[ptx] Multi-layer TIFFs on OS X

[Ian Wood]

After some discussions on the PTAssembler forum, i discovered that  
multi-layer TIFF output just renders the relevant part of each output  
image, saving on file size and resource usage.

Is there any easy way on OS X to split this file up for use with  
Enblend? Preview recognises all the layers but can't save individual  
ones, and anyway doesn't recognise any offset info that would be  
needed to pad out the canvas.

Ian