On Fri, 28 Feb 2025 21:35:30 GMT, John Hendrikx <jhendr...@openjdk.org> wrote:
>> In theory, all these calculations end up being used by the layoutChildren() >> which always (?) snap the values. >> >> So the question is whether this small error gets accumulated enough to shift >> the result to a wrong value. Given a typical number of children (<1000) and >> screen sizes (<10000), we might conclude that it's unlikely, since >> >> `Math.ulp(1000 * 10_000.0) = 1.862645149230957E-9` >> >> which is much smaller than the distance between pixels even at 1000% scale. > > That's absolutely true, however this can change quickly when large values are > added or subtracted to/from small values, or when division or multiplication > gets involved. So I'd say its relatively safe to do simple calculations with > near snapped values, but one must be careful with functions like `ceil` and > `floor` as they can amplify tiny floating-point errors due to their > discontinuous nature. For example, if take a snapped `spacing` and add a > snapped `left` and `right` margin, then call ceil on the result, it could go > like this: > > 1.99999999999999 + > 4.00000000000001 + > 4.00000000000001 = > 10.00000000000001 -> ceil -> 11 (instead of the expected 10) good point! This is exactly the reason for the code in ScaledMath:71 return Math.ceil(d - Math.ulp(d)) / scale; ------------- PR Review Comment: https://git.openjdk.org/jfx/pull/1723#discussion_r1976043283
