swift - Export video to the size of the UIImagePickerController view -

i have app user take video using uiimagepickercontroller.

then create watermark on top of recorded video , export avassetexportsession.

i'm guessing need use cgaffinetransformscale , transform export right size, i'm new transforming.

what kind of transform use crop video size of uiimagepickercontroller's view?

func imagepickercontroller(picker: uiimagepickercontroller, didfinishpickingmediawithinfo info: [string : anyobject]) {          let videopath = info[uiimagepickercontrollermediaurl] as! nsurl          let stringvideopath = videopath.path          //add watermark starting here          let videoasset = avurlasset(url: videopath)         let mixcomposition = avmutablecomposition()         let compositionvideotrack = mixcomposition.addmutabletrackwithmediatype(avmediatypevideo, preferredtrackid: kcmpersistenttrackid_invalid)         let clipvideotrack = videoasset.trackswithmediatype(avmediatypevideo)[0]         {         try compositionvideotrack.inserttimerange(cmtimerangemake(kcmtimezero, videoasset.duration), oftrack: clipvideotrack, attime: kcmtimezero)         } catch {             print("error")         }           compositionvideotrack.preferredtransform = clipvideotrack.preferredtransform           //create watermark image         let myimage = uiimage(named: "watermarkimage.png")         let alayer = calayer()         alayer.contents = myimage?.cgimage         alayer.frame = cgrectmake(5, 25, 100, 60)         alayer.opacity = 1.0          //sort layer         let videosize = clipvideotrack.naturalsize         let parentlayer = calayer()         let videolayer = calayer()         parentlayer.frame = cgrectmake(0, 0, videosize.height, videosize.width)         videolayer.frame = cgrectmake(0, 0, videosize.height, videosize.width)         parentlayer.addsublayer(videolayer)         parentlayer.addsublayer(alayer)          //create composition , add instructions insert layer          let videocomp = avmutablevideocomposition()         videocomp.rendersize = cgsize(width: videosize.height, height: videosize.width)         videocomp.frameduration = cmtimemake(1, 30)         videocomp.animationtool = avvideocompositioncoreanimationtool(postprocessingasvideolayer: videolayer, inlayer: parentlayer)          //instructions         let maininstruction = avmutablevideocompositioninstruction()         maininstruction.timerange = cmtimerangemake(kcmtimezero, mixcomposition.duration)         let videotrack = mixcomposition.trackswithmediatype(avmediatypevideo)[0]             let layerinstruction = avmutablevideocompositionlayerinstruction(assettrack: videotrack)  //i'm guessing have let zoomtransform = cgaffinetransformscale(compositionvideotrack.preferredtransform, 1, 1) , apply transform below instead of compositionvideotrack.preferredtransform   layerinstruction.settransform(compositionvideotrack.preferredtransform, attime: kcmtimezero)           maininstruction.layerinstructions = [layerinstruction]          videocomp.instructions = [maininstruction]          let assetexport = avassetexportsession(asset: mixcomposition, presetname: avassetexportpresetmediumquality)         assetexport?.videocomposition = videocomp         let exportpath = nstemporarydirectory().stringbyappendingstring("testvideo.mp4")         let exporturl = nsurl(fileurlwithpath: exportpath)          if nsfilemanager.defaultmanager().fileexistsatpath(exportpath) {             { try nsfilemanager.defaultmanager().removeitematpath(exportpath)} catch{}         }          assetexport?.outputfiletype = avfiletypempeg4         assetexport?.outputurl = exporturl         assetexport?.shouldoptimizefornetworkuse = true         assetexport?.exportasynchronouslywithcompletionhandler({ () -> void in             print("done")             uisavevideoatpathtosavedphotosalbum(exporturl.path!, self, nil, nil)         })          picker.dismissviewcontrolleranimated(true, completion: nil)         picker.view.superview?.removefromsuperview()      }