This sections lists various operations you can do with raster layers.
A raster layer consists of one or more raster bands — it is referred to as either single band or multi band raster. One band represents a matrix of values. Usual color image (e.g. aerial photo) is a raster consisting of red, blue and green band. Single band layers typically represent either continuous variables (e.g. elevation) or discrete variables (e.g. land use). In some cases, a raster layer comes with a palette and raster values refer to colors stored in the palette:
rlayer.width(), rlayer.height()
(812, 301)
rlayer.extent()
<qgis._core.QgsRectangle object at 0x000000000F8A2048>
rlayer.extent().toString()
u'12.095833,48.552777 : 18.863888,51.056944'
rlayer.rasterType()
2 # 0 = GrayOrUndefined (single band), 1 = Palette (single band), 2 = Multiband
rlayer.bandCount()
3
rlayer.metadata()
u'<p class="glossy">Driver:</p>...'
rlayer.hasPyramids()
False
ラスターレイヤーがロードされると、そのタイプに基づいてデフォルトのレンダラーが取得されます。ラスターレイヤーのプロパティまたはプログラムで変更できます。
To query the current renderer:
>>> rlayer.renderer()
<qgis._core.QgsSingleBandPseudoColorRenderer object at 0x7f471c1da8a0>
>>> rlayer.renderer().type()
u'singlebandpseudocolor'
To set a renderer use setRenderer() method of QgsRasterLayer. There are several available renderer classes (derived from QgsRasterRenderer):
シングルバンドラスターレイヤーはグレースケール(低い値=黒, 高い値=白)でも、シングルバンドの値に色を割り当てた疑似カラーでも表示できます。また、シングルバンドラスターはカラーマップでも表示できます。マルチバンドラスターは基本的にRGBカラーが割り当てて表示されますが、いずれかのバンドをグレースケールや疑似カラーでも表示できます。
The following sections explain how to query and modify the layer drawing style. After doing the changes, you might want to force update of map canvas, see Refreshing Layers.
Let’s say we want to render our raster layer (assuming one band only) with colors ranging from green to yellow (for pixel values from 0 to 255). In the first stage we will prepare QgsRasterShader object and configure its shader function:
>>> fcn = QgsColorRampShader()
>>> fcn.setColorRampType(QgsColorRampShader.INTERPOLATED)
>>> lst = [ QgsColorRampShader.ColorRampItem(0, QColor(0,255,0)), \
QgsColorRampShader.ColorRampItem(255, QColor(255,255,0)) ]
>>> fcn.setColorRampItemList(lst)
>>> shader = QgsRasterShader()
>>> shader.setRasterShaderFunction(fcn)
シェーダは、そのカラーマップによって指定された色をマッピングします。カラーマップは、画素値とそれに関連する色を持つ項目のリストとなっています。値の補間のモードは3つあります。
In the second step we will associate this shader with the raster layer:
>>> renderer = QgsSingleBandPseudoColorRenderer(layer.dataProvider(), 1, shader)
>>> layer.setRenderer(renderer)
The number 1 in the code above is band number (raster bands are indexed from one).
By default, QGIS maps the first three bands to red, green and blue values to create a color image (this is the MultiBandColor drawing style. In some cases you might want to override these setting. The following code interchanges red band (1) and green band (2):
rlayer.renderer().setGreenBand(1)
rlayer.renderer().setRedBand(2)
In case only one band is necessary for visualization of the raster, single band drawing can be chosen — either gray levels or pseudocolor.
If you do change layer symbology and would like ensure that the changes are immediately visible to the user, call these methods
if hasattr(layer, "setCacheImage"):
layer.setCacheImage(None)
layer.triggerRepaint()
The first call will ensure that the cached image of rendered layer is erased in case render caching is turned on. This functionality is available from QGIS 1.4, in previous versions this function does not exist — to make sure that the code works with all versions of QGIS, we first check whether the method exists.
ノート
This method is deprecated as of QGIS 2.18.0 and will produce a warning. Simply calling triggerRepaint() is sufficient.
The second call emits signal that will force any map canvas containing the layer to issue a refresh.
With WMS raster layers, these commands do not work. In this case, you have to do it explicitly
layer.dataProvider().reloadData()
layer.triggerRepaint()
In case you have changed layer symbology (see sections about raster and vector layers on how to do that), you might want to force QGIS to update the layer symbology in the layer list (legend) widget. This can be done as follows (iface is an instance of QgisInterface)
iface.legendInterface().refreshLayerSymbology(layer)
To do a query on value of bands of raster layer at some specified point
ident = rlayer.dataProvider().identify(QgsPoint(15.30, 40.98), \
QgsRaster.IdentifyFormatValue)
if ident.isValid():
print ident.results()
The results method in this case returns a dictionary, with band indices as keys, and band values as values.
{1: 17, 2: 220}