關於FFMPEG踩過的一些坑（ffmpeg：視頻無損轉碼黑屏，視頻轉碼失真，手機拍攝的視頻轉碼角度旋轉）

FFMPEG轉碼優化篇

在解決以上問題之前都需要通過ffmpeg對視頻的的編碼，屬性，以及各種參數有一個大致的瞭解。

通過命令解析視頻獲取視頻參數

ffprobe -v quiet -print_format json -show_format -show_streams  test.mp4

獲取的參數格式大致如下：

{
    "streams": [
        {
            "index": 0,
            "codec_name": "h264",
            "codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
            "profile": "Baseline",
            "codec_type": "video",
            "codec_time_base": "65899/3932160",
            "codec_tag_string": "avc1",
            "codec_tag": "0x31637661",
            "width": 720,
            "height": 480,
            "coded_width": 720,
            "coded_height": 480,
            "has_b_frames": 0,
            "sample_aspect_ratio": "1:1",
            "display_aspect_ratio": "3:2",
            "pix_fmt": "yuv420p",
            "level": 30,
            "chroma_location": "left",
            "refs": 1,
            "is_avc": "true",
            "nal_length_size": "4",
            "r_frame_rate": "30/1",
            "avg_frame_rate": "1966080/65899",
            "time_base": "1/16384",
            "start_pts": 0,
            "start_time": "0.000000",
            "duration_ts": 65899,
            "duration": "4.022156",
            "bit_rate": "491008",
            "bits_per_raw_sample": "8",
            "nb_frames": "120",
            "disposition": {
                "default": 1,
                "dub": 0,
                "original": 0,
                "comment": 0,
                "lyrics": 0,
                "karaoke": 0,
                "forced": 0,
                "hearing_impaired": 0,
                "visual_impaired": 0,
                "clean_effects": 0,
                "attached_pic": 0,
                "timed_thumbnails": 0
            },
            "tags": {
                "rotate": "90",
                "language": "eng",
                "handler_name": "VideoHandler"
            },
            "side_data_list": [
                {
                    "side_data_type": "Display Matrix",
                    "displaymatrix": "\n00000000:            0       65536
     0\n00000001:       -65536           0           0\n00000002:            0
         0  1073741824\n",
                    "rotation": -90
                }
            ]
        },
        {
            "index": 1,
            "codec_name": "aac",
            "codec_long_name": "AAC (Advanced Audio Coding)",
            "profile": "LC",
            "codec_type": "audio",
            "codec_time_base": "1/44100",
            "codec_tag_string": "mp4a",
            "codec_tag": "0x6134706d",
            "sample_fmt": "fltp",
            "sample_rate": "44100",
            "channels": 2,
            "channel_layout": "stereo",
            "bits_per_sample": 0,
            "r_frame_rate": "0/0",
            "avg_frame_rate": "0/0",
            "time_base": "1/44100",
            "start_pts": 0,
            "start_time": "0.000000",
            "duration_ts": 178385,
            "duration": "4.045011",
            "bit_rate": "112697",
            "max_bit_rate": "128000",
            "nb_frames": "175",
            "disposition": {
                "default": 1,
                "dub": 0,
                "original": 0,
                "comment": 0,
                "lyrics": 0,
                "karaoke": 0,
                "forced": 0,
                "hearing_impaired": 0,
                "visual_impaired": 0,
                "clean_effects": 0,
                "attached_pic": 0,
                "timed_thumbnails": 0
            },
            "tags": {
                "language": "eng",
                "handler_name": "SoundHandler"
            }
        }
    ],
    "format": {
        "filename": "1.mp4",
        "nb_streams": 2,
        "nb_programs": 0,
        "format_name": "mov,mp4,m4a,3gp,3g2,mj2",
        "format_long_name": "QuickTime / MOV",
        "start_time": "0.000000",
        "duration": "4.069000",
        "size": "309658",
        "bit_rate": "608813",
        "probe_score": 100,
        "tags": {
            "major_brand": "isom",
            "minor_version": "512",
            "compatible_brands": "isomiso2avc1mp41",
            "encoder": "Lavf57.71.100"
        }
    }
}

一，轉碼清晰度問題（無損轉碼黑屏，轉碼失真）

在我們轉碼時使用一般命令會發現轉碼之後畫面失真，清晰度變差，我們可以增加使用-vcodec copy指令來解決該問題，該指令的意思就是將視頻的原畫面直接拷貝過來，不會出現失真現象，使用該指令轉碼速度也會大大提升。

該指令存在一個問題，有時會出現轉碼之後畫面黑屏，只有聲音沒有畫面的現象。

通過命令解析視頻獲取視頻源文件參數

ffprobe -v quiet -print_format json -show_format -show_streams  test.mp4

注意觀察codec_long_name參數，如果包含AVC說明該視頻爲h264編碼格式，則支持-vcodec copy,否則不支持，所以會出現轉碼黑屏的現象。這時爲了最大限度的減少清晰度的損失，使用-q:v:0.01。

二.手機拍攝的視頻轉碼角度旋轉。

手機豎屏拍攝的視頻在電腦上會發現視頻的角度出現了問題

同樣使用指令解析獲取視頻參數，參數中會有一個rotate的屬性，該屬性代表視頻被旋轉的角度。

此時我們使用轉碼命令-metadata:s:v rotate=90，將視頻角度旋轉即可。