Clarify image export rules when sub-pixel position and/or size

[gpholman]

Please see the attached .xar. In it I have 4 xara objects and their .png exports. The objects differ slightly in that they are positioned on the page and/or sized either to whole pixels or sub-pixels. Click on each object under the 'Xara Objects' heading and look at the 'X' 'Y' 'W' and 'H' values to see what I mean. In the next column ('PNG Eqivalents') are the same objects as PNG format exports.

Now zoom in on the edges of the PNG objects and you'll notice some have sharp edges and others are slightly aliased (you'll probably need to go to 800% and above to see this). I know that Xara intentionally does this but I have the following questions:

1) Why Xara do this, what purpose does aliasing exports of sub-pixel positioned/sized objects serve?
2) Why has the 3rd box ('FRACTION PX POS, FRACTION PX DIMENSION') exported with a sharp edge? I was expecting aliased edges as it was positioned sub-pixel.
3) Can this 'feature' be disabled or is it up to the user to remember to check objects prior to export?