I'd like to start by mentioning that I'm just an amateur programmer of the past 2 years with no formal training and know very little about maximizing the potential of graphics hardware. I can write shaders and manipulate a multi-layered drawing environment, but I've basically stuck to minimalist pixel shaders.

I'm working on putting dynamic point light shadows in my 2d sidescroller, and have had it working to a reasonable degree. Just chucking it in without working on serious optimizations outside of basic culling, I can get 50 lights or so onscreen at once and still hover around 100 fps.

The only issue is that I'm on a very high end machine and would like to target the game at as many platforms I can, low and high end. The way I'm doing shadows involves a lot of masking before I can finally draw the light to my light layer.

Basically, my technique to achieveing such shadows is as follows.

See pics in this album

http://imgur.com/a/m2fWw#0

1. The dark gray represents the background tiles, the light gray represents the foreground tiles, and the yellow represents the shadow-emitting foreground tile.

2. I'll draw the light using a radial gradient and a color of choice

3. I'll then exclude light from the mask by drawing some geometry extending through the tile from my point light. I actually don't mask the light yet at this point, but I'm just illustrating the technique in this image

4. Finally, I'll re-include the foreground layer in my mask, as I only want shadows to collect on the background layer and finally multiply the light with it's mask to the light layer

My question is simple - How can I go about reducing the amount of render target switches I need to do to achieve the following:

a. Draw mask to exclude shadows from the foreground to it's own target once per frame

b. For each light that emits shadows,

-Begin light mask as full white

-Render shadow geometry as transparent with an opaque blendmode to eliminate shadowed areas from the mask

-Render foreground mask back over the light mask to reintroduce light to the foreground

c. Multiply light texture with it's individual mask to the main light layer.