Extract nodata values from metadata when possible

examples/cog-math-multisource.js

@@ -59,7 +59,7 @@ const map = new Map({
   ],
   view: new View({
     center: [1900000, 6100000],
-    zoom: 15,
+    zoom: 13,
     minZoom: 10,
   }),
 });

examples/cog-math.js

@@ -80,13 +80,11 @@ const map = new Map({
           {
             // visible red, band 1 in the style expression above
             url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B04.tif',
-            nodata: 0,
             max: 10000,
           },
           {
             // near infrared, band 2 in the style expression above
             url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B08.tif',
-            nodata: 0,
             max: 10000,
           },
         ],

examples/cog.html

@@ -4,8 +4,7 @@ title: Cloud Optimized GeoTIFF (COG)
 shortdesc: Rendering a COG as a tiled layer.
 docs: >
   Tiled data from a Cloud Optimized GeoTIFF (COG) can be rendered as a layer.  In this
-  example, a single 3-band GeoTIFF is used to render RGB data.  The `nodata` property is
-  used to avoid rendering pixels where all three bands are 0.
+  example, a single 3-band GeoTIFF is used to render RGB data.
 tags: "cog"
 ---
 <div id="map" class="map"></div>

examples/cog.js

@@ -26,7 +26,6 @@ const map = new Map({
         sources: [
           {
             url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/TCI.tif',
-            nodata: 0,
           },
         ],
       }),

src/ol/source/GeoTIFF.js

@@ -191,6 +191,12 @@ class GeoTIFFSource extends DataTile {
      */
     this.samplesPerPixel_;
 
+    /**
+     * @type {Array<Array<number>>}
+     * @private
+     */
+    this.nodataValues_;
+
     /**
      * @type {boolean}
      * @private
@@ -249,6 +255,7 @@ class GeoTIFFSource extends DataTile {
     let tileSizes;
     let resolutions;
     const samplesPerPixel = new Array(sources.length);
+    const nodataValues = new Array(sources.length);
     let minZoom = 0;
 
     const sourceCount = sources.length;
@@ -261,8 +268,14 @@ class GeoTIFFSource extends DataTile {
       const sourceTileSizes = new Array(imageCount);
       const sourceResolutions = new Array(imageCount);
 
+      nodataValues[sourceIndex] = new Array(imageCount);
+
       for (let imageIndex = 0; imageIndex < imageCount; ++imageIndex) {
         const image = images[imageIndex];
+        const nodataValue = image.getGDALNoData();
+        nodataValues[sourceIndex][imageIndex] =
+          nodataValue === null ? NaN : nodataValue;
+
         const wantedSamples = this.sourceInfo_[sourceIndex].bands;
         samplesPerPixel[sourceIndex] = wantedSamples
           ? wantedSamples.length
@@ -351,13 +364,39 @@ class GeoTIFFSource extends DataTile {
     }
 
     this.samplesPerPixel_ = samplesPerPixel;
-    const sourceInfo = this.sourceInfo_;
-    for (let sourceIndex = 0; sourceIndex < sourceCount; ++sourceIndex) {
-      if (sourceInfo[sourceIndex].nodata !== undefined) {
+    this.nodataValues_ = nodataValues;
+
+    // decide if we need to add an alpha band to handle nodata
+    outer: for (let sourceIndex = 0; sourceIndex < sourceCount; ++sourceIndex) {
+      // option 1: source is configured with a nodata value
+      if (this.sourceInfo_[sourceIndex].nodata !== undefined) {
         this.addAlpha_ = true;
         break;
       }
+
+      const values = nodataValues[sourceIndex];
+
+      // option 2: check image metadata for limited bands
+      const bands = this.sourceInfo_[sourceIndex].bands;
+      if (bands) {
+        for (let i = 0; i < bands.length; ++i) {
+          if (!isNaN(values[bands[i]])) {
+            this.addAlpha_ = true;
+            break outer;
           }
+        }
+        continue;
+      }
+
+      // option 3: check image metadata for all bands
+      for (let imageIndex = 0; imageIndex < values.length; ++imageIndex) {
+        if (!isNaN(values[imageIndex])) {
+          this.addAlpha_ = true;
+          break outer;
+        }
+      }
+    }
+
     const additionalBands = this.addAlpha_ ? 1 : 0;
     this.bandCount =
       samplesPerPixel.reduce((accumulator, value) => {
@@ -410,6 +449,7 @@ class GeoTIFFSource extends DataTile {
 
     const pixelCount = size[0] * size[1];
     const dataLength = pixelCount * bandCount;
+    const nodataValues = this.nodataValues_;
 
     return Promise.all(requests).then(function (sourceSamples) {
       const data = new Uint8ClampedArray(dataLength);
@@ -430,8 +470,6 @@ class GeoTIFFSource extends DataTile {
           const gain = 255 / (max - min);
           const bias = -min * gain;
 
-          const nodata = source.nodata;
-
           for (
             let sampleIndex = 0;
             sampleIndex < samplesPerPixel[sourceIndex];
@@ -444,6 +482,17 @@ class GeoTIFFSource extends DataTile {
             if (!addAlpha) {
               data[dataIndex] = value;
             } else {
+              let nodata = source.nodata;
+              if (nodata === undefined) {
+                let bandIndex;
+                if (source.bands) {
+                  bandIndex = source.bands[sampleIndex];
+                } else {
+                  bandIndex = sampleIndex;
+                }
+                nodata = nodataValues[sourceIndex][bandIndex];
+              }
+
               if (sourceValue !== nodata) {
                 transparent = false;
                 data[dataIndex] = value;

test/browser/spec/ol/source/geotiff.test.js

@@ -25,7 +25,9 @@ describe('ol.source.GeoTIFF', function () {
 
   it('configures itself from source metadata', function (done) {
     source.on('change', () => {
-      expect(source.bandCount).to.be(3);
+      expect(source.addAlpha_).to.be(true);
+      expect(source.bandCount).to.be(4);
+      expect(source.nodataValues_).to.eql([[0]]);
       expect(source.getTileGrid().getResolutions().length).to.be(1);
       expect(source.projection.getCode()).to.be('EPSG:4326');
       done();