[Qgis-developer] Re: Raster layer display control from Plugin
Peter Ersts
ersts at amnh.org
Mon Sep 6 21:12:52 EDT 2010
Howdy JP,
Not quite sure what the question is at this stage. However, here are
some general comments.
When the raster is first loaded, the min max values for each band are
provided by GDAL but are estimates. You will not get the exact min max
values until the band statistics are generated, which depending on the
size of the image can take be slow. The raster has to be rendered at
least once for computeMinimumMaximumFromLastExtent() to provide
meaningful values.
Re:
"On the other hand,
resultsLayer.setColorShadingAlgorithm("PseudoColorShader")
Doesn't seem to do anything at all. Strange."
Pseudo color shading does not just use the minimum and maximum values,
it requires you to define the number of standard deviations to apply.
Also the pseudo color shader has to be used with the correct drawing
style, i.e., QgsRasterLayer.SingleBandPseudoColor
-pete
On 09/05/2010 10:52 AM, JP Glutting wrote:
> Ok, I figured this out. It was as simple as:
>
> *resultsLayer.setContrastEnhancementAlgorithm(QgsContrastEnhancement.StretchToMinimumMaximum)*
>
> That computes the min and max, and stretches the grayscale (to black
> and white for boolean rasters).
>
> In the mean time I was futzing around with:
>
> * band = resultsLayer.bandNumber(resultsLayer.grayBandName()) *
> * minVal = resultsLayer.minimumValue(band)*
> * maxVal = resultsLayer.maximumValue(band)*
>
> and
>
> * resultsLayer.setMinimumValue(band, minVal, generateLookupTableFlag)*
> * resultsLayer.setMaximumValue(band, maxVal,
> generateLookupTableFlag)*
>
> but it turns out that *resultsLayer.**setMaximumValue(band) *returns
> the theoretical maximum (255) rather than the real maximum (1), which
> is not what I wanted. You can get the correct value with
>
> * maxVal = resultsLayer.bandStatistics(band).maximumValue*
>
> However, it says that calling bandStatistics is very CPU intensive,
> since it also calculates a lot of other stuff. As it turns out, the
> StretchToMinimumMaximum algorithm seems to calculate the minmax
> anyway, so none of that is needed.
>
> My big mistake, as I was rushing through this, was to confuse extent
> and values, as I was trying to copy from a tutorial and wasn't paying
> close attention. Once I figured that out, it was easy.
>
> On the other hand,
>
> *resultsLayer.setColorShadingAlgorithm("PseudoColorShader")*
>
> Doesn't seem to do anything at all. Strange.
>
> Cheers,
> JP
>
> On Sat, Sep 4, 2010 at 6:35 PM, Benoit de Cabissole
> <benoit at exigesa.com <mailto:benoit at exigesa.com>> wrote:
>
> Hi JP,
> I would suggest asking your question on the Qgis-developer list
> instead of this one.
> Below is what I've done to display a custom colormap on a raster.
> Could it be adapted to your problem?
> # Display the raster with the selected colour table:
> #
> # - tell the layer to use a QgsColorRampShader function
> theLayer.setColorShadingAlgorithm(
> QgsRasterLayer.ColorRampShader )
> # - get a pointer to the raster shader function (QgsColorRampShader)
> myColorRampShader =
> theLayer.rasterShader().rasterShaderFunction()
> # - set parameters for the QgsColorRampShader function
> myColorRampShader.setColorRampType( QgsColorRampShader.DISCRETE )
> myColorRampShader.setColorRampItemList( theTBL )
> theLayer.setDrawingStyle( QgsRasterLayer.SingleBandPseudoColor )
> # - refresh map & legend
> if hasattr(theLayer, "setCacheImage"):
> theLayer.setCacheImage( None )
> theLayer.triggerRepaint()
> self.iface.legendInterface().refreshLayerSymbology( theLayer )
> # - tell QGIS that it needs to ask user to save changes
> self.iface.mapCanvas().setDirty( True )
> Cheers,
> Benoit
>
> -----Original Message-----
> *From:* qgis-user-bounces at lists.osgeo.org
> <mailto:qgis-user-bounces at lists.osgeo.org>
> [mailto:qgis-user-bounces at lists.osgeo.org
> <mailto:qgis-user-bounces at lists.osgeo.org>]*On Behalf Of *JP
> Glutting
> *Sent:* Saturday, 04 September 2010 17:47
> *To:* Qgis-user at lists.osgeo.org <mailto:Qgis-user at lists.osgeo.org>
> *Subject:* [Qgis-user] Re: Raster layer display control from
> Plugin
>
> No takers? No hints? I have been looking all over the place,
> and I am stuck. If it is something absurdly simple, just point
> me in the right direction.
>
> Any ideas?
>
> Thanks,
> JP
>
> On Fri, Sep 3, 2010 at 7:50 AM, JP Glutting
> <jpglutting at gmail.com <mailto:jpglutting at gmail.com>> wrote:
>
> Although here (http://blog.qgis.org/node/94) it seems to
> indicate that SingleBandPseudoColor is a constant:
>
> mypLayer->setColorRampingType(QgsRasterLayer::BLUE_GREEN_RED);
> mypLayer->setDrawingStyle(QgsRasterLayer::SINGLE_BAND_PSEUDO_COLOR);
> std::deque myLayerSet;
>
> which is what I was thinking in the first place, and here
> (
> http://doc.qgis.org/stable/classQgsRasterLayer.html#36796f1a303dac9848ba3dce3e5527dc7b7c9814c053986846b579119d2e5be9 )
> DrawingStyle is described as an enumerator, which seems
> coherent. I am not sure how to do this from Python.
