FFmpeg Reinit context to 1920x1088问题描述-白红宇

场景

指定海康摄像机的分辨率是1920*1080P，I帧间隔是25，视频帧率是25fps。查看AVFormatContext结构体中streams数组中的第一个元素codec

codec_id=AV_CODEC_ID_H264, width=1920, height=1080, coded_with=1920, coded_height=1088, pix_fmt=AV_PIX_FMT_YUVJ420P

疑问

coded_height为什么不是1080

解答

当前SPS的帧的宽 = (sps_info.pic_width_in_mbs_minus1 + 1) * 16

当前SPS的帧的高 = (sps_info.pic_height_in_map_units_minus1 + 1) * 16

通过如下代码计算得到的宽高（1888 x 1920）却不完全等同于源视频的宽高（1920 x 1080）。

根据编码规则计算到的宽高是1920x1088，而MP4中读取的是1920x1080。那么，如何FFmpeg是如何修正SPS里的宽高计算呢？

实际上，SPS还有crop系列字段，由crop表示帧是否被裁剪、crop_left、crop_right、crop_top和crop_bottom表示要裁剪的值得到正确的宽高值。

相关知识

sps和pps的结构参考：h264编码里面的Sequence parameter set RBSP syntax

0x00000001或者0x000001是起始码，0x67是sps的开头，0x68是pps的开头。

0x42代表profile_idc,后面八位是constraint_set0_flag和reserved_zero_4bits，都设为0，0x0a是level_idc,

接着后面为图方便能用0表示的都用了。这里要注意是ue(v)表示该域是可变位，使用的指数-哥伦布编码

我的目的主要是设置正确帧高度和帧高度，所以只要填充 pic_width_in_mbs_minus1和 pic_height_in_map_units_minus1，

将它们的十六进制数写入sps_pps，

如果宽度和高度不是16的倍数，要填frame_cropping_flag

参考

http://m.blog.csdn.net/stn_lcd/article/details/74390636

http://m.blog.csdn.net/leixiaohua1020/article/details/45001033

static int h264_init_ps(H264Context *h, const H264SliceContext *sl, int first_slice)

{

const SPS *sps;

int needs_reinit = 0, must_reinit, ret;

if (first_slice) {

av_buffer_unref(&h->ps.pps_ref);

h->ps.pps = NULL;

h->ps.pps_ref = av_buffer_ref(h->ps.pps_list[sl->pps_id]);

if (!h->ps.pps_ref)

return AVERROR(ENOMEM);

h->ps.pps = (const PPS*)h->ps.pps_ref->data;

}

if (h->ps.sps != (const SPS*)h->ps.sps_list[h->ps.pps->sps_id]->data) {

av_buffer_unref(&h->ps.sps_ref);

h->ps.sps = NULL;

h->ps.sps_ref = av_buffer_ref(h->ps.sps_list[h->ps.pps->sps_id]);

if (!h->ps.sps_ref)

return AVERROR(ENOMEM);

h->ps.sps = (const SPS*)h->ps.sps_ref->data;

if (h->mb_width != h->ps.sps->mb_width ||

h->mb_height != h->ps.sps->mb_height ||

h->cur_bit_depth_luma != h->ps.sps->bit_depth_luma ||

h->cur_chroma_format_idc != h->ps.sps->chroma_format_idc

)

needs_reinit = 1;

if (h->bit_depth_luma != h->ps.sps->bit_depth_luma ||

h->chroma_format_idc != h->ps.sps->chroma_format_idc)

needs_reinit = 1;

}

sps = h->ps.sps;

must_reinit = (h->context_initialized &&

( 16*sps->mb_width != h->avctx->coded_width

|| 16*sps->mb_height != h->avctx->coded_height

|| h->cur_bit_depth_luma != sps->bit_depth_luma

|| h->cur_chroma_format_idc != sps->chroma_format_idc

|| h->mb_width != sps->mb_width

|| h->mb_height != sps->mb_height

));

if (h->avctx->pix_fmt == AV_PIX_FMT_NONE

|| (non_j_pixfmt(h->avctx->pix_fmt) != non_j_pixfmt(get_pixel_format(h, 0))))

must_reinit = 1;

if (first_slice && av_cmp_q(sps->sar, h->avctx->sample_aspect_ratio))

must_reinit = 1;

if (!h->setup_finished) {

h->avctx->profile = ff_h264_get_profile(sps);

h->avctx->level = sps->level_idc;

h->avctx->refs = sps->ref_frame_count;

h->mb_width = sps->mb_width;

h->mb_height = sps->mb_height;

h->mb_num = h->mb_width * h->mb_height;

h->mb_stride = h->mb_width + 1;

h->b_stride = h->mb_width * 4;

h->chroma_y_shift = sps->chroma_format_idc <= 1; // 400 uses yuv420p

h->width = 16 * h->mb_width;

h->height = 16 * h->mb_height;

ret = init_dimensions(h);

if (ret < 0)

return ret;

if (sps->video_signal_type_present_flag) {

h->avctx->color_range = sps->full_range > 0 ? AVCOL_RANGE_JPEG

: AVCOL_RANGE_MPEG;

if (sps->colour_description_present_flag) {

if (h->avctx->colorspace != sps->colorspace)

needs_reinit = 1;

h->avctx->color_primaries = sps->color_primaries;

h->avctx->color_trc = sps->color_trc;

h->avctx->colorspace = sps->colorspace;

}

if (!h->context_initialized || must_reinit || needs_reinit) {

int flush_changes = h->context_initialized;

h->context_initialized = 0;

if (sl != h->slice_ctx) {

av_log(h->avctx, AV_LOG_ERROR,

"changing width %d -> %d / height %d -> %d on "

"slice %d\n",

h->width, h->avctx->coded_width,

h->height, h->avctx->coded_height,

h->current_slice + 1);

return AVERROR_INVALIDDATA;

}

av_assert1(first_slice);

if (flush_changes)

ff_h264_flush_change(h);

if ((ret = get_pixel_format(h, 1)) < 0)

return ret;

h->avctx->pix_fmt = ret;

av_log(h->avctx, AV_LOG_VERBOSE, "Reinit context to %dx%d, "

"pix_fmt: %s\n", h->width, h->height, av_get_pix_fmt_name(h->avctx->pix_fmt));

if ((ret = h264_slice_header_init(h)) < 0) {

av_log(h->avctx, AV_LOG_ERROR,

"h264_slice_header_init() failed\n");

return ret;

}

return 0;

}

/* This function is called right after decoding the slice header for a first

* slice in a field (or a frame). It decides whether we are decoding a new frame

* or a second field in a pair and does the necessary setup.

static int h264_field_start(H264Context *h, const H264SliceContext *sl, const H2645NAL *nal, int first_slice)

被调用

int ff_h264_queue_decode_slice(H264Context *h, const H2645NAL *nal)

被调用

static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)

本文转自fengyuzaitu 51CTO博客，原文链接：http://blog.51cto.com/fengyuzaitu/2055607，如需转载请自行联系原作者