"Picture in picture" alpha blend

    Feb 10 2017 | 4:45 pm
    I'm trying to put a small matrix (320x240) in the center of a large one (640x480) and use the alpha channel of the smaller one to fade it in (from 0. to 1. transparency). To do this, I'm sending the large one into a [[jit.matrix 4 char 640 480 @dstdimstart 160 120 @dstdimend 480 360]], with [usedstdim 0] and [thru 0], then, the second one with [usedstdim 1] and [thru 1].
    When I ramp the alpha down to zero by multiplying it by a 1.->0. float using [[jit.op @op * pass pass pass]], I would expect it to fade out. However, I'm getting a black box, as if jit.matrix is ignoring the alpha channel. I've tried jit.alphablend with no results, but I'm probably using it wrong. Ideas?