Delphi, TBitmap (rgb) 到 YCbCr 颜色格式

Question

有一个来自 http://www.delphiffmpeg.com 的视频编码示例- 需要将一组TBitmaps转换成YCbCr(YUV)，我们应该怎么做呢？该示例包含虚拟颜色:

  (* encode 1 second of video *)
  idx := 1;
  for i := 0 to 25 - 1 do
  begin
    av_init_packet(@pkt);
    pkt.data := nil;    // packet data will be allocated by the encoder
    pkt.size := 0;

    //fflush(stdout);
    (* prepare a dummy image *)
    (* Y *)
    for y := 0 to c.height - 1 do
      for x := 0 to c.width - 1 do
        PByte(@PAnsiChar(frame.data[0])[y * frame.linesize[0] + x])^ := x + y + i * 3;

    (* Cb and Cr *)
    for y := 0 to c.height div 2 - 1 do
      for x := 0 to c.width div 2 - 1 do
      begin
        PByte(@PAnsiChar(frame.data[1])[y * frame.linesize[1] + x])^ := 128 + y + i * 2;
        PByte(@PAnsiChar(frame.data[2])[y * frame.linesize[2] + x])^ := 64 + x + i * 5;
      end;

    frame.pts := i;

    (* encode the image *)
    ret := avcodec_encode_video2(c, @pkt, frame, @got_output);
    if ret < 0 then
    begin
      Writeln(ErrOutput, 'Error encoding frame');
      ExitCode := 1;
      Exit;
    end;

    if got_output <> 0 then
    begin
      Writeln(Format('Write frame %d (size=%d)', [idx, pkt.size]));
      FileWrite(f, pkt.data^, pkt.size);
      av_packet_unref(@pkt);
      Inc(idx);
    end;
  end;

但是我们需要将位图转换为 YCbCr..而不是用虚拟图像填充像素。这是完整的源代码:

(*
 * Video encoding example
 *)
procedure video_encode_example(const filename: string; codec_id: TAVCodecID);
const
  endcode: array[0..3] of Byte = ( 0, 0, 1, $b7 );
var
  codec: PAVCodec;
  c: PAVCodecContext;
  idx, i, ret, x, y, got_output: Integer;
  f: THandle;
  frame: PAVFrame;
  pkt: TAVPacket;
begin
  Writeln(Format('Encode video file %s', [filename]));

  (* find the mpeg1 video encoder *)
  codec := avcodec_find_encoder(codec_id);
  if not Assigned(codec) then
  begin
    Writeln(ErrOutput, 'Codec not found');
    ExitCode := 1;
    Exit;
  end;

  c := avcodec_alloc_context3(codec);
  if not Assigned(c) then
  begin
    Writeln(ErrOutput, 'Could not allocate video codec context');
    ExitCode := 1;
    Exit;
  end;

  (* put sample parameters *)
  c.bit_rate := 400000;
  (* resolution must be a multiple of two *)
  c.width := 352;
  c.height := 288;
  (* frames per second *)
  c.time_base.num := 1;
  c.time_base.den := 25;
  (* emit one intra frame every ten frames
   * check frame pict_type before passing frame
   * to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
   * then gop_size is ignored and the output of encoder
   * will always be I frame irrespective to gop_size
   *)
  c.gop_size := 10;
  c.max_b_frames := 1;
  c.pix_fmt := AV_PIX_FMT_YUV420P;

  if codec_id = AV_CODEC_ID_H264 then
    av_opt_set(c.priv_data, 'preset', 'slow', 0);

  (* open it *)
  if avcodec_open2(c, codec, nil) < 0 then
  begin
    Writeln(ErrOutput, 'Could not open codec');
    ExitCode := 1;
    Exit;
  end;

  f := FileCreate(filename);
  if f = INVALID_HANDLE_VALUE then
  begin
    Writeln(ErrOutput, Format('Could not open %s', [filename]));
    ExitCode := 1;
    Exit;
  end;

  frame := av_frame_alloc();
  if not Assigned(frame) then
  begin
    Writeln(ErrOutput, 'Could not allocate video frame');
    ExitCode := 1;
    Exit;
  end;
  frame.format := Ord(c.pix_fmt);
  frame.width  := c.width;
  frame.height := c.height;

  (* the image can be allocated by any means and av_image_alloc() is
   * just the most convenient way if av_malloc() is to be used *)
  ret := av_image_alloc(@frame.data[0], @frame.linesize[0], c.width, c.height,
                       c.pix_fmt, 32);
  if ret < 0 then
  begin
    Writeln(ErrOutput, 'Could not allocate raw picture buffer');
    ExitCode := 1;
    Exit;
  end;

