"luminosity" composite model and unsharp mask

[Peter Smyth]

Hey there,

I've got a couple of questions about the use of Krita, apologies if
this is the wrong list, I didn't see another.  I'm using Krita 1.6.3.

Both questions regard unsharp mask.  I'm trying to sharpen only the
luminosity information in a picture I have.  There are a couple of
ways to do this, you can convert to L*a*b* and then just sharpen the L
channel.  In version 1.9.5 this seems to be possible, as you can turn
off the a and b  channels, but I couldn't get it to go long enough
without crashing.  Is it possible in 1.6?  The other way I have seen
recommended is to create a duplicate layer, sharpen as normal and then
blend using the luminosity mode in the layers window (this relates to
photoshop, obviously).  This last way is even better as there are no
colour space conversions to perform.  Is something like this possible
in Krita?  An example of this can be found down the bottom of the page
at: http://www.cambridgeincolour.com/tutorials/unsharp-mask.htm

The other question I have relates to the unsharp mask dialog itself.
I'm assuming "half size" is the same as "radius" in other programs
(well, it sounds like it should be).  The rule of thumb I've heard and
used before for setting radius is to divide your ppi figure by 200.
So, if you are aiming to print at 300dpi your radius setting for
unsharp mask would be 1.5, for 76ppi (nominal) for screen would get
you a figure of 0.4 (ish).  Is there a reason Krita only accepts
integers, or is "half size" different to radius?

Cheers

Peter.