Hey, thanks! I think I managed to trick your set up into working for me! I even tried wrapping up as a module, but it only works once. And that's enough for me. For now. Maybe someday I'll poke around a little more? 
[Something to do with how surfaces are assigned/referenced, I think...]
The way surfaces are referenced is a trick to prevent attract mode from delaying the update of nested surfaces. I think it might be useful if I try to describe how my blur works...
There's only one shader, that can be tweaked with parameters for matrix size, gaussian blulr sigma and an offset vector. The offsect vector should be (0 , 1/height) for the vertical blur and (1/width, 0) for horizontal blur. If this value is not correct you'll end up with artifacts.
The "nominal" surface stack should be like this (this will be changed for a reason, more on that later):
surface1 as base surface,
surface2 inside surface1,
picture inside surface 2
Surface1 and surface2 are the base surfaces to which you'll apply the horizontal and vertical blur shader. They should be of a known width and height and are used not only to stack the shaders, but also to force a downsample of the picture, so regardless of the picture size, you'll always apply the shader to a known size surface. If you are using the blur to generate a drop shadow this base surface should be larger than the picture to avoid clipping.
Then to avoid the delay in AM "actual" picture and surface creation is shuffled a bit so everything is created at the fe.add_ level and later "cloned" inside the surface. That's the way you build your stack is reversed, you have to create first the items that are on top, and create them on the layout level:
- create the topmost surface (surface2) as fe.add_surface etc
- add image to the surface
- create the bottom surface (surface1) as fe.add_surface etc
- add shaders to your surfaces (not to the image)
- use the Oomek trick like this:
surface2.visible = false
surface2= surface1.add_clone(surface2)
surface2.visible = true
surface1.set_pos(actual surface position and size)
the trick allows you to reposition the surfaces, but they will be updated all together since they were created as fe.add_surface and not as surface1.add_surface
The point of all this nesting is to avoid applying the shaders to the image resolution, but to the surface resolution, so you can control that and even use it to downsample.
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