项目场景：

最近学习使用ffmpeg在做一个视频缩略图的例子。在这里做一个记录。

参考大神的文章：https://blog.csdn.net/leixiaohua1020/article/details/15811977

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需求描述：

• 需要获取指定时间点处的视频缩略图。
• 缩略图需要保持原有的长宽比列，缩放至480x360，缺失的部分使用纯色填充。

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实现计划：

1. 根据视频的分辨率和要求的480x360计算处，缩放后的视频分辨率。
2. 视频需要解复用并且解码视频，使用ffmpeg直接软解。
3. 视频需要缩放，考虑过avfilter过滤器，但是部分图片缩放后会花屏【还没有找到问题原因】。暂时先使用swscale来实现。
4. 拿到缩放后的数据，分辨率可能不完全等于480x360，这个时候需要在数据外部填充一些纯色。

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具体实现：

1.计算缩放后的视频分辨率

//视频原始大小
m_VideoWidth = pCodecCtx->width;
m_VideoHeight = pCodecCtx->height;//calculate
//m_width 和 m_height 是用户规定的大小，这里设置为480x360
//m_SwsWidth 和 m_SwsHeight是视频缩放后的大小
if (m_VideoWidth <= m_width && m_VideoHeight <= m_height)
{m_SwsWidth  = m_VideoWidth;m_SwsHeight = m_VideoHeight;
}else if(m_VideoWidth <= m_width && m_VideoHeight > m_height)
{float cc_h = (double)m_VideoHeight / m_height;m_SwsHeight = m_height;m_SwsWidth  = m_VideoWidth / cc_h;
}else if(m_VideoWidth > m_width && m_VideoHeight <= m_height)
{float cc_w = (double)m_VideoWidth / m_width;m_SwsWidth  = m_width;m_SwsHeight = m_VideoHeight / cc_w;
}else{//m_VideoWidth > m_width && m_VideoHeight > m_heightfloat cc_w = (double)m_VideoWidth / m_width;float cc_h = (double)m_VideoHeight / m_height;if(cc_w >= cc_h){m_SwsWidth  = m_width;m_SwsHeight = m_VideoHeight / cc_w;}else{m_SwsHeight = m_height;m_SwsWidth  = m_VideoWidth / cc_h;}
}printf("video:width = %d, height = %d.\n", m_VideoWidth, m_VideoHeight);
printf("user:width = %d, height = %d.\n", m_width, m_height);
printf("SWS:width = %d, height = %d.\n", m_SwsWidth, m_SwsHeight);

2.由于我使用的是ffmpeg 4.3.2版本，所以解码过程如下：

AVPacket packet;
AVFrame *frame;frame = av_frame_alloc();/* read all packets */while (1) {if ((ret = av_read_frame(fmt_ctx, &packet)) < 0)break;if (packet.stream_index == video_stream_index) {ret = avcodec_send_packet(dec_ctx, &packet);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Error while sending a packet to the decoder\n");break;}while (ret >= 0) {ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {break;} else if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Error while receiving a frame from the decoder\n");goto end;}//这里我们拿到了一包解码后的数据frame.av_frame_unref(frame);}}av_packet_unref(&packet);}

3.使用swscale进行格式转换和缩放。

AVFrame *pRgba = av_frame_alloc();int video_size = av_image_get_buffer_size(AV_PIX_FMT_RGBA, m_SwsWidth, m_SwsHeight, 1);
uint8_t * picture_buf = (uint8_t *)av_malloc(video_size);
//picture_buf 指向pRgba的data
av_image_fill_arrays(pRgba->data, pRgba->linesize, picture_buf, AV_PIX_FMT_RGBA, m_SwsWidth, m_SwsHeight, 1);
//视频像素数据格式转换上下文
SwsContext *sws_ctx = sws_getContext(dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt, /*INPUT  W H FMT*/m_SwsWidth, m_SwsHeight, AV_PIX_FMT_RGBA,                 /*OUTPUT W H FMT*/SWS_BICUBIC,                                              /*拉伸算法*/NULL, NULL, NULL);                                        /*其他参数*/
//将AVFrame转成视频像素数YUV420P格式
sws_scale(sws_ctx, (const uint8_t * const *)frame->data, frame->linesize,0, dec_ctx->height, pRgba->data, pRgba->linesize);
sws_freeContext(sws_ctx);
av_frame_free(&pRgba);

4.拿到picture_buf后，需要进行判断并填充。测试视频为3840x2160
了解raga数据格式，参考：[FFmpeg] RGBA 和 YUV 存储方式

• m_width == m_SwsWidth && m_height == m_SwsHeight:
  这个是最简单的，不需要任何处理。

• m_width == m_SwsWidth && m_height > m_SwsHeight:
  宽度相同，但是高度不匹配，需要对上下分别填充 m_width * (m_height - m_SwsHeight)/2 边框。

