GeoTools入门（七）-- 操作栅格数据

1. 概述

在前面的章节，我们主要讲述了GeoTools对于矢量(主要是Shape)数据的操作。在地理信息系统的世界里，还有一类很重要的数据类型，那便是栅格数据。虽然GeoTools对于栅格数据的支持并没有gdal强大，但既然他作为GeoTools的一部分，我们还是又必要了解一下它。

2. GridCoverage

GeoTools对于栅格数据的支持主要又由GridCoverage实现的。作为一名程序员，我们习惯于处理诸如JPEG、GIF或者PNG等格式的栅格数据。在地理空间方面，有一个Coverage个概念，他是空间定位要素的集合。非正式地，我们将地图和Coverage视为等同，当然，这是相对于地理意思而非编程思维上。

GridCoverage是Coverage的一个特例，他要求栅格要以矩形的形式填充到Coverage的区域中。在我们的java代码中，我们可以使用位图图形作为GridCoverage的后台数据结构，并使用其他元素记录特定坐标系中的空间边界。

这里有许多种GridCoverage的文件格式，最常用的如一下三类。

World+ 图像

一种普通的图像格式，如jpeg或png，它有一个sidecar 文件来描述它的位置，还有一个prjsidecar文件来定义地图投影，就像shapefile使用的那样。请注意，尽管jpeg格式由于下载量小而很常见；运行时的性能非常糟糕，因为整个图像都需要读入内存。TIFF等格式没有此限制。
GeoTiff

具有存储在图像元数据字段中的地理空间信息的普通tiff图像。这通常在安全的前提下表现出良好的性能；特别是如果它已经准备了一个内部覆盖（可用于缩小）或内部平铺（允许快速平移时放大）。当计算机的磁盘速度比CPU快时，性能最佳。
JPEG2000

他jpeg的扩展，使用小波压缩来处理大量图像。文件格式还支持可用于存储地理空间信息的元数据字段。当您CPU速度比磁盘访问更快的情况下，这种格式的性能最好。

针对上面三种数据个数的描述，我们知道，第一种数据格式基本是被摒弃的那种。GeoTiff和JPEG2000的区别是要看IO性能VS CPU性能。

3. GridCoverage对栅格数据的操作

在本篇文章中，我们主要关注GridCoverage对于栅格数据的加载，波段提取，显示样式的创建。

3.1 栅格数据的加载

AbstractGridFormat format = GridFormatFinder.findFormat(rasterFile);
// this is a bit hacky but does make more geotiffs work
Hints hints = new Hints();
if (format instanceof GeoTiffFormat) {hints = new Hints(Hints.FORCE_LONGITUDE_FIRST_AXIS_ORDER, Boolean.TRUE);
}
reader = format.getReader(rasterFile, hints);

3.2 波段提取

GridCoverage2D cov = reader.read(null);
int numBands = cov.getNumSampleDimensions();
for (int i = 0; i < numBands; i++) {GridSampleDimension dim = cov.getSampleDimension(i);//这里依次输出 RED_BAND  GREEN_BANK BLUE_BANKSystem.out.println(dim.getDescription().toString());
}

3.3 样式创建

3.3.1 创建灰度的样式

private Style createGreyscaleStyle(int band) {StyleFactory sf = CommonFactoryFinder.getStyleFactory();FilterFactory2 ff = CommonFactoryFinder.getFilterFactory2();ContrastEnhancement ce = sf.contrastEnhancement(ff.literal(1.0), ContrastMethod.NORMALIZE);SelectedChannelType sct = sf.createSelectedChannelType(String.valueOf(band), ce);RasterSymbolizer sym = sf.getDefaultRasterSymbolizer();ChannelSelection sel = sf.channelSelection(sct);sym.setChannelSelection(sel);return SLD.wrapSymbolizers(sym);
}

