ios - Efficient use of Core Image with AV Foundation -

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i'm writing ios app applies filters existing video files , outputs results new ones. initially, tried using brad larson's nice framework, gpuimage. although able output filtered video files without effort, output wasn't perfect: videos proper length, frames missing, , others duplicated (see issue 1501 more info). plan learn more opengl es can better investigate dropped/skipped frames issue. however, in meantime, i'm exploring other options rendering video files.

i'm familiar core image, decided leverage in alternative video-filtering solution. within block passed avassetwriterinput requestmediadatawhenreadyonqueue:usingblock:, filter , output each frame of input video file so:

cmsamplebufferref samplebuffer = [self.assetreadervideooutput copynextsamplebuffer]; if (samplebuffer != null) {     cmtime presentationtimestamp = cmsamplebuffergetoutputpresentationtimestamp(samplebuffer);      cvpixelbufferref inputpixelbuffer = cmsamplebuffergetimagebuffer(samplebuffer);     ciimage* frame = [ciimage imagewithcvpixelbuffer:inputpixelbuffer];     // cifilter created outside "isreadyformoremediadata" loop     [screenblend setvalue:frame forkey:kciinputimagekey];      cvpixelbufferref outputpixelbuffer;     cvreturn result = cvpixelbufferpoolcreatepixelbuffer(null, assetwriterinputpixelbufferadaptor.pixelbufferpool, &outputpixelbuffer);      // verify everything's gonna ok     nsassert(result == kcvreturnsuccess, @"cvpixelbufferpoolcreatepixelbuffer failed error code");     nsassert(cvpixelbuffergetpixelformattype(outputpixelbuffer) == kcvpixelformattype_32bgra, @"wrong pixel format");      [self.coreimagecontext render:screenblend.outputimage tocvpixelbuffer:outputpixelbuffer];     bool success = [assetwriterinputpixelbufferadaptor appendpixelbuffer:outputpixelbuffer withpresentationtime:presentationtimestamp];     cvpixelbufferrelease(outputpixelbuffer);     cfrelease(samplebuffer);     samplebuffer = null;     completedorfailed = !success; }

this works well: rendering seems reasonably fast, , resulting video file doesn't have missing or duplicated frames. however, i'm not confident code efficient be. specifically, questions are

  1. does approach allow device keep frame data on gpu, or there methods (e.g. imagewithcvpixelbuffer: or render:tocvpixelbuffer:) prematurely copy pixels cpu?
  2. would more efficient use cicontext's drawimage:inrect:fromrect: draw opengles context?
  3. if answer #2 yes, what's proper way pipe results of drawimage:inrect:fromrect: cvpixelbufferref can appended output video file?

i've searched example of how use cicontext drawimage:inrect:fromrect: render filtered video frames, haven't found any. notably, source gpuimagemoviewriter similar, since a) don't understand yet, , b) it's not working quite right use case, i'm wary of copying solution.


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