QGIS-2.8-PyQGISDeveloperCookbook-en(17)

为开发人员准备的文档,涵盖QGIS的多种功能实现和代码。

Toreturnbacktodefaultgraylevels,use:

4.2.2MultiBandRasters

Bydefault,QGISmapsthe rstthreebandstored,greenandbluevaluestocreateacolorimage(thisisthe

MultiBandColordrawingstyle.Insomecasesyoumightwanttooverridethesesetting.Thefollowingcode

interchangesredband(1)andgreenband(2):

4.3RefreshingLayers

Ifyoudochangelayersymbologyandwouldlikeensurethatthechangesareimmediatelyvisibletotheuser,call

these

methods

The rstcallwillensurethatthecachedimageofrenderedlayeriserasedincaserendercachingisturnedon.

ThisfunctionalityisavailablefromQGIS1.4,inpreviousversionsthisfunctiondoesnotexist—tomakesure

thatthecodeworkswithallversionsofQGIS,we rstcheckwhetherthemethodexists.

Thesecondcallemitssignalthatwillforceanymapcanvascontainingthelayertoissuearefresh.

WithWMSrasterlayers,thesecommandsdonotwork.Inthiscase,

youhavetodoitexplicitly

Incaseyouhavechangedlayersymbology(seesectionsaboutrasterandvectorlayersonhowtodothat),you

mightwanttoforceQGIStoupdatethelayersymbologyinthelayerlist(legend)widget.Thiscanbedoneas

follows(ifaceisaninstanceofQgisInterface)

4.4QueryValues

Todoaqueryonvalueofbandsofrasterlayeratsomespeci edpoint

Theresultsmethodinthiscasereturnsa

dictionary,withbandindicesaskeys,andbandvaluesasvalues.