How to optimize this sequence of operations to produce one output image

Hi,

I have 4 png images that I want to merge into one. I want (255,255,255) to
be transparent in all
images, and layers 1,2,3 need all colors to be partially transparent (here I
have set it to 50%), so
that features from layers one,two and three are visible in the output image.

The following code produces the output image I need, but how can this be
streamlined/optimized, for both
execution speed and number of lines of code? I need to do this many times
with different images.
Perlmagick or MSL solutions are welcome as well.


*******************************************************
convert -transparent 'rgb(255,255,255)' top.png top1.png
convert -transparent 'rgb(255,255,255)' layer-1.png 1.png
convert -transparent 'rgb(255,255,255)' layer-2.png 2.png
convert -transparent 'rgb(255,255,255)' layer-3.png 3.png


composite -dissolve 50x100 1.png background.png  tmp1.png
composite -dissolve 50x100 2.png tmp1.png tmp2.png
composite -dissolve 50x100 3.png tmp2.png tmp3.png

convert -layers merge tmp3.png top1.png out.png
**********************************************************
single resulting output image is out.png

The files are located here, but I guess they are probably not needed to
answer this:

http://drop.io/i4wpshm#

Thanks.

GK