Veejay has some support for alpha channel compositing

In general, you will need to add an alpha channel to your playing sample or stream by using one of the "Alpha:" filters in veejay

Then, the alpha channel will be combined by FX that can deal with Alpha. Some of such effects have a mode parameter "Alpha" that functions like an on/off switch but others require an alpha channel to work. 

By default, the alpha channel is set to 0 (completely invisible) so such effects will always result in a black screen if you have not added an alpha channel




The 'Select by Color Key' operator
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Use this filter to create an alpha channel from the foreground object.
The filter requires an existing alpha channel to decide which pixel from source B to composite in. Pixels with an alpha of 0 are skipped.
Using the parameters R,G,B and Angle you can select which pixels belong to the background and key them out, leaving a mask of the foreground object.


The 'Select by Chroma Key' operator
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Use this filter to create an alpha channel from the foreground object.
The filter may use an existing alpha channel to decide which pixel from source B to composite in. Pixels with an alpha of 0 are skipped optionally.



Luma Key
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Luma Key in Mode 3 will composite-in pixels from source B using its alpha channel


Alpha Blend
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Image in source B will be blended on top of source A using its alpha channel



Black and White Mask by Threshold
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This filter creates a black/white image from a minimum and maximum threshold value. You can set a mode parameter so that the render result is written as an Alpha channel



LVD Scale0Tilt / Crop,Scale,Tilt
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This filter is a port of the frei0r filter "scale0tilt.so"
You can use it to crop,scale, tilt an image from source B over source A
If the Alpha paramater is set to 1, the final result will be an opaque pixel with the transparency of each pixel determined by the alpha channel values


Flatten Image
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Use this to multiply the alpha channel against a black background


Alpha Fill
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Solid value fill of the alpha channel, 0 - 255


Set from Image / Mixing source
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Use the luminance channel of the image as a new Alpha channel


You can use the FX switch parameter to scale the values to full range (0-255) if needed.
If you have no result in the Alpha channel, flip this parameter.


Alpha to Greyscale
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Use this FX to display the alpha channel as a greyscale image.