3.3.2 创建RGB样式

StyleFactory sf = CommonFactoryFinder.getStyleFactory();
FilterFactory2 ff = CommonFactoryFinder.getFilterFactory2();
SelectedChannelType[] sct = new SelectedChannelType[cov.getNumSampleDimensions()];
ContrastEnhancement ce = sf.contrastEnhancement(ff.literal(1.0), ContrastMethod.NORMALIZE);
for (int i = 0; i < 3; i++) {sct[i] = sf.createSelectedChannelType(String.valueOf(channelNum[i]), ce);
}
RasterSymbolizer sym = sf.getDefaultRasterSymbolizer();
ChannelSelection sel = sf.channelSelection(sct[RED], sct[GREEN], sct[BLUE]);
sym.setChannelSelection(sel);
return SLD.wrapSymbolizers(sym);

通过上面的例子，我们可以简单画个时序图及流程图方便大家记忆：

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4. 示例

前面的示例中，我们查看了如何读取和显示Shape文件。对于ImageLab.java，我们将通过显示一个三波段的全球卫星图像，并将其与shapefile中的国家边界进行叠加，将光栅数据添加到混合中。

4.1 pom引入

<dependencies><dependency><groupId>org.geotools</groupId><artifactId>gt-shapefile</artifactId><version>${geotools.version}</version></dependency><dependency><groupId>org.geotools</groupId><artifactId>gt-swing</artifactId><version>${geotools.version}</version></dependency><dependency><groupId>org.geotools</groupId><artifactId>gt-epsg-hsql</artifactId><version>${geotools.version}</version></dependency><dependency><groupId>org.geotools</groupId><artifactId>gt-geotiff</artifactId><version>${geotools.version}</version></dependency><dependency><groupId>org.geotools</groupId><artifactId>gt-image</artifactId><version>${geotools.version}</version></dependency>
</dependencies>
<repositories><repository><id>osgeo</id><name>OSGeo Release Repository</name><url>https://repo.osgeo.org/repository/release/</url><snapshots><enabled>false</enabled></snapshots><releases><enabled>true</enabled></releases></repository><repository><id>osgeo-snapshot</id><name>OSGeo Snapshot Repository</name><url>https://repo.osgeo.org/repository/snapshot/</url><snapshots><enabled>true</enabled></snapshots><releases><enabled>false</enabled></releases></repository>
</repositories>

4.2 创建ImageLab类

public class ImageLab {private StyleFactory sf = CommonFactoryFinder.getStyleFactory();private FilterFactory2 ff = CommonFactoryFinder.getFilterFactory2();private JMapFrame frame;private GridCoverage2DReader reader;public static void main(String[] args) throws Exception {ImageLab me = new ImageLab();me.getLayersAndDisplay();}
}

4.3 选择Image和shape文件

 private void getLayersAndDisplay() throws Exception {List<Parameter<?>> list = new ArrayList<>();list.add(new Parameter<>("image",File.class,"Image","GeoTiff or World+Image to display as basemap",new KVP(Parameter.EXT, "tif", Parameter.EXT, "jpg")));list.add(new Parameter<>("shape",File.class,"Shapefile","Shapefile contents to display",new KVP(Parameter.EXT, "shp")));JParameterListWizard wizard =new JParameterListWizard("Image Lab", "Fill in the following layers", list);int finish = wizard.showModalDialog();if (finish != JWizard.FINISH) {System.exit(0);}File imageFile = (File) wizard.getConnectionParameters().get("image");File shapeFile = (File) wizard.getConnectionParameters().get("shape");displayLayers(imageFile, shapeFile);}

