.. only:: html .. index:: pair: Raster; Raster layers .. _raster: ********************* Using Raster Layers ********************* .. warning:: |outofdate| .. contents:: :local: The code snippets on this page needs the following imports if you're outside the pyqgis console: .. code-block:: python from qgis.core import ( QgsRasterLayer, QgsColorRampShader, QgsSingleBandPseudoColorRenderer ) .. index:: Raster layers; Details Layer Details ============= A raster layer consists of one or more raster bands --- referred to as single band and multi band rasters. One band represents a matrix of values. A color image (e.g. aerial photo) is a raster consisting of red, blue and green bands. Single band rasters 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 the raster values refer to the colors stored in the palette. The following code assumes ``rlayer`` is a :class:`QgsRasterLayer ` object. .. code-block:: python rlayer = QgsProject.instance().mapLayersByName('srtm')[0] # get the resolution of the raster in layer unit rlayer.width(), rlayer.height() (919, 619) # get the extent of the layer as QgsRectangle rlayer.extent() # get the extent of the layer as Strings rlayer.extent().toString() '20.0685680819999988,-34.2700107699999990 : 20.8394528430000001,-33.7507750070000014' # get the raster type: 0 = GrayOrUndefined (single band), 1 = Palette (single band), 2 = Multiband rlayer.rasterType() 0 # get the total band count of the raster rlayer.bandCount() 1 # get all the available metadata as a QgsLayerMetadata object rlayer.metadata() '' .. index:: Raster layers; Renderer Renderer ======== When a raster layer is loaded, it gets a default renderer based on its type. It can be altered either in the layer properties or programmatically. To query the current renderer: .. code-block:: python rlayer.renderer() rlayer.renderer().type() 'singlebandgray' To set a renderer, use the :meth:`setRenderer ` method of :class:`QgsRasterLayer `. There are a number of renderer classes (derived from :class:`QgsRasterRenderer `): * :class:`QgsMultiBandColorRenderer ` * :class:`QgsPalettedRasterRenderer ` * :class:`QgsSingleBandColorDataRenderer ` * :class:`QgsSingleBandGrayRenderer ` * :class:`QgsSingleBandPseudoColorRenderer ` Single band raster layers can be drawn either in gray colors (low values = black, high values = white) or with a pseudocolor algorithm that assigns colors to the values. Single band rasters with a palette can also be drawn using the palette. Multiband layers are typically drawn by mapping the bands to RGB colors. Another possibility is to use just one band for drawing. .. index:: Raster layers; Single band Single Band Rasters ------------------- Let's say we want a render single band raster layer with colors ranging from green to yellow (corresponding to pixel values from 0 to 255). In the first stage we will prepare a :class:`QgsRasterShader ` object and configure its shader function: .. code-block:: python 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 pixel values with associated colors. There are three modes of interpolation: * linear (``Interpolated``): the color is linearly interpolated from the color map entries above and below the pixel value * discrete (``Discrete``): the color is taken from the closest color map entry with equal or higher value * exact (``Exact``): the color is not interpolated, only pixels with values equal to color map entries will be drawn In the second step we will associate this shader with the raster layer: .. code-block:: python renderer = QgsSingleBandPseudoColorRenderer(rlayer.dataProvider(), 1, shader) rlayer.setRenderer(renderer) The number ``1`` in the code above is the band number (raster bands are indexed from one). Finally we have to use the :meth:`triggerRepaint ` method to see the results: .. code-block:: python rlayer.triggerRepaint() .. index:: Raster layers; Multi band Multi Band Rasters ------------------ By default, QGIS maps the first three bands to red, green and blue 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): .. code-block:: python rlayer_multi = QgsProject.instance().mapLayersByName('multiband')[0] rlayer_multi.renderer().setGreenBand(1) rlayer_multi.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. We have to use :meth:`triggerRepaint ` to update the map and see the result: .. code-block:: python rlayer_multi.triggerRepaint() .. index:: pair: Raster layers; Refreshing .. index:: pair: Raster layers; Querying Query Values ============ Raster values can be queried using the :meth:`sample ` method of the :class:`QgsRasterDataProvider ` class. You have to specify a :class:`QgsPointXY ` and the band number of the raster layer you want to query. The method returns a tuple with the value and ``True`` or ``False`` depending on the results: .. code-block:: python val, res = rlayer.dataProvider().sample(QgsPointXY(20.50, -34), 1) Another method to query raster values is using the :meth:`identify ` method that returns a :class:`QgsRasterIdentifyResult ` object. .. code-block:: python ident = rlayer.dataProvider().identify(QgsPointXY(20.5, -34), QgsRaster.IdentifyFormatValue) if ident.isValid(): print(ident.results()) In this case, the :meth:`results ` method returns a dictionary, with band indices as keys, and band values as values. For instance, something like ``{1: 323.0}`` .. Substitutions definitions - AVOID EDITING PAST THIS LINE This will be automatically updated by the find_set_subst.py script. If you need to create a new substitution manually, please add it also to the substitutions.txt file in the source folder. .. |outofdate| replace:: `Despite our constant efforts, information beyond this line may not be updated for QGIS 3. Refer to https://qgis.org/pyqgis/master for the python API documentation or, give a hand to update the chapters you know about. Thanks.`