[TCLCORE] Purpose of mmObjType vs pixelObjType

[Alexandre F. <ale...@gm...>]

Hi,

In the absence of reaction to my question "Why millimeters", I have
spent several minutes in apnea in tkObj.c et al.
Now I am really perplex. I have the impression that:

     - pixelObjType is the primary multi-unit storage type. It holds a
double internal value, and an integer "returnValue".

     - mmObjType is a secondary type with roughly the same properties,
the only differences being the interpretation (scale) of the double
value *and* the fact that the returnValue is double too.

     - in tkObj.c the code comments are completely confusing because
they are visibly copied/pasted from the pixelObjType, and frequently
misuse the word "pixel" where "mm" is meant.

    - I cannot seem to find a documentation page giving the contracts
nor even rationale for these types. Something like "the unit is
sticky" or "no loss of precision while staying iso-unit", or the
like...

Can you help me there ? I have the vague impression that the initial
problem in tkCanvUtil.c (useless roundtrip to mm for [coords]) would
be trivially avoided by using the pixel type (especially the
GetDoublePixels variant, which does not go through the integer
bottleneck) and not the MM type. But the problem is that this would
completely void the usefulness of the MM type (the other use is in
text tabs; similar migration could be done harmlessly I guess)...

What am I missing ?

Thanks in advance,

-Alex