The following table summarizes results of running the tests in buffertests.zip
dist  GEOS 3.3.0 + patch  GEOS (b) OLD reduc stratg  GEOS (b) NEW reduc stratg  JTS (c) 
0.35  Same output as 3.2.0  ok on 1st attempt, no reduction  ok on 1st attempt, no reduction  ok on 1st attempt, no reduction 
0.75  Same output as 3.2.0  ok on 1st reduction (1e10)  ok on 1st reduction (1e5)  ok on 1st reduction (1e5) 
1.01  Same output as 3.2.0  ok on 2nd reduction (1e9)  fail on 5th reduction (1e1) (d)  ok on 1st reduction (1e5) 
1.1 (a)  Same output as 3.2.0  ok on 2nd reduction (1e9)  ok on 1st reduction (1e5)  ok on 1st reduction (1e5) 
1.5  Half incorrect (i)  ok on 2nd reduction (1e9)  fail on 6th reduction (1e0) (e)  ok on 1st reduction (1e5) 
2  Incorrect output(ii)  fail on 11th reduction (1e0) (f)  fail on 6th reduction (1e0) (f)  ok on 1st reduction (1e5) 
5  Incorrect output(ii)  fail on 11th reduction (1e0) (g)  fail on 6th reduction (1e0) (g)  ok on 1st reduction (1e5) 
10  Incorrect output(ii)  fail on 12th reduction (1e1)(h)  fail on 7th reduction (1e1)(h)  ok on 1st reduction (1e5) 
(a) aka TestBufferJagged?.xml
(b) GEOS 3.3.2SVN r3528 / 3.4.0SVN r3527 on a 64bit system
(c) JTS rev.480
(d) symDiffArea frac: 0.00970281 tolerated 0.001
(e) symDiffArea frac: 0.0731868 tolerated 0.001
(f) symDiffArea frac: 0.0704188 tolerated 0.001
(g) symDiffArea frac: 0.0429171 tolerated 0.001
(h) Expected result is of type Polygon; obtained result is of type MultiPolygon?
(i) Same as GEOS 3.2.0 in xmltester. Incorrect in GEOS 3.3.0 client  same problem as (ii).
(ii) Output seems to have lost way too much precision.
MCIndexSnapRounder
Comparing operations of MCIndexSnapRounder against bufrob4.wkt noding. The data in bufrob4.wkt is rounded to the precision grid of 1e5.
After the fix to HotPixel? in r3528 there are still differences in the number of coordinates found for each noded edge.
 scale=1e2
 GEOS computes 25 edges 3 of which composed by 3 vertices and 22 of 2 vertices.
 JTS computes 25 edges 3 of which composed by 3 vertices, 3 by 1 vertex and 19 by 2.
 The LineString generated using all the points, according to Equals, is the same. The difference is in SegmentNodeList.createSplitEdge refusing (in GEOS) to produce 1vertex SegmentString?.
 scale=1e1
 GEOS computes 52 edges
 JTS computes 53 edges
 Result don't change dropping the additional check in SegmentNodeList.createSplitEdge
 scale=1e5
 GEOS computes 19 edges
 JTS computes 20 edges
 Result don't change dropping the additional check in SegmentNodeList.createSplitEdge
Actually the above runs aren't expected to be meaningful as MCIndexSnapRounder is expected to receive input already rounded to the given precision grid, which in our case would be 1e5. Passing that scale to the noder gives 178 edges with both GEOS and JTS.
ScaledNoder?
Having detected a difference in noding I've produced a testcase to read a geometry and perform noding on its linework. The JTS version is ScaledNoderTest.java
Resources
diffnoding.gz contains the offsetcurve produced by both GEOS and JTS with reduction scale 1e5 and distance 1.01 against input.
The curve, passed to BufferBuilder::computeNodedEdges, results in edges with a differently computed _depth_ between JTS and GEOS.
Attachments (7)

diffnoding.wkt.gz (24.6 KB)  added by 5 years ago.
offsetcurve produced by both JTS and GEOS when buffering input by 1.01 distance. GEOS and JTS compute edges depth differently starting from this same input.

bufrob3.wkt (7.8 KB)  added by 5 years ago.
Using ScaledNoder? with scale=1e5 nodes this with edges: JTS 333, GEOS 330

bufrob3_5.wkt (3.0 KB)  added by 5 years ago.
further simplification: 147 edges with JTS, 146 with GEOS

buffertests.zip (63.3 KB)  added by 5 years ago.
All buffer tests, with correct expected results

bufrob3_5_5.wkt (3.8 KB)  added by 5 years ago.
Version of bufrob3_5 with all coordinates snapped to a 1e5 grid

ScaledNoderTest.java (5.5 KB)  added by 5 years ago.
Updated version of ScaledNoderTest? for JTS, supports both WKT and HexWKB in input

bufrob4.wkt (821 bytes)  added by 5 years ago.
14 edges with JTS, 13 with GEOS  input snapped to 1e5
Download all attachments as: .zip