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• DXGI Desktop Duplication : encoding frames to send them over the network

  31 août 2018, par prazuber

  I’m trying to write an app which will capture a video stream of the screen and send it to a remote client. I’ve found out that the best way to capture a screen on Windows is to use DXGI Desktop Duplication API (available since Windows 8). Microsoft provides a neat sample which streams duplicated frames to screen. Now, I’ve been wondering what is the easiest, but still relatively fast way to encode those frames and send them over the network.

  The frames come from AcquireNextFrame with a surface that contains the desktop bitmap and metadata which contains dirty and move regions that were updated. From here, I have a couple of options :

  1. Extract a bitmap from a DirectX surface and then use an external library like ffmpeg to encode series of bitmaps to H.264 and send it over RTSP. While straightforward, I fear that this method will be too slow as it isn’t taking advantage of any native Windows methods. Converting D3D texture to a ffmpeg-compatible bitmap seems like unnecessary work.
  2. From this answer : convert D3D texture to IMFSample and use MediaFoundation’s SinkWriter to encode the frame. I found this tutorial of video encoding, but I haven’t yet found a way to immediately get the encoded frame and send it instead of dumping all of them to a video file.

  Since I haven’t done anything like this before, I’m asking if I’m moving in the right direction. In the end, I want to have a simple, preferably low latency desktop capture video stream, which I can view from a remote device.

  Also, I’m wondering if I can make use of dirty and move regions provided by Desktop Duplication. Instead of encoding the frame, I can send them over the network and do the processing on the client side, but this means that my client has to have DirectX 11.1 or higher available, which is impossible if I would want to stream to a mobile platform.

• How to use scale filter in doc/examples/transcoding.c

  19 janvier 2018, par siods333333

  I tried to replace the "null" filter with the "scale=320x180" filter in the doc/examples/transcoding.c file, it only resulted into the this error message :

  [libx264 @ 03303ee0] Input picture width (640) is greater than stride (256)
  
  Error occurred: Generic error in an external library

  What is wrong ? Look that init_filters happens after open_output_file, the encoder is already set before it even knows the resolution of the output.

  How to do this properly ?

  Look at this piece of code, I don’t get what it’s talking about, fiters ain’t going to magically set the correct resolution :

          /* In this example, we transcode to same properties (picture size,
  
           * sample rate etc.). These properties can be changed for output
  
           * streams easily using filters */
  
          if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
  
              enc_ctx->height = dec_ctx->height;
  
              enc_ctx->width = dec_ctx->width;

  Found this in the ffmpeg.c itself :

      enc_ctx->width  = av_buffersink_get_w(ost->filter->filter);
  
      enc_ctx->height = av_buffersink_get_h(ost->filter->filter);

• Background version of uploaded video creation in Elixir with Arc and FFmpeg

  12 décembre 2017, par Levit

  I tried to make video uploading on Elixir phoenix project using Arc. I need to have different size versions of video. So I used FFmpeg to do this :

  def transform(:large, {file, scope}) do
  
    {:ffmpeg, fn(input, output) -> "-i #{input} -c:a copy -s 1920x1080 -f mp4 #{output}" end, :mp4}
  
  end

  However, it takes a lot of time to create this versions even to small videos. So, I decided to make version creation on background. I want Arc to upload video, then I give the response with uploaded original while other versions are creating.

  I was hoping to find such option at https://github.com/stavro/arc but I didn’t succed. I only found "To disable asynchronous processing, add @async false to your upload definition" but I don’t want to disable it, right ?

  I tried to use Exq https://github.com/akira/exq for background processing but I didn’t menage to use it in uploader.

  Could anybody tell me how it should be made in a proper way or give me some dirty hack advice to make it work. Thanks.

  I tried Task as was advised in comments, but I am not sure how to use them in this case. When I try

  {:ffmpeg, fn(input, output) -> Task.async(fn -> "-i #{input} -c:a copy -s 1920x1080 -f mp4 #{output}" end) end, :mp4}

  or

  {:ffmpeg, Task.async(fn -> fn(input, output) ->  "-i #{input} -c:a copy -s 1920x1080 -f mp4 #{output}" end end), :mp4}

  I got "protocol String.Chars not implemented for %Task".

  When I try

  {:ffmpeg, Task.async(fn(input, output) ->  "-i #{input} -c:a copy -s 1920x1080 -f mp4 #{output}" end), :mp4}

  I got "#Function<20.83953603/2 in MyWebSite.Content.transform/2> with arity 2 called with no arguments". I tried to pass function as an argument with "&" but it fails as well.

  My uploader :

  defmodule MyWebSite.Content do
  
  use Arc.Definition
  
  use Arc.Ecto.Definition
  
  
  
  @acl :public_read
  
  
  
  @versions [:original,  :huge]
  
  
  
  def transform(:huge, {file, scope}) do
  
    {:ffmpeg, Task.async(fn(input, output) ->  "-i #{input} -c:a copy -s 1920x1080 -f mp4 #{output}" end), :mp4}
  
  end
  
  
  
  def s3_object_headers(version, {file, scope}) do
  
    [timeout: 3_000_00, content_type: Plug.MIME.path(file.file_name)]
  
  end
  
  end