Pixel Mapper snap to fixture

[htevents]

Hello all,

I have a few fixtures containing 25 cells each (5 by 5). I use the pixel mapper to have a background colour (layer 1) and the pixel mapper to have effects on them (layer 2). On both I use the layer masters, to control intensity of the layer and easily select colours for the background an the effect.

What I would like is a random white flash over the background colour that snaps to the fixtures. With the pixel mapper, I can have a block randomly spawn, but it doesn't snap to the fixture, but it just spawns on random cells. (English is not my native language, so I hope I can explain myself correctly).
If I use the key frame shape, I can not "fade"/release back to my background colour, same for the shape generator, only thing I can do is to make a chase/cue list to achieve the wanted effect, but that is more time consuming, so I hoped I could do it with the pixel mapper, but maybe I am overlooking something in the pixel mapper to achieve the desired effect. I would prefer the pixel mapper, because I can control the intensity of the white flash with the layer master.