使用ffmpeg编码时丢失质量

我正在使用ffmpeg的c库从视频中读取帧，并创建一个应该与输入相同的输出文件。然而，在这个过程中的某个地方，一些质量会丢失，结果"不那么清晰"。我的猜测是问题出在编码上，帧压缩过大(也是因为文件的大小显著减小)编码器中是否有一些参数允许我控制结果的质量我发现AVCodecContext有一个compression_level成员，但更改它似乎没有任何效果。

我在这里发布了我的部分代码，以防有帮助。我想说，当我设置编解码器时，OutputVideoBuilder的init函数中必须更改一些内容。传递给该方法的AVCodecContext与InputVideoHandler相同。以下是我为包装ffmpeg功能而创建的两个主要类：

// This class opens the video files and sets the decoder
class InputVideoHandler {
public:
InputVideoHandler(char* name);
~InputVideoHandler();
AVCodecContext* getCodecContext();
bool readFrame(AVFrame* frame, int* success);
private:
InputVideoHandler();
void init(char* name);
AVFormatContext* formatCtx;
AVCodec* codec;
AVCodecContext* codecCtx;
AVPacket packet;
int streamIndex;
};
void InputVideoHandler::init(char* name) {
streamIndex = -1;
int numStreams;
if (avformat_open_input(&formatCtx, name, NULL, NULL) != 0)
throw std::exception("Invalid input file name.");
if (avformat_find_stream_info(formatCtx, NULL)<0)
throw std::exception("Could not find stream information.");
numStreams = formatCtx->nb_streams;
if (numStreams < 0)
throw std::exception("No streams in input video file.");
for (int i = 0; i < numStreams; i++) {
if (formatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
streamIndex = i;
break;
}
}
if (streamIndex < 0)
throw std::exception("No video stream in input video file.");
// find decoder using id
codec = avcodec_find_decoder(formatCtx->streams[streamIndex]->codec->codec_id);
if (codec == nullptr)
throw std::exception("Could not find suitable decoder for input file.");
// copy context from input stream
codecCtx = avcodec_alloc_context3(codec);
if (avcodec_copy_context(codecCtx, formatCtx->streams[streamIndex]->codec) != 0)
throw std::exception("Could not copy codec context from input stream.");
if (avcodec_open2(codecCtx, codec, NULL) < 0)
throw std::exception("Could not open decoder.");
}
// frame must be initialized with av_frame_alloc() before!
// Returns true if there are other frames, false if not.
// success == 1 if frame is valid, 0 if not.
bool InputVideoHandler::readFrame(AVFrame* frame, int* success) {
*success = 0;
if (av_read_frame(formatCtx, &packet) < 0)
return false;
if (packet.stream_index == streamIndex) {
avcodec_decode_video2(codecCtx, frame, success, &packet);
}
av_free_packet(&packet);
return true;
}
// This class opens the output and write frames to it
class OutputVideoBuilder{
public:
OutputVideoBuilder(char* name, AVCodecContext* inputCtx);
~OutputVideoBuilder();
void writeFrame(AVFrame* frame);
void writeVideo();
private:
OutputVideoBuilder();
void init(char* name, AVCodecContext* inputCtx);
void logMsg(AVPacket* packet, AVRational* tb);
AVFormatContext* formatCtx;
AVCodec* codec;
AVCodecContext* codecCtx;
AVStream* stream;
};
void OutputVideoBuilder::init(char* name, AVCodecContext* inputCtx) {
if (avformat_alloc_output_context2(&formatCtx, NULL, NULL, name) < 0)
throw std::exception("Could not determine file extension from provided name.");
codec = avcodec_find_encoder(inputCtx->codec_id);
if (codec == nullptr) {
throw std::exception("Could not find suitable encoder.");
}
codecCtx = avcodec_alloc_context3(codec);
if (avcodec_copy_context(codecCtx, inputCtx) < 0)
throw std::exception("Could not copy output codec context from input");
codecCtx->time_base = inputCtx->time_base;
codecCtx->compression_level = 0;
if (avcodec_open2(codecCtx, codec, NULL) < 0)
throw std::exception("Could not open encoder.");
stream = avformat_new_stream(formatCtx, codec);
if (stream == nullptr) {
throw std::exception("Could not allocate stream.");
}
stream->id = formatCtx->nb_streams - 1;
stream->codec = codecCtx;
stream->time_base = codecCtx->time_base;

av_dump_format(formatCtx, 0, name, 1);
if (!(formatCtx->oformat->flags & AVFMT_NOFILE)) {
if (avio_open(&formatCtx->pb, name, AVIO_FLAG_WRITE) < 0) {
throw std::exception("Could not open output file.");
}
}
if (avformat_write_header(formatCtx, NULL) < 0) {
throw std::exception("Error occurred when opening output file.");
}
}
void OutputVideoBuilder::writeFrame(AVFrame* frame) {
AVPacket packet = { 0 };
int success;
av_init_packet(&packet);
if (avcodec_encode_video2(codecCtx, &packet, frame, &success))
throw std::exception("Error encoding frames");
if (success) {
av_packet_rescale_ts(&packet, codecCtx->time_base, stream->time_base);
packet.stream_index = stream->index;
logMsg(&packet,&stream->time_base);
av_interleaved_write_frame(formatCtx, &packet);
}
av_free_packet(&packet);
}

这是读取和写入帧的主要功能的一部分：

while (inputHandler->readFrame(frame,&gotFrame)) {
if (gotFrame) {
try {
outputBuilder->writeFrame(frame);
}
catch (std::exception e) {
std::cout << e.what() << std::endl;
return -1;
}
}
}