Right, I meant "exact" in the Racket sense of "exact rational."
The broader issue I'm thinking about is what kind of units to use in a typesetting system in order to get the best balance of precision and speed. For instance, the flexibility of a 64-bit flonum doesn't necessarily buy you anything over a 64-bit fixnum, since typesetting systems have a practical limit on both precision (in the subpixel direction) and scale (in the megapixel direction). TeX, for instance, is based on a "scaled point" which represents 1/65536th of a point, with a max dimension of 2^30 scaled points, or about 19 feet. One could imagine a 64-bit version of this concept that extends both the scale and precision (to ludicrous degrees) while remaining a fixnum (which I gather from the docs are typically cheapest). On Mon, Dec 1, 2014 at 2:01 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > We should probably improve the contracts on `racket/draw` to promise > flonum results for text metrics. The intent is to make metric-derived > calculations have a predictable cost, instead of potentially triggering > expensive exact arithmetic. > > When you say that Pango produces "exact" results, do you mean "integer" > or "exact rational"? The latter is certainly true: Pango's raw API > produces integers to be divided by 1024 to convert to drawing units. > Taking that conversion into account, Pango doesn't always produce > integer drawing units (at least on some platforms; I'm not sure about > all). Even so, the intent is that representing an integer divided by > 1024 as a flonum will not normally lose any prevision (i.e., for normal > drawing sizes), and so `inexact->exact` on the immediate result from > `racket/draw` recovers the exact Pango result when exact arithmetic is > specifically wanted. > > At Mon, 1 Dec 2014 11:56:12 -0800, Matthew Butterick wrote: > > The `get-text-extent` method in racket/draw does not contractually > guarantee > > either exact or inexact numbers, though in practice I find it produces > inexact. > > > > This function, however, calls into the Pango text-layout system. I find > that > > when I invoke Pango's text measuring directly through the FFI, it > produces > > exact results. > > > > Is this difference in behavior deliberate, or does `get-text-extent` > preserve > > exactness under certain circumstances? > > > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users >
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