.. 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 <qgis.core.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()
<QgsRectangle: 20.06856808199999875 -34.27001076999999896, 20.83945284300000012 -33.75077500700000144>
# 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()
'<qgis._core.QgsLayerMetadata object at 0x13711d558>'
.. 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()
<qgis._core.QgsSingleBandGrayRenderer object at 0x7f471c1da8a0>
rlayer.renderer().type()
'singlebandgray'
To set a renderer, use the :meth:`setRenderer <qgis.core.QgsRasterLayer.setRenderer>`
method of :class:`QgsRasterLayer <qgis.core.QgsRasterLayer>`. There are a
number of renderer classes (derived from :class:`QgsRasterRenderer
<qgis.core.QgsRasterRenderer>`):
* :class:`QgsMultiBandColorRenderer <qgis.core.QgsMultiBandColorRenderer>`
* :class:`QgsPalettedRasterRenderer <qgis.core.QgsPalettedRasterRenderer>`
* :class:`QgsSingleBandColorDataRenderer <qgis.core.QgsSingleBandColorDataRenderer>`
* :class:`QgsSingleBandGrayRenderer <qgis.core.QgsSingleBandGrayRenderer>`
* :class:`QgsSingleBandPseudoColorRenderer <qgis.core.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 <qgis.core.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 <qgis.core.QgsMapLayer.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 <qgis.core.QgsMapLayer.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 <qgis.core.QgsRasterDataProvider.sample>` method of
the :class:`QgsRasterDataProvider <qgis.core.QgsRasterDataProvider>` class.
You have to specify a :class:`QgsPointXY <qgis.core.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
<qgis.core.QgsRasterDataProvider.identify>` method that returns a
:class:`QgsRasterIdentifyResult <qgis.core.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 <qgis.core.QgsRasterIdentifyResult.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.`