[GIS] pyQGIS changing Paletted Raster Layers to Grayscale

colorpyqgispythonqgisraster

I've written a plugin which imports a number of colour geoTifs (map tiles) and works correctly but it would be great to display these as Grayscale after loading in.

The geoTifs load as paletted images (under Render Type) and I am looking to have them display as they do when you manually change the layer properties to set Grayscale to any of the three options (by lightness, by luminosity, by averaqe).

I've tried many things which haven't worked including things like layer.setDrawingStyle('PalettedSingleBandGray') which gives a very strange looking image – I assume it's only using information from a single colour channel?

I understand that these Grayscale options are relatively new to QGIS and hence their implementation through pyQGIS may not be straightforward, but I'm completely stuck with this now.

Following the response from xunilk, I've had a further play with this and whilst this does work, it doesn't quite do what I'm after.

Running xunilk's script for a colour tif gives:

Desired result (settings in image 3):

Setting grayscale OR desaturating the image as in image 3 give result like that in image 2.

Importantly, note that when SingleBandGrayRenderer is used (manually selected or through the script), even the manual selection of the grayscale option as in image 3 above gives a result that looks like image 1.

It seems that it needs to be loaded using QgsPalettedRasterRenderer with relevant rendering options specified?

Best Answer

Next script works for me.

layer=iface.activeLayer()

myGrayRenderer = QgsSingleBandGrayRenderer(layer.dataProvider(), 
                                           1)

layer.setRenderer(myGrayRenderer)

renderer = layer.renderer()
provider = layer.dataProvider()

layer_extent = layer.extent()
uses_band = renderer.usesBands()

myType = renderer.dataType(uses_band[0])

stats = provider.bandStatistics(uses_band[0], 
                                QgsRasterBandStats.All, 
                                layer_extent, 
                                0)

myEnhancement = QgsContrastEnhancement(myType)

contrast_enhancement = QgsContrastEnhancement.StretchToMinimumMaximum

myEnhancement.setContrastEnhancementAlgorithm(contrast_enhancement,True)
myEnhancement.setMinimumValue(stats.minimumValue)
myEnhancement.setMaximumValue(stats.maximumValue)

layer.renderer().setContrastEnhancement(myEnhancement)
layer.triggerRepaint()

I tested it for changing an object of QgsSingleBandPseudoColorRenderer class (in your case it would be a QgsPalettedRasterRenderer object) in a QgsSingleBandGrayRenderer class object.

Before running the script at the Python Console:

After running the script:

Editing Note:

I tested the script with the same raster and a paletted render type (see next image) and it worked too.

Related Solutions

[GIS] Suitable Raster processing methods to extract field boundaries from images

I'm not sure how well this will work without trying on your specific raster, but a few pointers. I assume from your image you used Threshold in GIMP to give you a binary raster (inside field/outside field)

Removing noise

You can use GDAL Sieve to remove the 'salt and pepper' noise - those little holes within the fields.

You can do this from QGIS (Raster > Analysis > Sieve). That should fill any 'holes' (smaller than an area you specify) with the value of the surrounding raster area. If you're lucky and guess the minimum area well, it might also clean up the noise in the gaps between fields too.

Using QGIS to get edges

Depending on how successful that is, and how well you choose the sieve settings, you may be able to convert the noise-free raster straight to polygons. There seems to be a lot of noise in the gaps between fields so there's a good chance you end up with some fields joined together.

Using SAGA to get edges

If you get a reasonably clean noise-free image from Sieve

SAGA GIS has a Raster Connectivty Filter (Grid > Filter > Connectivity Analysis). This will give two outputs

a raster, with each contiguous area of pixels given its own ID
a shapefile with the boundary of each contiguous area of pixels

Using GIMP to get edges

GIMP has a number of noise-removal and edge filters (e.g. Sobel, Difference of Guassians etc), you might want to try these too.

Post-processing

You might also need to simplify geometries to get rid of the 'staircasing' (the polygon edges will trace round pixel boundaries)

[GIS] PyQGIS save raster as rendered image, then use GDAL tools on it

You should understand that when you save the file from the QgsRenderer as in your code:

piperenderer = pipelayer.renderer()
pipe.set(piperenderer.clone())
file_writer.writeRaster(pipe...

You are not saving the raster styled, instead you are saving the actual RGB file that represents that visualization, so it is not classified or styled, but it is like you've taken a print screen of the rendered image.

So gdalwarp and translate can't change this, as it is hardcoded in the image's bands, what must be happening is that they are clipping by a single band or so, I have to check it thorughly.

EDIT

I can see in the xml file you provided that the raster has now only one band

<PAMRasterBand band="1">

There should be the tags:

<PAMRasterBand band="2">
<PAMRasterBand band="3">

So the problem is in gdalogr:warpreproject. I used the same code you provided in a raster I have but the output is RGB, I don't know what's going on, maybe you should give a sample of your raster file.

NOTE

The xml has no style data, as I said the raster you firstly saved is not a styled layer, but an actual RGB image representing the styled raster.