Objective c : fastest method to overlay a video -

after recording video, want overlay dynamic uiview updated related current video frame timestamp. in fact trying same thing vidometer application. following apple example, able extract video frame buffer , overlay uiimage of uiview.

the steps :

1. extract video frame:

   cmsamplebufferref samplebuffer = [assetreadervideooutput copynextsamplebuffer]; if (cancelled) {     completedorfailed = yes;     [assetwritervideoinput markasfinished]; } cvpixelbufferref pixelbuffer = cmsamplebuffergetimagebuffer(samplebuffer); 

2. update uiview's subviews following frame timestamp:

   mergetime = cmtimegetseconds(cmsamplebuffergetpresentationtimestamp(samplebuffer)); [self updatewidget:mergetime*1000]; 

3. get uiimage of uiview:

   msurcoucheimage = [self imagefromsurcouche]; 

   with

   -(uiimage*)imagefromsurcouche{ cgsize msize; msize = cgsizemake(self.msurcouche.bounds.size.width, self.msurcouche.bounds.size.height); uigraphicsbeginimagecontextwithoptions(msize, no, 0.0); if (videoorientation == uiinterfaceorientationlandscaperight) {     cgcontexttranslatectm(uigraphicsgetcurrentcontext(), msize.width, msize.height);     cgcontextrotatectm(uigraphicsgetcurrentcontext(), m_pi); } [self.msurcouche.layer renderincontext:uigraphicsgetcurrentcontext()]; uiimage* image = uigraphicsgetimagefromcurrentimagecontext(); uigraphicsendimagecontext(); return image; } 

4. apply filter keep alpha of uiimage:

   cvpixelbufferlockbaseaddress(pixelbuffer, 0);                         inputimage = [ciimage imagewithcvpixelbuffer:pixelbuffer options:options]; filter = [cifilter filterwithname:@"cisourceovercompositing"]; [filter setvalue:maskimage forkey:kciinputimagekey]; [filter setvalue:inputimage forkey:kciinputbackgroundimagekey]; outputimage = [filter outputimage]; cvpixelbufferunlockbaseaddress(pixelbuffer, 0); 

   with

   colorspace = cgcolorspacecreatedevicergb(); options = [nsdictionary dictionarywithobject:(__bridge id)colorspace forkey:kciimagecolorspace]; 

5. render uiimage video frame buffer

   [cicontext render:outputimage tocvpixelbuffer:pixelbuffer bounds:[outputimage extent] colorspace:cgcolorspacecreatedevicergb()]; 

   with

   eaglcontext = [[eaglcontext alloc] initwithapi:keaglrenderingapiopengles2]; cicontext = [cicontext contextwitheaglcontext:eaglcontext options:@{kcicontextworkingcolorspace : [nsnull null]}]; if (eaglcontext != [eaglcontext currentcontext])    [eaglcontext setcurrentcontext:eaglcontext]; 

i trying reach same performance vidometer application have overlay duration less video duration, far method.

question 1: method best performing?

question 2: see method use avmutablecomposition, don't think can synchronize uiview video frame timestamps. can i?