问题描述
我正在尝试初始化FFMPEG H264编解码器上下文,像这样用SPS帧填充extradata字段:
I am trying to initialize an FFMPEG H264 codec context filling the extradata field with the SPS frame like this :
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
int main()
{
const char sps[] = {0x00, 0x00, 0x00, 0x01, 0x67, 0x42, 0x00, 0x0a, 0xf8, 0x41, 0xa2};
av_register_all();
av_log_set_level(AV_LOG_DEBUG);
AVCodec *const codec = avcodec_find_decoder(CODEC_ID_H264);
if (codec != NULL)
{
AVCodecContext* ctx = avcodec_alloc_context3(codec);
ctx->debug = ~0;
ctx->extradata = (uint8_t *)av_malloc(sizeof(sps) + FF_INPUT_BUFFER_PADDING_SIZE);
ctx->extradata_size = sizeof(sps);
memcpy(ctx->extradata,sps,sizeof(sps));
memset(&ctx->extradata[ctx->extradata_size], 0, FF_INPUT_BUFFER_PADDING_SIZE);
if (avcodec_open2(ctx, codec, NULL) < 0)
{
fprintf(stderr, "Failed to open codec\n");
}
else
{
char buf[1024];
avcodec_string(buf,sizeof(buf),ctx,1);
fprintf(stderr, "%s\n", buf);
}
avcodec_close(ctx);
av_free(ctx);
}
}
程序的输出为:
输出显示 h264_ps.c (mb_width = 8,mb_height = 6,crop_left = crop_right = crop_top = crop_bottom = 0).
The output show that sps was decoded with the needed information to compute width and height by h264_ps.c (mb_width=8, mb_height=6, crop_left=crop_right=crop_top=crop_bottom=0).
然后我期望调用avcodec_string获得宽度和高度.
有没有一种方法可以不解码帧?
Then I was expecting to get the width and the height calling avcodec_string.
Is there a way to do this without decoding frames ?
推荐答案
为解码器提供虚拟缓冲区可以获取SPS中编码的大小.
使用空的PPS和空的非IDR SLICE:
Feeding the decoder with a dummy buffer make possible to get the size encoded in the SPS.
Using a null PPS and an empty non-IDR SLICE :
int got_frame = 0;
AVPacket pkt;
av_init_packet(&pkt);
char buffer[] = {0x00, 0x00, 0x00, 0x01, 0x68, 0xff, 0x00, 0x00, 0x00, 0x01, 0x21};
pkt.data = buffer;
pkt.size = sizeof(buffer);
AVFrame* frame = av_frame_alloc();
avcodec_decode_video2(ctx, frame, &got_frame, &pkt);
av_frame_free(&frame);
fprintf(stderr, "size: %dx%d\n", ctx->width, ctx->height);
运行此代码会显示错误,但可以从SPS中提取宽度和高度:
Running this code print errors but it allow to extract width and height from the SPS :
[h264 @ 0xd0d040] Missing reference picture, default is 0
[h264 @ 0xd0d040] decode_slice_header error
size: 128x96
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