[postgis-users] Surplus pixels in raster for geometry smaller than pixel scale

[David Swain]

Hi all,

I hope this is the right place and approach to get support for my issue. Apologies if it isn't.
Question also posted to the Gis reddit if I'm more likely to get help as replies on there (https://www.reddit.com/r/gis/comments/8dnsr2/could_a_more_experienced_giser_tell_me_why_i_get/)

In PostGis I run the following query:

    SELECT
      (ST_MetaData(ST_AddBand(ST_MakeEmptyRaster(1, 1, 2, 2, 10::FLOAT, 10::FLOAT, 0, 0), '1BB'))).*
    UNION ALL
    SELECT
      (ST_MetaData(ST_AsRaster(
          ST_MakeEnvelope(2, 2, 3, 3),
          0.5 :: FLOAT, 0.5 :: FLOAT,
          0, 0, '1BB', 1, 0, 0, 0, true
      ))).*
    UNION ALL
    SELECT
      (ST_MetaData(ST_AsRaster(
          ST_MakeEnvelope(2, 2, 3, 3),
          1 :: FLOAT, 1 :: FLOAT,
          0, 0, '1BB', 1, 0, 0, 0, true
      ))).*
    UNION ALL
    SELECT
      (ST_MetaData(ST_AsRaster(
          ST_MakeEnvelope(2, 2, 3, 3),
          10 :: FLOAT, 10 :: FLOAT,
          0, 0, '1BB', 1, 0, 0, 0, true
      ))).*
    ;

That gives me results:

    upperleftx | upperlefty | width | height | scalex | scaley | skewx | skewy | srid | numbands
    -----------+------------+-------+--------+--------+--------+-------+-------+------+----------
             2 |          2 |     1 |      1 |     10 |     10 |     0 |     0 |    0 |        1
             2 |          2 |     2 |      2 |    0.5 |    0.5 |     0 |     0 |    0 |        1
             2 |          2 |     1 |      1 |      1 |      1 |     0 |     0 |    0 |        1
             0 |        -10 |     2 |      2 |     10 |     10 |     0 |     0 |    0 |        1

What gets me is the final row. Why would the biggest pixel scale give a 2x2 raster?
If I dump the points I see

    {{NULL,NULL},{1,NULL}}

So it's created some null pixels for no reason that I understand.
Is this a bug? Could someone explain this behaviour for me?


David Swain

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