>
> Cheers,
> JP
>
> On Fri, Sep 3, 2010 at 7:43 AM, JP Glutting
> <jpglutting at gmail.com <mailto:jpglutting at gmail.com>> wrote:
>
> Actually, I am not even sure that first part is the
> way to do it. I tried this:
>
>
> resultsLayer.setDrawingStyle(QtCore.QString('SingleBandPseudoColor'))
> resultsLayer.setCacheImage(None)
> resultsLayer.triggerRepaint()
>
> (passing the 'SingleBandPseudoColor' style as a
> string) and it makes the raster invisible. It still
> shows up black in the Layers Panel, but it doesn't
> show in the main window until you change the
> properties manually (and it is Grayscale when you do).
> It feels like I am pretty close, but I am not sure how
> to interpret this code from the QGIS documentation:
>
> myRasterLayer->setDrawingStyle
> <http://classQgsRasterLayer.html#3a923f732bedd87d0b920c5552215434>(QgsRasterLayer::SingleBandPseudoColor
> <http://classQgsRasterLayer.html#36796f1a303dac9848ba3dce3e5527dc7b7c9814c053986846b579119d2e5be9>);
>
> (I never learned more than the basics of C++, and that
> was a long time ago). The source code seems to
> indicate that the format needs to be passed as a
> string (of course, when the layer is generated):
>
> 00204QgsRasterLayer <http://classQgsRasterLayer.html>(int dummy,
> 00205const QString& baseName = QString(),
> 00206const QString& path = QString(),
> 00207const QString& providerLib = QString(),
> 00208const QStringList& layers = QStringList(),
> 00209const QStringList& styles = QStringList(),
> 00210const QString& format = QString(),
>
> 00211 const QString & crs = QString() );
>
> Thanks,
> JP
>
> On Fri, Sep 3, 2010 at 3:04 AM, JP Glutting
> <jpglutting at gmail.com <mailto:jpglutting at gmail.com>>
> wrote:
>
> Hello,
>
> I am working on a plugin (I mentioned it on the
> list earlier, but it isn't relevant to the
> question I have now). I have the results written
> to a raster file, and I need to display it. I am
> using this code:
>
> resultsLayer =
> qgis.core.QgsRasterLayer(self.query.results_file,
> QtCore.QFileInfo(self.query.results_file).baseName())
>
> qgis.core.QgsMapLayerRegistry.instance().addMapLayer(resultsLayer)
>
>
> which works fine for opening the file, but I would
> like to fine-tune the display so the user doesn't
> have to reset the properties (in my test exaple
> the values are 0 and 1 and the display is
> essentially all black). I would like to either
> display the results in pseudocolor directly, or in
> grayscale with the scale stretched to the min and
> max extent of the raster.
>
> I tried the psuedocolor with this code:
>
>
> resultsLayer.setDrawingStyle(qgis.core.QgsRasterLayer.SingleBandPseudoColor)
> resultsLayer.setCacheImage(None)
> resultsLayer.triggerRepaint()
>
> which doesn't seem to do anything at all, and I am
> just guessing, really.
>
> I found a nice tutorial about how to calculate the
> min and max extent of a raster and adjust the
> display here:
>
> http://linfiniti.com/2010/08/a-simple-qgis-python-tutorial/
>
> and I tried the following code:
>
> band =
> resultsLayer.bandNumber(resultsLayer.grayBandName())
> extentMin = 0.0
> extentMax = 0.0
> generateLookupTableFlag = False
> extentMin, extentMax =
> resultsLayer.computeMinimumMaximumFromLastExtent(band)
> resultsLayer.setMinimumValue(band,
> extentMin, generateLookupTableFlag)
> resultsLayer.setMaximumValue(band,
> extentMax, generateLookupTableFlag)
> resultsLayer.setStandardDeviations(0.0)
>
> resultsLayer.setUserDefinedGrayMinimumMaximum( True )
> resultsLayer.setCacheImage(None)
> resultsLayer.triggerRepaint()
>
> but that fails with the following error:
>
> Traceback (most recent call last):
> File
> "/Users//.qgis/python/plugins/mcelite/MCELiteDialog.py",
> line 361, in accept
> extentMin, extentMax =
> resultsLayer.computeMinimumMaximumFromLastExtent(band)
> TypeError: 'float' object is not iterable
>
>
> and I don't understand what the float object is,
> exactly.
>
>
> Any help or suggestions much appreciated.
>
>
> Cheers,
>
> JP
>
>
>
>
>
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--
====================================
Peter J. Ersts, Software Developer
American Museum of Natural History
Center for Biodiversity and Conservation
Central Park West at 79th Street
New York, New York 10024
Web: http://biodiversityinformatics.amnh.org
Web: http://cbc.amnh.org
Open Source,
...evolving through community cooperation to change the world bit by bit
Quantum GIS Raster Development Team.
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