Update README.md

README.md

@@ -8,7 +8,7 @@ npm install geotiff-palette
 
 # usage
 ## downloaded GeoTIFF
-```
+```javascript
 const { readFileSync } = require('fs');
 const { fromArrayBuffer } = require('geotiff');
 const { getPalette } = require('geotiff-palette');
@@ -20,17 +20,28 @@ const image = await geotiff.getImage();
 const palette = await getPalette(image);
 console.log(palette);
 ```
-palette will look something like this:
-```
+palette contains an array of [R, G, B, A] values and looks like this:
+```javascript
 [
   [ 112, 108, 96, 255 ],  [ 112, 104, 80, 255 ],  [ 104, 104, 104, 255 ],
   [ 96, 88, 52, 255 ],    [ 104, 104, 112, 255 ], [ 128, 120, 108, 255 ],
   [ 160, 160, 160, 255 ], [ 152, 152, 144, 255 ], [ 104, 96, 72, 255 ],
   [ 96, 80, 44, 255 ],    [ 40, 48, 56, 255 ],    [ 152, 144, 144, 255 ],
   [ 32, 48, 56, 255 ],    [ 120, 120, 96, 255 ],  [ 112, 120, 104, 255 ],
-  ... 256 total items
+  // ... 256 total items
 ]
 ```
 
+## getting a color
+The index number in the palette corresponds to a raster value. 
+For example, if you have an 8-bit single-band GeoTIFF and you want to get the color for the first pixel with value 130
+```javascript
+const [ band ] = await image.readRasters();
+const value = band[0]; // first pixel in the band, which is usually in the top-left of the image
+// value is 130
+const color = palette[value];
+// color is [ 112, 108, 96, 255 ], which is the same as `rgba(112, 108, 96, 1)`
+```
+
 # support
 Post an issue at https://github.com/GeoTIFF/geotiff-palette or email the package author at daniel.j.dufour@gmail.com