[postgis-tickets] [PostGIS] #5382: ST_AsMVTGeom output is snapped to grid incorrectly

[PostGIS]

#5382: ST_AsMVTGeom output is snapped to grid incorrectly
---------------------+---------------------------
 Reporter:  mknycha  |      Owner:  pramsey
     Type:  defect   |     Status:  new
 Priority:  medium   |  Milestone:  PostGIS 3.3.3
Component:  postgis  |    Version:  3.3.x
 Keywords:           |
---------------------+---------------------------
 Steps to replicate:

 - Run:
 {{{#!sql
 SELECT ST_AsText(ST_AsMVTGeom(
     ST_GeomFromText('POLYGON((0 0, 4 0, 6 6, -1 5, 0 0))'),
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(10, 10)),
     20, 0, true));
 }}}

 Actual output:

 {{{
 POLYGON((12 8,8 20,0 20,0 9,12 8))
 }}}

 Expected output:

 {{{
 POLYGON((12 8,8 20,0 20,0 10,12 8))
 }}}

 Explanation:

 They way I understand ST_AsMVTGeom works, is that after the transformation
 and clipping, it snaps the output to the grid of integer points. The
 problem is, that is does not round the integer coordinates correctly.
 For example, the output for:

 {{{
 SELECT ST_AsText(ST_AsMVTGeom(
     ST_GeomFromText('POLYGON((0 0, 4 0, 6 6, -3 5, 0 0))'),
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(10, 10)),
     20, 0, false));
               st_astext
 --------------------------------------
  POLYGON((12 8,8 20,0 20,-6 10,12 8))
 (1 row)
 }}}

 Comparing to the clipped output:

 {{{
 SELECT ST_AsText(ST_AsMVTGeom(
     ST_GeomFromText('POLYGON((0 0, 4 0, 6 6, -3 5, 0 0))'),
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(10, 10)),
     20, 0, true));
              st_astext
 ------------------------------------
  POLYGON((12 8,8 20,0 20,0 9,12 8))
 (1 row)
 }}}
 makes sense, because looking at the bottom edge of the unclipped output,
 after clipping by the bounding box the bottom left corner should be closer
 to (0 9) than (0 10).

 Outputs comparison (orange unclipped, red clipped):
 [[Image(https://i.ibb.co/Gk5Qccn/Screenshot-2023-05-16-at-16-53-39.png)]]


 But consider what happens for another output:

 {{{
 SELECT ST_AsText(ST_AsMVTGeom(
     ST_GeomFromText('POLYGON((0 0, 4 0, 6 6, -1 5, 0 0))'),
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(10, 10)),
     20, 0, false));
               st_astext
 --------------------------------------
  POLYGON((12 8,8 20,0 20,-2 10,12 8))
 (1 row)
 }}}
 Comparing to the clipped one:

 {{{
 SELECT ST_AsText(ST_AsMVTGeom(
     ST_GeomFromText('POLYGON((0 0, 4 0, 6 6, -1 5, 0 0))'),
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(10, 10)),
     20, 0, true));
              st_astext
 ------------------------------------
  POLYGON((12 8,8 20,0 20,0 9,12 8))
 (1 row)
 }}}
 Now, after clipping and before snap to grid, the left bottom corner should
 be closer to (0 10). That's why I think the output polygon should contain
 (0 10), instead of (0 9).

 Image with outputs comparison (orange unclipped, red clipped):
 [[Image(https://i.ibb.co/xs2dWH8/Screenshot-2023-05-16-at-17-07-02.png)]]

 Hopefully I got that right, thanks for your open source project
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5382>
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