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• Does Apple AS assembler replace certain NEON instructions with equivalent ones on iOS ?

  5 avril 2012, par Yi Wang

  I was trying to use ffmpeg on iOS and was debugging a crash in the optimized arm code. I have discovered that some unsigned (.u16, .u32) instruction have been replaced by signed ones (.i16, .i32). It's easy to see because disassembled instruction on GDB doesn't quite match the source code.

  For example,

  vrshrn.u32 -> vrshrn.i32
  
  vrshrn.u16 -> vrshrn.i16
  
  vadd.u16 -> vadd.i16

  My questions :

  1. Is this behavior correct and expected ? If not, how do we correct it ?
  2. If they are equivalent, why do we nee need the unsigned ones at all ? Is it because that way the code is more explicit ?
  3. Is this behavior expected with other platform's toolkit ? For example, Android's toolkit ? (I have heard Apple's AS is an ancient one)

• How to write frames to a video file ?

  23 décembre 2013, par Mike Chen

  I am currently writing an application that read frames from camera, modify them, and save them into a video file. I'm planing to do it with ffmpeg. There's rarely a documentation about ffmpeg. I can't find a way. Does any know how to do it ?

  I need it to be done on unix, and in C or C++. Does any can provide some instructions ?

  Thanks.

  EDIT :

  Sorry, I haven't write clearly. I want some developer APIs to write frames to a video file. I open up camera stream, I get every single frame, then I save them into a video file with those APIs available in ffmpeg's public apis. So using command line tool actually doesn't help me. And I've seen output_example.c under the ffmpeg src folder. It's pretty great that I may copy some parts of the code directly without change. And I am still looking for a easier way.

  Also, I'm thinking of porting my app to iPhone, as far as I know, only ffmpeg has been ported on iPhone. GStreamer is based on glib, and it's all GNU stuff. I'm not sure if I can get it work on iPhone. So ffmpeg is still the best choice for now.

  Any comments is appreciated.

• Make MP4 video using JPEG 2000 frames and ROI

  21 mars 2012, par Leandro

  I'm having a hard time trying to figure out how to solve the following problem.

  I have a set of jp2 frames. I want to save them as a .mp4 video for archiving (which is great because it supports motion compensation) and then be able to extract the frames as jp2 files again.

  I have this working with a set of tools/libraries like ffmpeg. The issue is that I also want to include a ROI in the original jp2 images.

  When I do that, the video shows nothing where the ROI originally was, and when I use ffmpeg to go back to the original files, the frames are still broken.

  I tried going from JPEG2000 w\ ROI to another format before making the mp4 file, but every tool I use seems to have the same results : the ROI gets translated as a square w/o nothing there.

  Any help would be greatly appreciated.

  Thanks !