RFR: JDK-8306990: The guarantees given by Region's floor and ceiling functions should work for larger values [v2]

[Andy Goryachev]

On Thu, 27 Apr 2023 19:04:39 GMT, John Hendrikx <jhendrikx 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.
>
> John Hendrikx has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Print seed if test fails
>  - Merge branch 'master' into feature/ceil-fix
>  - Fix Region ceil/floor function guarantees to be more accurate

modules/javafx.graphics/src/test/java/test/javafx/scene/layout/RegionTest.java line 1281:

> 1279:         }
> 1280: 
> 1281:         random.setSeed(seed);

it might be better to set the seed only once at the beginning.  otherwise the next 2 tests use the same sequence of doubles due to the seed being the same.  
or is this intentional?

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PR Review Comment: https://git.openjdk.org/jfx/pull/1118#discussion_r1179576437