/*** -------------* |    top    |* -------------* -------------* |    pic    |* -------------* -------------* |   buttom  |* -------------*/ 
int diff_h       = m_height - m_SwsHeight;
int diff_top     = diff_h/2;
int diff_buttom  = diff_h - diff_top; //这里是防止diff_h 是一个奇数。int pos = 0; //用来标记当前行的位置
//create top
for(int i = 0; i < diff_top; i++)
{pos = i * m_width * 4;for(int j  = 0; j < m_width; j++){m_picture_buf2[pos + j * 4 + 0] = 235;//Rm_picture_buf2[pos + j * 4 + 1] = 142;//Gm_picture_buf2[pos + j * 4 + 2] = 85;//Bm_picture_buf2[pos + j * 4 + 3] = 0;//A 0:完全透明  ===>>  1：完全不透明}
}
//original buf
memcpy(picture_buf2 + diff_top * m_width * 4, picture_buf, m_SwsWidth * m_SwsHeight * 4);//create buttom
for(int i = diff_top + m_SwsHeight; i < m_height; i++)
{pos = i * m_width * 4;for(int j = 0; j < m_width; j++){m_picture_buf2[pos + j * 4 + 0] = 235;//Rm_picture_buf2[pos + j * 4 + 1] = 142;//Gm_picture_buf2[pos + j * 4 + 2] = 85;//Bm_picture_buf2[pos + j * 4 + 3] = 0;//A 0:完全透明  ===>>  1：完全不透明}
}

原图缩放后：

加上边框后：

• m_width > m_SwsWidth && m_height == m_SwsHeight:
  高度相同，但是宽度不匹配，需要对左右分别填充 (m_width - m_SwsWidth )/2 * m_height 边框。

/*** |--left--||--pic--||--right--|*/ 
int diff_w      = m_width - m_SwsWidth;
int diff_left   = diff_w/2;
int diff_right  = diff_w - diff_left;
int pos         = 0;//原图的位置
int pos_line    = 0;//行位置for(int i = 0; i < m_height; i++)
{pos_line = i * m_width * 4;for(int j = 0; j < m_width; j++){if(j < diff_left || j >= (diff_left + m_SwsWidth)){m_picture_buf2[pos_line + j * 4 + 0] = 235; //Rm_picture_buf2[pos_line + j * 4 + 1] = 142; //Gm_picture_buf2[pos_line + j * 4 + 2] = 85;  //Bm_picture_buf2[pos_line + j * 4 + 3] = 0;   //A}else{m_picture_buf2[pos_line + j * 4 + 0] = picture_buf[pos ++];//Rm_picture_buf2[pos_line + j * 4 + 1] = picture_buf[pos ++];//Gm_picture_buf2[pos_line + j * 4 + 2] = picture_buf[pos ++];//Bm_picture_buf2[pos_line + j * 4 + 3] = picture_buf[pos ++];//A}}
}

• m_width > m_SwsWidth && m_height > m_SwsHeight:
  宽度和高度不匹配，需要对上下m_width * (m_height - m_SwsHeight)/2 边框，左右 (m_width - m_SwsWidth )/2 * m_SwsHeight。

/*** |----------------------top-----------------------|* |--left--||------------pic------------||--right--|* |-------------------buttom-----------------------|*/ 
//top & buttom 's height
int diff_h      = m_height - m_SwsHeight;
int diff_top    = diff_h/2;
int diff_buttom = diff_h - diff_top;
//left & right 's width
int diff_w      = m_width - m_SwsWidth;
int diff_left   = diff_w/2;
int diff_right  = diff_w - diff_left;
int pos         = 0;
int pos_line    = 0;for(int i = 0; i < m_height; i++)
{pos_line = i * m_width * 4;for(int j = 0; j < m_width; j++){if(j < diff_left || j >= (diff_left + m_SwsWidth) || i < diff_top || i >= (diff_top + m_SwsHeight)){m_picture_buf2[pos_line + j * 4 + 0] = 235; //Rm_picture_buf2[pos_line + j * 4 + 1] = 142; //Gm_picture_buf2[pos_line + j * 4 + 2] = 85;  //Bm_picture_buf2[pos_line + j * 4 + 3] = 0;   //A}else{m_picture_buf2[pos_line + j * 4 + 0] = picture_buf[pos ++];//Rm_picture_buf2[pos_line + j * 4 + 1] = picture_buf[pos ++];//Gm_picture_buf2[pos_line + j * 4 + 2] = picture_buf[pos ++];//Bm_picture_buf2[pos_line + j * 4 + 3] = picture_buf[pos ++];//A}}
}