I've been playing with PS Elements (came with my camera) so I recently
found it and played with the background erase tool.

Its a fixed aspect brush (only a circle, though you can vary its
size).  It samples the current circle continously, and uses its
average (center weighted I think) to determine what the current
"color" is for a color-to-alpha type removal.

There's a threshold parameter which determines how close something has
to be to get the color-to-alpha treatment.  With a bad threshold, the
tool is pretty unusable (becomes either like a bad eraser, or a
paintbrush in color to alpha mode).  I find I have to change it a lot,
even on the same image.
There's also a Continuous or Discontinous paramterer, but I'm not able
to determine what this does simply by playing.

A typical usage pattern is to trace around the outside of an object
you want to cut out, just barely touching the object with the outside
of the brush (the idea is that this will stay out of the threshold and
remove everything else).

Its not a smooth drawing tool - the outside generally looks like a
bunch of semi-circles (I think it requires having a center pixel
non-alpha).  However, the inside will hopefully be fairly
representitive of the outline of the object.

It also suffers from the same problem as color to alhpa, in that it
tends to leave garbage around (with a low alpha) that you have to go
back and touch up later.  JPEG noise is especially prevalent, so its
best to run a noise filter over it before doing this.

If we want to make a killer tool for background erasing, we should
really be looking at knock-out or an equivalent, but the background
erase tool isn't bad for quick removal (and certainly is more
applicable than something like color-to-alpha in a typical
real-photograph situation).

Hope this helps!

Seth Burgess
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