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
When a raster layer is loaded, it gets a default renderer based on its type. It can be altered either in raster layer properties or programmatically.
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):
단일 밴드 래스터 레이어를 회색조(낮은 값=검은색, 높은 값=하얀색)로 그릴 수도 있고, 단일 밴드의 값에 색상을 배정하는 의사색채 알고리즘으로 그릴 수도 있습니다. 계단식 값(palette)을 가진 단일 밴드 래스터는 그 값에 지정된 색상표를 적용해서 그릴 수 있습니다. 다중 밴드 레이어는 일반적으로 밴드들을 RGB 색상에 매핑해서 그리지만 단 하나의 밴드만 사용해서 회색조나 의사색채 알고리즘으로 그리는 것도 가능합니다.
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)
The shader maps the colors as specified by its color map. The color map is provided as a list of items with pixel value and its associated color. There are three modes of interpolation of values:
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.
주석
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}