Text Effects - Debugging rounding errors

The first of the text effects is an effect where glyphs are swapped our randomly. Having fixed the more obvious (large scale) problem in the positioning (as described here) there was an outstanding more subtle bug in the positioning of the effect glyphs.

One of my test files is based on some effect text that is precisely positioned on top of some normal text. When the effect ends the text come to rest so it exactly covers the normal text. In the screen shot below you can see that there is a vertical and horizontal offse that looks a bit like a drop-shadow.These offset problems were, as I suspected, due to rounding errors. The PC version of Cello deals exclusively with integer values when positioning text. CoreText, on the other hand, deals with floating point values. The output format deals with integer TWIPS. The issue with the effects is that Cello carries about bound rectangles for the effects (in this case glyph-by-glyph) that are stored as integers before being multiplied by 20 to become TWIPS.

Once I uncovered the problem (it took a while to work out where it was going wrong) I replaced the CRect (a MFC integer rectangle) with a floating point equivalent. Ultimately I need to go through all of the text calculations and make sure that floating point is used - but this is something for my to-do list rather than for now. At the moment I want things to work - but also the minimum of disruption in getting the effects to build and work.