RFR: JDK-8306990: The guarantees given by Region's floor and ceiling functions should work for larger values

[John Hendrikx]

On Thu, 27 Apr 2023 11:41:40 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Region has floor and ceiling functions that ensure that calling them twice in a row will yield the same result:
>> 
>>      ceil(x) = ceil(ceil(x))
>> 
>> However, due to use of a constant `EPSILON` which is added/subtracted before doing the rounding, this only works for small numbers (in the range of 0-50 approximately).  For larger values and scales, rounding errors can easily occur.  This is visible as artifacts on screen where controls are a pixel wider than they should be.
>> 
>> The use of the `EPSILON` constant is incorrect, as its value depends on the magnitude of the value in question (as magnitude increases, the fractional precision decreases).
>> 
>> The Math class offers the function `ulp` that should be used here.  It represents the smallest possible change in value for a given double.
>> 
>> Extending the existing test case `snappingASnappedValueGivesTheSameValueTest` to use larger magnitude numbers exposes the problems.
>
> @andy-goryachev-oracle can you also review this?

@kevinrushforth I think this one is ready to go, and should help to already solve some of the off-by-one pixel errors for controls that are larger than 100 pixels (in any direction) that make use of the ceiling or floor functions

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PR Comment: https://git.openjdk.org/jfx/pull/1118#issuecomment-1556904744