guide lines: desktop vs document coordinate system

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guide lines: desktop vs document coordinate system

Peter Moulder
Someone has bug #1222819: “guides have different orgin than objects”:
horizontal guideline positions are currently stored as distance up from
the bottom of the document (i.e. desktop coordinate system), whereas
shape positions’ y coordinates are represented as distance from the top
of the document.

The result is that if you snap a shape’s position to a horizontal
guideline and then resize the document (File>Document preferences,
Custom page size, then change the page height), then the shape and the
guideline will no longer appear to be at the same position.

In my local copy I've mostly fixed this so that the guideline's position
from the top of the document is preserved (as for shapes) rather than
from the bottom.

However, this does raise a user interface issue of how to report to the
user what the guide line's position is:

 * For dialog boxes and status bar, should we report positions in the
 * document coordinate system or in the desktop coordinate system?
 * The argument for reporting document coordinate system is that this is
 * the one that we honour when the document is resized, so in some sense
 * it is the true (or at least robust) position of the guide.  OTOH,
 * resizing a document is rare.
 * The argument for reporting desktop coordinate system is that this is
 * what's used by the rulers and "current position" indication.  This is
 * the coordinate system that we usually report to users, so is less
 * confusing to users.
 * ...
 * (Hopefully, future versions of inkscape will get rid of the separate
 * desktop coordinate system, to remove this problem.)



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