document cog generation and requirements

README.md

@@ -22,6 +22,15 @@ docker run --rm -ti -v $PWD:/host --workdir /host ghcr.io/osgeo/gdal \
 
 Full documentation is available at [gdal.org/drivers/vector/pmtiles.html](https://gdal.org/drivers/vector/pmtiles.html).
 
+
+## COG conversion
+Raster images should be converted a 4 bands RGBA GEOTiff using EPSG:3857.
+
+If you have a single-band greyscale GEOTiff you can use `generate_cog.sh` to produce a valid Cloud Optimized GEOTiff.
+
+**NOTE**: COG layers are static, style must be pre-applyied.
+
+
 ## Running the webapp
 
 The only requirement is having a web server which supports HTTP bytes serving/ranged requests, such as NGINX.

generate_cog.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -ex
+
+export MASK=$(mktemp .cog.XXXXXXX.tif -p .)
+export NORMALIZED=$(mktemp .cog.XXXXXXX.tif -p .)
+export RGBA=$(mktemp .cog.XXXXXXX.tif -p .)
+
+gdal_translate -scale 0 1 255 255 $1 $MASK
+gdal_translate -scale 0 1 0 255 $1 $NORMALIZED
+gdal_merge.py -separate -o $RGBA -of GTiff $NORMALIZED $NORMALIZED $NORMALIZED $MASK
+gdal_translate $RGBA $2 -of COG -co TARGET_SRS=EPSG:3857 -co ADD_ALPHA=NO -co COMPRESS=LZW -co LEVEL=9
+rm $MASK $NORMALIZED $RGBA;