feat(cogify): add background color support for overriding transparent pixels BM-1146

[tawera-manaena]

Motivation

As a Basemaps user, when I view the NZTopo Raster Maps from afar (low zoom level), I can see black edge artifacts around the perimeter of the imagery.

Topo 50	Topo250
Appears between zoom levels 0-6 inclusive. Hard to notice when zoom level is >= 6.5	Appears between zoom levels 0-4 inclusive. Hard to notice when zoom level is >= 4.5

You may need to open each image/link in a new tab to see the black lines more clearly.

Problem

We suspect the issue resides in the way the basemaps/tiler composes a small tile. The resizing down of many pixels into one, causes artifacts to appear where opaque pixels meet transparent pixels.

Solution

There is already a 'workaround' fix that treats the symptoms of the black edge artifacts. This work, however, seeks to treat the source of the issue by providing a mechanism to replace all transparent pixels with opaque pixels. The effect being to erase such areas where opaque pixels meet transparent pixels.

Modifications

basemaps/cogify

• We have added a background parameter to the cover command.

  • When the user provides the parameter, the create function will perform two additional gdal steps after gdalwarp and before gdal_translate, to ensure that any and all transparent pixels that would otherwise appear in the resulting tile, are replaced with the provided background colour.

    • gdal_create - To create a background image where all pixels are set to the provided background colour.
    • gdalbuildvrt - To layer the background image behind the source GeoTIFF files after reprojection.

Verification

For a given tile	What cogify usually makes	What cogify can now make
Example shown: 9-4-325 - Web Mercator (EPSG:3857)	No transparent pixels are overwritten.	All transparent pixels are overwritten with the provided background colour.