I thought it would be useful to link here to the blog post on using the camera as a texture fill: http://www.coronalabs.com/blog/2013/12/19/holiday-surpril-time-effects/
The code snippet to set this up with a filter effect looks like the following:
local shape = display.newRect( 0, 0, 320, 480 ) shape.anchorX = 0 shape.anchorY = 0 shape.fill = { type="camera" } shape.fill.effect = "filter.sobel"