4.4 显示地图

private void displayLayers(File rasterFile, File shpFile) throws Exception {AbstractGridFormat format = GridFormatFinder.findFormat(rasterFile);Hints hints = new Hints();if (format instanceof GeoTiffFormat) {hints = new Hints(Hints.FORCE_LONGITUDE_FIRST_AXIS_ORDER, Boolean.TRUE);}reader = format.getReader(rasterFile, hints);// 创建一个灰度样式Style rasterStyle = createGreyscaleStyle(1);// 连接shape图层FileDataStore dataStore = FileDataStoreFinder.getDataStore(shpFile);SimpleFeatureSource shapefileSource = dataStore.getFeatureSource();//创建一个黄色边界且没有填充的简单样式Style shpStyle = SLD.createPolygonStyle(Color.YELLOW, null, 0.0f);// 创建一个两个图层的地图final MapContent map = new MapContent();map.setTitle("ImageLab");Layer rasterLayer = new GridReaderLayer(reader, rasterStyle);map.addLayer(rasterLayer);Layer shpLayer = new FeatureLayer(shapefileSource, shpStyle);map.addLayer(shpLayer);//设置地图界面的样式frame = new JMapFrame(map);frame.setSize(800, 600);frame.enableStatusBar(true);frame.enableToolBar(true);JMenuBar menuBar = new JMenuBar();frame.setJMenuBar(menuBar);JMenu menu = new JMenu("Raster");menuBar.add(menu);menu.add(new SafeAction("Grayscale display") {public void action(ActionEvent e) throws Throwable {Style style = createGreyscaleStyle();if (style != null) {((StyleLayer) map.layers().get(0)).setStyle(style);frame.repaint();}}});menu.add(new SafeAction("RGB display") {public void action(ActionEvent e) throws Throwable {Style style = createRGBStyle();if (style != null) {((StyleLayer) map.layers().get(0)).setStyle(style);frame.repaint();}}});frame.setVisible(true);
}

4.4.1 创建选择灰度的对话框

private Style createGreyscaleStyle() {GridCoverage2D cov = null;try {cov = reader.read(null);} catch (IOException giveUp) {throw new RuntimeException(giveUp);}int numBands = cov.getNumSampleDimensions();Integer[] bandNumbers = new Integer[numBands];for (int i = 0; i < numBands; i++) {bandNumbers[i] = i + 1;}Object selection =JOptionPane.showInputDialog(frame,"Band to use for greyscale display","Select an image band",JOptionPane.QUESTION_MESSAGE,null,bandNumbers,1);if (selection != null) {int band = ((Number) selection).intValue();return createGreyscaleStyle(band);}return null;
}

4.4.2 创建灰度样式

private Style createGreyscaleStyle(int band) {ContrastEnhancement ce = sf.contrastEnhancement(ff.literal(1.0), ContrastMethod.NORMALIZE);SelectedChannelType sct = sf.createSelectedChannelType(String.valueOf(band), ce);RasterSymbolizer sym = sf.getDefaultRasterSymbolizer();ChannelSelection sel = sf.channelSelection(sct);sym.setChannelSelection(sel);return SLD.wrapSymbolizers(sym);
}

4.5 创建RGB样式

private Style createRGBStyle() {GridCoverage2D cov = null;try {cov = reader.read(null);} catch (IOException giveUp) {throw new RuntimeException(giveUp);}//我们需要至少三个波段的样式int numBands = cov.getNumSampleDimensions();if (numBands < 3) {return null;}//获取各个波段的名称String[] sampleDimensionNames = new String[numBands];for (int i = 0; i < numBands; i++) {GridSampleDimension dim = cov.getSampleDimension(i);sampleDimensionNames[i] = dim.getDescription().toString();}final int RED = 0, GREEN = 1, BLUE = 2;int[] channelNum = {-1, -1, -1};for (int i = 0; i < numBands; i++) {String name = sampleDimensionNames[i].toLowerCase();if (name != null) {if (name.matches("red.*")) {channelNum[RED] = i + 1;} else if (name.matches("green.*")) {channelNum[GREEN] = i + 1;} else if (name.matches("blue.*")) {channelNum[BLUE] = i + 1;}}}if (channelNum[RED] < 0 || channelNum[GREEN] < 0 || channelNum[BLUE] < 0) {channelNum[RED] = 1;channelNum[GREEN] = 2;channelNum[BLUE] = 3;}//我们使用selected通道创建RasterSymbolizer样式SelectedChannelType[] sct = new SelectedChannelType[cov.getNumSampleDimensions()];ContrastEnhancement ce = sf.contrastEnhancement(ff.literal(1.0), ContrastMethod.NORMALIZE);for (int i = 0; i < 3; i++) {sct[i] = sf.createSelectedChannelType(String.valueOf(channelNum[i]), ce);}RasterSymbolizer sym = sf.getDefaultRasterSymbolizer();ChannelSelection sel = sf.channelSelection(sct[RED], sct[GREEN], sct[BLUE]);sym.setChannelSelection(sel);return SLD.wrapSymbolizers(sym);
}