[#1] Colorize raster using pgraster mapnik plugin

- This code was possible thanks to a pairing session with @dlebrero

- We found that there is no need to convert the raster to Byte type
  if we use the band number in the datasouce configuration

  https://github.com/mapnik/mapnik/wiki/PgRaster

  band - integer - request for a specific raster band index (1-based).
  0 means to read all bands. Note that a band read from a single
  band raster gets interpreted as Grayscale if band=0 is specified
  while they retain their original value when explicitly referenced
  with the "band" parameter.
  This affects effectiveness of RasterColorizer:

  https://github.com/mapnik/mapnik/wiki/RasterColorizer

- We still need to test if the same behavior applies to a raster that
  has been reprojected to Web mercator (EPSG:3857) with gdalwarp

- We're using `pgraster` plugin. We need to double if this is the same
  plugin used in Windshaft or if it is `postgis`

README.md

@@ -6,6 +6,10 @@ Example of Mapnik raster colorizer
 
     docker-compose up --build -d
 
+### Run test
+
+    docker-compose run mapnik /usr/local/src/run.sh test.js
+
 ## Raster dataset license
 
 [WorldPop](http://www.worldpop.org.uk/data/summary/?doi=10.5258/SOTON/WP00234) datasets

docker-compose.yml

@@ -14,3 +14,6 @@ services:
       context: ./mapnik
     links:
       - postgres:postgres
+    volumes:
+      - ./src:/usr/local/src
+      - ./rasters:/rasters

postgres/raster-import.sh

@@ -20,8 +20,8 @@ tar xvfz "${ARCHIVE}"
 
 raster2pgsql -C -I -Y "${GEOTIFF}" -t 128x128 sle_pop | psql -U postgres -h localhost -d "${DB_NAME}"
 
-# Reproject and compress
+# Reproject
 
-gdalwarp -ot Byte -co COMPRESS=LZW -t_srs EPSG:3857 "${GEOTIFF}" "sle-wm.tif"
+gdalwarp -t_srs EPSG:3857 "${GEOTIFF}" "sle-wm.tif"
 
 raster2pgsql -C -I -Y "sle-wm.tif" -t 128x128 sle_pop_wm | psql -U postgres -h localhost -d "${DB_NAME}"

src/pg.xml

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Map srs="+init=epsg:4326" background-color="white">
+  <Style name="style">
+    <Rule>
+      <RasterSymbolizer opacity="1" scaling="bilinear">
+	<RasterColorizer default-mode="linear" default-color="black" epsilon="0.5">
+          <stop color="yellow" value="0" />
+	  <stop color="green" value="240" />
+	</RasterColorizer>
+      </RasterSymbolizer>
+    </Rule>
+  </Style>
+
+
+  <Layer name="sierra_leone" srs="+init=epsg:4326">
+    <StyleName>style</StyleName>
+    <Datasource>
+      <Parameter name="type">pgraster</Parameter>
+      <Parameter name="host">postgres</Parameter>
+      <Parameter name="dbname">sierra_leone</Parameter>
+      <Parameter name="user">postgres</Parameter>      
+      <Parameter name="password">sa</Parameter>
+      <Parameter name="table">sle_pop</Parameter>
+      <Parameter name="raster_table">sle_pop</Parameter>
+      <Parameter name="raster_field">rast</Parameter>
+      <Parameter name="estimate_extent">true</Parameter>
+      <Parameter name="band">1</Parameter>
+      <Parameter name="srid">4326</Parameter>
+    </Datasource>
+  </Layer>
+</Map>

src/run.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+set -eu
+
+cd /usr/local/src
+
+FILE=$(node "${1}")
+
+curl --upload-file "/tmp/${FILE}" "https://transfer.sh/${FILE}" && echo

src/test.js

@@ -0,0 +1,27 @@
+var mapnik = require('mapnik');
+var fs = require('fs');
+
+mapnik.Logger.setSeverity(mapnik.Logger.DEBUG);
+
+// register fonts and datasource plugins
+mapnik.register_default_fonts();
+mapnik.register_default_input_plugins();
+
+var map = new mapnik.Map(1000, 1000);
+var file = Date.now() + '.png'
+
+map.load('./pg.xml', function(err,map) {
+    if (err) throw err;
+    map.zoomAll();
+    var im = new mapnik.Image(1000, 1000);
+    map.render(im, function(err,im) {
+	if (err) throw err;
+	im.encode('png', function(err,buffer) {
+            if (err) throw err;
+            fs.writeFile('/tmp/' + file, buffer, function(err) {
+		if (err) throw err;
+		console.log(file);
+            });
+	});
+    });
+});