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Utiliser des couches raster

Cette section liste différentes opérations réalisables avec des couches raster.

Détails d’une couche

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()
u'12.095833,48.552777 : 18.863888,51.056944'
>>> rlayer.rasterType()
2  # 0 = GrayOrUndefined (single band), 1 = Palette (single band), 2 = Multiband
>>> rlayer.bandCount()
>>> rlayer.metadata()
u'<p class="glossy">Driver:</p>...'
>>> rlayer.hasPyramids()

Drawing Style

When a raster layer is loaded, it gets a default drawing style based on its type. It can be altered either in raster layer properties or programmatically. The following drawing styles exist:

Index Constant: QgsRasterLater.X


1 SingleBandGray Single band image drawn as a range of gray colors
2 SingleBandPseudoColor Single band image drawn using a pseudocolor algorithm
3 PalettedColor “Palette” image drawn using color table
4 PalettedSingleBandGray “Palette” layer drawn in gray scale
5 PalettedSingleBandPseudoColor “Palette” layerdrawn using a pseudocolor algorithm
7 MultiBandSingleBandGray Layer containing 2 or more bands, but a single band drawn as a range of gray colors
8 MultiBandSingleBandPseudoColor Layer containing 2 or more bands, but a single band drawn using a pseudocolor algorithm
9 MultiBandColor Layer containing 2 or more bands, mapped to RGB color space.

To query the current drawing style:

>>> rlayer.drawingStyle()

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 for values from the single band. Single band rasters with a palette can be additionally drawn using their palette. Multiband layers are typically drawn by mapping the bands to RGB colors. Other possibility is to use just one band for gray or pseudocolor drawing.

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 Rafraîchir les couches.

TODO: contrast enhancements, transparency (no data), user defined min/max, band statistics

Single Band Rasters

They are rendered in gray colors by default. To change the drawing style to pseudocolor:

>>> rlayer.setDrawingStyle(QgsRasterLayer.SingleBandPseudoColor)
>>> rlayer.setColorShadingAlgorithm(QgsRasterLayer.PseudoColorShader)

The PseudoColorShader is a basic shader that highlighs low values in blue and high values in red. Another, FreakOutShader uses more fancy colors and according to the documentation, it will frighten your granny and make your dogs howl.

There is also ColorRampShader which maps the colors as specified by its color map. It has three modes of interpolation of values:

  • linear (INTERPOLATED): resulting color is linearly interpolated from the color map entries above and below the actual pixel value
  • discrete (DISCRETE): color is used from the color map entry with equal or higher value
  • exact (EXACT): color is not interpolated, only the pixels with value equal to color map entries are drawn

To set an interpolated color ramp shader ranging from green to yellow color (for pixel values from 0 to 255):

>>> rlayer.setColorShadingAlgorithm(QgsRasterLayer.ColorRampShader)
>>> lst = [ QgsColorRampShader.ColorRampItem(0, QColor(0,255,0)), \
    QgsColorRampShader.ColorRampItem(255, QColor(255,255,0)) ]
>>> fcn = rlayer.rasterShader().rasterShaderFunction()
>>> fcn.setColorRampType(QgsColorRampShader.INTERPOLATED)
>>> fcn.setColorRampItemList(lst)

Pour retourner aux niveaux de gris par défaut, utilisez:

>>> rlayer.setDrawingStyle(QgsRasterLayer.SingleBandGray)

Multi Band Rasters

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.setGreenBandName(rlayer.bandName(1))
>>> rlayer.setRedBandName(rlayer.bandName(2))

In case only one band is necessary for visualization of the raster, single band drawing can be chosen — either gray levels or pseudocolor, see previous section:

>>> rlayer.setDrawingStyle(QgsRasterLayer.MultiBandSingleBandPseudoColor)
>>> rlayer.setGrayBandName(rlayer.bandName(1))
>>> rlayer.setColorShadingAlgorithm(QgsRasterLayer.PseudoColorShader)
>>> # now set the shader

Rafraîchir les couches

Si vous avez effectué des modifications sur le style d’une couche et tenez à ce qu’elles soient automatiquement visibles pour l’utilisateur, appelez ces méthodes:

if hasattr(layer, "setCacheImage"): layer.setCacheImage(None)

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.

La deuxième commande émet un signal forçant l’actualisation de tout canevas de carte contenant la couche.

With WMS raster layers, these commands do not work. In this case, you have to do it explicitily:


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):


Query Values

To do a query on value of bands of raster layer at some specified point:

ident = rlayer.dataProvider().identify(QgsPoint(15.30,40.98), \
if ident.isValid():
  print ident.results()

The results method in this case returs a dictionary, with band indices as keys, and band values as values.

{1: 17, 2: 220}