QGIS-2.8-PyQGISDeveloperCookbook-en(16)

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

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9QgsRasterLater.XSingleBandPseudoColorPalettedColorPalettedSingleBandGrayPalettedSingleBandPseudo-ColorMultiBandSingleBandGrayMultiBandSingle-BandPseudoColorMultiBandColor

Singlebandimagedrawnusingapseudocoloralgorithm“Palette”imagedrawnusingcolortable“Palette”layerdrawningrayscale“Palette”layerdrawnusingapseudocoloralgorithmLayercontaining2ormorebands,butasinglebanddrawnasarangeofgraycolorsLayercontaining2ormorebands,butasinglebanddrawnusingapseudocoloralgorithmLayercontaining2ormorebands,mappedtoRGBcolorspace.

Toquerythecurrentdrawingstyle:

Singlebandrasterlayerscanbedrawneitheringraycolors(lowvalues=black,highvalues=white)orwitha

pseudocoloralgorithmthatassignscolorsforvaluesfromthesingleband.Singlebandrasterswithapalettecan

beadditionallydrawnusingtheirpalette.MultibandlayersaretypicallydrawnbymappingthebandstoRGB

colors.Otherpossibilityistousejustonebandforgrayorpseudocolordrawing.

Thefollowingsectionsexplainhowtoqueryandmodifythelayerdrawingstyle.Afterdoingthechanges,you

mightwanttoforceupdateofmapcanvas,seeRefreshingLayers.

TODO:contrastenhancements,transparency(nodata),userde nedmin/max,bandstatistics

4.2.1SingleBandRasters

Theyarerenderedingraycolorsbydefault.Tochangethedrawingstyletopseudocolor:

ThePseudoColorShaderisabasicshaderthathighlightslowvaluesinblueandhighvaluesinred.Thereis

alsoColorRampShaderwhichmapsthecolorsasspeci edbyitscolormap.Ithasthreemodesof

interpolationofvalues:

linear(INTERPOLATED):resultingcolorislinearlyinterpolatedfromthecolormapentriesaboveand

belowtheactualpixelvalue

discrete(DISCRETE):colorisusedfromthecolormapentrywithequalorhighervalue

exact(EXACT):colorisnotinterpolated,onlythepixelswithvalueequaltocolormapentriesaredrawn

Tosetaninterpolatedcolorrampshaderrangingfromgreentoyellowcolor(forpixelvaluesfrom0to255):