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• How do I calculate optimal dimensions and bitrate for displaying a video on an iPhone ?

  6 février 2020, par wachutu

  I’m currently developing a mobile app that will have a library of 2-5 minute videos (approx 100 in total) and going through the process of determining which versions of the videos to have ready to serve to different mobile devices. In my research, I have noticed that there is a lot of room to play with video settings such as dimensions and bitrate.

  As a first test, I am attempting to find the minimum video size I can deliver to an iPhone XS with dimensions 1125x2436 without losing any noticeable quality. I started by scaling the video to 1125x2436 and creating versions with 5 different bitrates ranging from 500kbps-4400kbps. I noticed that at 1500kbps, the video looks great and the size is cut 1/3 so that was a good start.

  Then after doing some reading, I saw that in adaptive bitrate scenarios Apple recommends delivering video of lower bitrate AND lower resolution. So in my next test I just cut both in half - scaled to 562x1218 and bitrate at 750kbps and noticed the video also looked great on the iPhone. So 1125x2436 at 750kbps looks bad, but 562x1218 at 750kbps looks great on the same device. To some extent this makes sense to me as you need less bits to fill a smaller screen but what I’m not understanding is how the scaling plays a factor. Shouldn’t it essentially pixelate because the resolution is 1/2 of the iPhone dimensions ? And at a higher level, is there a somewhat concrete way to figure out this optimal resolution / bitrate balance given the dimensions of a device ? We want to most modern smartphones (iPhone 6 and later, Samsung Galaxy, etc.) so we need to be prepared for a range of dimensions (aspect ratios 9:16 or 6:13).

• Using OpenMAX (IL ?) for audio/video decoding on Android

  14 septembre 2012, par Christopher Corsi

  Many of the newer hardware platforms running Android, in particular NVIDIA's Tegra 2, support OpenMAX for media acceleration. It's effectively impossible on today's devices to decode 720p video without this support, but the number of demuxers supported on Android are quite slim. The only public API I've been able to find has been through the MediaPlayer class in the Android SDK. There are multiple places in the Android source tree with OpenMAX related tidbits, however.

  On my device (Samsung Galaxy Tab 10.1) I've got access to hardware decoders through a multitude of OpenMAX libs in /system/lib, and it would be great to interface my video application with these. Can anyone point me to information on implementing a decoder powered by OpenMAX ? I've found the documentation from Khronos, but nothing in the way of example code or tutorials. I've already got demuxing and even software decoding taken care of (via libavcodec/libavformat), I'd just like to put hooks in to enable hardware encoding. I'm also assuming here it would be necessary to link directly to the ones available on the device, which makes it pretty lackluster in terms of portability, but it works.

  Alternatively, I'm interested in anything anyone knows about private APIs for accessing the video decoding available on Tegra 2 devices. Especially if there's a vdpau interface like what NVIDIA implements for desktop linux distributions, since there's plenty available for that - but I wasn't able to find shared libraries that indicate that support.

• ffserver : Clear avio context after closing it

  1er novembre 2015, par Michael Niedermayer

  ffserver : Clear avio context after closing it
  Fixes : ==13287== Invalid read of size 4
  
  ==13287==    at 0x45161A : flush_buffer (aviobuf.c:143)
  
  ==13287==    by 0x451971 : avio_flush (aviobuf.c:200)
  
  ==13287==    by 0x512CCF : av_write_trailer (mux.c:1016)
  
  ==13287==    by 0x41A5E0 : close_connection (ffserver.c:853)
  
  ==13287==    by 0x421EDC : rtsp_cmd_interrupt (ffserver.c:3245)
  
  ==13287==    by 0x420B9C : rtsp_parse_request (ffserver.c:2854)
  
  ==13287==    by 0x41A9C2 : handle_connection (ffserver.c:930)
  
  ==13287==    by 0x41A04B : http_server (ffserver.c:700)
  
  ==13287==    by 0x423A60 : main (ffserver.c:3897)
  
  ==13287==  Address 0xb6cd258 is 88 bytes inside a block of size 192 free’d
  
  ==13287==    at 0x4C2B5D9 : free (vg_replace_malloc.c:446)
  
  ==13287==    by 0x1004DAC : av_free (mem.c:239)
  
  ==13287==    by 0x454835 : avio_close_dyn_buf (aviobuf.c:1170)
  
  ==13287==    by 0x41F385 : http_prepare_data (ffserver.c:2368)
  
  ==13287==    by 0x41F59B : http_send_data (ffserver.c:2416)
  
  ==13287==    by 0x41ABE2 : handle_connection (ffserver.c:986)
  
  ==13287==    by 0x41A04B : http_server (ffserver.c:700)
  
  ==13287==    by 0x423A60 : main (ffserver.c:3897)
  Reviewed-by : "Reynaldo H. Verdejo Pinochet" <reynaldo@osg.samsung.com>
  
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] ffserver.c