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5. Utilizarea straturilor raster

Sugestie

Fragmentele de cod de pe această pagină necesită următoarele importuri, dacă vă aflați în afara consolei pyqgis:

from qgis.core import (
    QgsRasterLayer,
    QgsProject,
    QgsPointXY,
    QgsRaster,
    QgsRasterShader,
    QgsColorRampShader,
    QgsSingleBandPseudoColorRenderer,
    QgsSingleBandColorDataRenderer,
    QgsSingleBandGrayRenderer,
)

from qgis.PyQt.QtGui import (
    QColor,
)

5.1. Detaliile stratului

Un strat raster constă dintr-una sau mai multe benzi raster — denumite raster cu o singură bandă și cu benzi multiple. O bandă reprezintă o matrice de valori. O imagine color (de exemplu, o fotografie aeriană) este un raster format din benzi roșii, albastre și verzi. Rasterele cu o singură bandă reprezintă, de obicei, fie variabile continue (de ex.: altitudinea), fie variabile discrete (de ex.: utilizarea terenului). În unele cazuri, un strat raster vine cu o paletă, iar valorile raster se referă la culorile stocate în paleta respectivă.

The following code assumes rlayer is a QgsRasterLayer object.

rlayer = QgsProject.instance().mapLayersByName('srtm')[0]
# get the resolution of the raster in layer unit
print(rlayer.width(), rlayer.height())

919 619

# get the extent of the layer as QgsRectangle
print(rlayer.extent())

<QgsRectangle: 20.06856808199999875 -34.27001076999999896, 20.83945284300000012 -33.75077500700000144>

# get the extent of the layer as Strings
print(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
print(rlayer.rasterType())

RasterLayerType.GrayOrUndefined

 # get the total band count of the raster
print(rlayer.bandCount())

1

# get the first band name of the raster
print(rlayer.bandName(1))

Band 1: Height

# get all the available metadata as a QgsLayerMetadata object
print(rlayer.metadata())

<qgis._core.QgsLayerMetadata object at 0x13711d558>

5.2. Render

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.

Pentru a interoga renderul curent:

print(rlayer.renderer())

<qgis._core.QgsSingleBandGrayRenderer object at 0x7f471c1da8a0>

print(rlayer.renderer().type())

singlebandgray

To set a renderer, use the setRenderer() method of QgsRasterLayer. There are a number of renderer classes (derived from QgsRasterRenderer):

• QgsHillshadeRenderer

• QgsMultiBandColorRenderer

• QgsPalettedRasterRenderer

• QgsRasterContourRenderer

• QgsSingleBandColorDataRenderer

• QgsSingleBandGrayRenderer

• 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.

5.2.1. Rastere cu o singură bandă

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 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 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:

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 triggerRepaint() method to see the results:

rlayer.triggerRepaint()

5.2.2. Rastere multibandă

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):

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 triggerRepaint() to update the map and see the result:

rlayer_multi.triggerRepaint()

5.3. Interogarea valorilor

Raster values can be queried using the sample() method of the QgsRasterDataProvider class. You have to specify a 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:

val, res = rlayer.dataProvider().sample(QgsPointXY(20.50, -34), 1)

Another method to query raster values is using the identify() method that returns a QgsRasterIdentifyResult object.

ident = rlayer.dataProvider().identify(QgsPointXY(20.5, -34), QgsRaster.IdentifyFormatValue)

if ident.isValid():
  print(ident.results())

{1: 323.0}

In this case, the results() method returns a dictionary, with band indices as keys, and band values as values. For instance, something like {1: 323.0}

5.4. Editing raster data

You can create a raster layer using the QgsRasterBlock class. For example, to create a 2x2 raster block with one byte per pixel:

block = QgsRasterBlock(Qgis.Byte, 2, 2)
block.setData(b'\xaa\xbb\xcc\xdd')

Raster pixels can be overwritten thanks to the writeBlock() method. To overwrite existing raster data at position 0,0 by the 2x2 block:

provider = rlayer.dataProvider()
provider.setEditable(True)
provider.writeBlock(block, 1, 0, 0)
provider.setEditable(False)