  (* encode 1 second of video *)
  idx := 1;
  for i := 0 to 25 - 1 do
  begin
    av_init_packet(@pkt);
    pkt.data := nil;    // packet data will be allocated by the encoder
    pkt.size := 0;

    //fflush(stdout);
    (* prepare a dummy image *)
    (* Y *)
    for y := 0 to c.height - 1 do
      for x := 0 to c.width - 1 do
        PByte(@PAnsiChar(frame.data[0])[y * frame.linesize[0] + x])^ := x + y + i * 3;

    (* Cb and Cr *)
    for y := 0 to c.height div 2 - 1 do
      for x := 0 to c.width div 2 - 1 do
      begin
        PByte(@PAnsiChar(frame.data[1])[y * frame.linesize[1] + x])^ := 128 + y + i * 2;
        PByte(@PAnsiChar(frame.data[2])[y * frame.linesize[2] + x])^ := 64 + x + i * 5;
      end;

    frame.pts := i;

    (* encode the image *)
    ret := avcodec_encode_video2(c, @pkt, frame, @got_output);
    if ret < 0 then
    begin
      Writeln(ErrOutput, 'Error encoding frame');
      ExitCode := 1;
      Exit;
    end;

    if got_output <> 0 then
    begin
      Writeln(Format('Write frame %d (size=%d)', [idx, pkt.size]));
      FileWrite(f, pkt.data^, pkt.size);
      av_packet_unref(@pkt);
      Inc(idx);
    end;
  end;

  (* get the delayed frames *)
  repeat
    //fflush(stdout);

    ret := avcodec_encode_video2(c, @pkt, nil, @got_output);
    if ret < 0 then
    begin
      Writeln(ErrOutput, 'Error encoding frame');
      ExitCode := 1;
      Exit;
    end;

    if got_output <> 0 then
    begin
      Writeln(Format('Write frame %d (size=%d)', [idx, pkt.size]));
      FileWrite(f, pkt.data^, pkt.size);
      av_packet_unref(@pkt);
      Inc(idx);
    end;
  until got_output = 0;

  (* add sequence end code to have a real mpeg file *)
  FileWrite(f, endcode[0], SizeOf(endcode));
  FileClose(f);

  avcodec_close(c);
  av_free(c);
  av_freep(@frame.data[0]);
  av_frame_free(@frame);
  Writeln('');
end;

是的，我们知道这个公式，但是我们应该如何处理 (* Cb and Cr *) 循环到 c.height div 2 - 1 和 c.width div 2 - 1 ？我们所有的实验都制作了正确的图像几何形状但颜色不正确......这是我们所拥有的:

( Y )
    for y := 0 to c.height - 1 do
    begin
      Line := image.ScanLine[y];
      for x := 0 to c.width - 1 do
      begin
        Yy := Round(Line[x].R*0.29900 + Line[x].G*0.58700 + Line[x].B*0.11400);
        PByte(@PAnsiChar(frame.data[0])[y * frame.linesize[0] + x])^ := Yy;
      end;
    end;
    ( Cb and Cr )
    for y := 0 to c.height div 2 - 1 do
    begin
      Pixels := image.ScanLine[y];
      for x := 0 to c.width div 2 - 1 do
      begin
        Cb := Round(Line[x].R  -0.16874 - Line[x].G  0.33126 + Line[x].B * 0.50000) + 128;
        Cr := Round(Line[x].R   0.50000 - Line[x].G  0.41869 - Line[x].B * 0.08131) + 64;
        PByte(@PAnsiChar(frame.data[1])[y * frame.linesize[1] + x])^ := Cr;
        PByte(@PAnsiChar(frame.data[2])[y * frame.linesize[2] + x])^ := Cb;
        //PByte(@PAnsiChar(frame.data[1])[y  frame.linesize[1] + x])^ := 128 + y + i  2;
        //PByte(@PAnsiChar(frame.data[2])[y  frame.linesize[2] + x])^ := 64 + x + i  5;
      end;
    end;

这应该如何解决？

Answer 1

不难找到 RGB-YCbCr 转换的公式:

Y  = R *  0.29900 + G *  0.58700 + B *  0.11400
Cb = R * -0.16874 + G * -0.33126 + B *  0.50000 + 128
Cr = R *  0.50000 + G * -0.41869 + B * -0.08131 + 128

它已准备好在代码中实现。使用 Scanline用于快速访问每个像素的 R、G、B 分量的属性