A video matrix, such as comes out of jit.movie, has four planes of data, representing alpha, red, green, and blue. The alpha channel contains information about the opacity of the other three planes—a factor that can be used when mixing/compositing with another image. Normally the values in the alpha channel are all set to 1., meaning full opacity. However, if some values in the alpha channel are 0., the image will be transparent in those pixels, allowing another image to show through.
This patch uses jit.lcd to draw white text on a black background, uses that monochrome image as the color and alpha channels, colorizes the text using jit.scalebias, and then does an alphablend of the text with a movie.
This patch shows how to use a mask in the alpha channel to impose a non-rectangular image on top of another image. For this patch to work, you will need to download the mask file called "mask.png" (which was made using the patch in "Make an elliptical mask in Jitter").
One way to superimpose 2D graphics over a video is to draw into a jit.lcd object and then use alpha masking to overlay the jit.lcd contents on top of the video image.