from region selections (to, for instance, turn a black background into an alpha-on area). But painting with alpha strokes still kinda confounds me. I personaly want to explore doing alpha "touch-ups" on movies that I've already added alpha to. I would like to load a movie with an embedded alpha channel into a layer (movie layer) then go in with either an alpha eraser or an alpha paint brush to clean up the channel, then record the frame a la a regular movie layer edit... then flatten the movie. Is this possible? Do movie layers convey the alpha channel info into the flattened movie?
You can load a movie with an embedded alpha channel as a movie layer. You can then enable alpha for view, and turn on the paint synth alpha enable. You can then paint with alpha enabled paint strokes. And if you record your painted changes into a frame in the movie, the movie frame embedded alpha channel gets updated accordingly. And when you flatten the movie file, the alpha channel info is recorded into the flattened movie file.
Thanks very much for this tutorial on this. It was very helpful.