c - 缓冲区源是否将像素格式更改为 `pix_fmt` 还是仅读取输入，就好像它是该格式一样？

Question

我在学习libav和相关的事情。
我遇到了this ffmpeg 文档中的示例:

     /* buffer video source: the decoded frames from the decoder will be inserted here. */
     snprintf(args, sizeof(args),
             "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
             dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
             time_base.num, time_base.den,
             dec_ctx->sample_aspect_ratio.num, dec_ctx->sample_aspect_ratio.den);
  
     ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                                        args, NULL, filter_graph);

如果我指定 pix_fmt除了 dec_ctx->pix_fmt (例如 rgb24 )，此缓冲区源是否会将像素格式转换为 rgb24 , 还是会像在 rgb24 中一样读取源代码？格式，从而导致错误解码？
如果我设置不同的 video_size 会发生什么？ ?

Answer 1

指定 pix_fmt到 rgb24，将像素格式转换为 rgb24。

在没有太多关于过滤图的知识的情况下，我们可以执行代码并检查输出。
为了显示输出帧，我将“Trivial ASCII grayscale display”替换为 OpenCV方法cv::imshow .
建立一个同时使用 Libav 和 OpenCV 的项目需要一些初步的努力......
这是带有原始代码的输出帧(输出为灰度):

这是带有 AV_PIX_FMT_BGR24 的输出帧像素格式(输出为 BGR):

注意:使用 BGR(而不是 RGB)，因为 OpenCV 的颜色排序约定是 BGR。

使用 BGR 格式时，会出现警告消息:

Changing video frame properties on the fly is not supported by all filters.

设置 pix_fmt 可能不是最佳实践。以这种方式...

完整代码:

//https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/filtering_video.c

/*
 * Copyright (c) 2010 Nicolas George
 * Copyright (c) 2011 Stefano Sabatini
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

 /**
  * @file
  * API example for decoding and filtering
  * @example filtering_video.c
  */

#define _XOPEN_SOURCE 600 /* for usleep */
#ifdef __GNUC__
#include <unistd.h>
#else
//Windows - use C++11 for usleep
#include <chrono>
#include <thread>
#endif

#include <stdio.h>
#include <stdlib.h>

#include <opencv2/opencv.hpp>
#include <opencv2/highgui.hpp>

extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavfilter/buffersink.h>
#include <libavfilter/buffersrc.h>
#include <libavutil/opt.h>
}


#define DO_TEST_PIX_FMT_BGR24   //If defined, test pixel format BGR24


const char* filter_descr = "scale=78:24,transpose=cclock";
/* other way:
   scale=78:24 [scl]; [scl] transpose=cclock // assumes "[in]" and "[out]" to be input output pads respectively
 */

static AVFormatContext* fmt_ctx;
static AVCodecContext* dec_ctx;
AVFilterContext* buffersink_ctx;
AVFilterContext* buffersrc_ctx;
AVFilterGraph* filter_graph;
static int video_stream_index = -1;
static int64_t last_pts = AV_NOPTS_VALUE;

static int open_input_file(const char* filename)
{
    AVCodec* dec;
    int ret;

    if ((ret = avformat_open_input(&fmt_ctx, filename, NULL, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open input file\n");
        return ret;
    }

    if ((ret = avformat_find_stream_info(fmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find stream information\n");
        return ret;
    }

    /* select the video stream */
    ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &dec, 0);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find a video stream in the input file\n");
        return ret;
    }
    video_stream_index = ret;

    /* create decoding context */
    dec_ctx = avcodec_alloc_context3(dec);
    if (!dec_ctx)
        return AVERROR(ENOMEM);
    avcodec_parameters_to_context(dec_ctx, fmt_ctx->streams[video_stream_index]->codecpar);

    /* init the video decoder */
    if ((ret = avcodec_open2(dec_ctx, dec, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open video decoder\n");
        return ret;
    }

    return 0;
}

static int init_filters(const char* filters_descr)
{
    char args[512];
    int ret = 0;
    const AVFilter* buffersrc = avfilter_get_by_name("buffer");
    const AVFilter* buffersink = avfilter_get_by_name("buffersink");
    AVFilterInOut* outputs = avfilter_inout_alloc();
    AVFilterInOut* inputs = avfilter_inout_alloc();
    AVRational time_base = fmt_ctx->streams[video_stream_index]->time_base;

#ifdef DO_TEST_PIX_FMT_BGR24
    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_BGR24, AV_PIX_FMT_NONE };  //Replace AV_PIX_FMT_GRAY8 with AV_PIX_FMT_BGR24
#else
    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_GRAY8, AV_PIX_FMT_NONE };
#endif
    

    filter_graph = avfilter_graph_alloc();
    if (!outputs || !inputs || !filter_graph) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* buffer video source: the decoded frames from the decoder will be inserted here. */
#ifdef DO_TEST_PIX_FMT_BGR24
    snprintf(args, sizeof(args),
        "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
        dec_ctx->width, dec_ctx->height, (int)AV_PIX_FMT_BGR24 /*dec_ctx->pix_fmt*/,    //Replace dec_ctx->pix_fmt with AV_PIX_FMT_BGR24
        time_base.num, time_base.den,
        dec_ctx->sample_aspect_ratio.num, dec_ctx->sample_aspect_ratio.den);
#else
    snprintf(args, sizeof(args),
        "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
        dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
        time_base.num, time_base.den,
        dec_ctx->sample_aspect_ratio.num, dec_ctx->sample_aspect_ratio.den);
#endif

    ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
        args, NULL, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source\n");
        goto end;
    }

    /* buffer video sink: to terminate the filter chain. */
    ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
        NULL, NULL, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink\n");
        goto end;
    }

    ret = av_opt_set_int_list(buffersink_ctx, "pix_fmts", pix_fmts,
        AV_PIX_FMT_NONE, AV_OPT_SEARCH_CHILDREN);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format\n");
        goto end;
    }

    /*
     * Set the endpoints for the filter graph. The filter_graph will
     * be linked to the graph described by filters_descr.
     */

     /*
      * The buffer source output must be connected to the input pad of
      * the first filter described by filters_descr; since the first
      * filter input label is not specified, it is set to "in" by
      * default.
      */
    outputs->name = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx = 0;
    outputs->next = NULL;

    /*
     * The buffer sink input must be connected to the output pad of
     * the last filter described by filters_descr; since the last
     * filter output label is not specified, it is set to "out" by
     * default.
     */
    inputs->name = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx = 0;
    inputs->next = NULL;

    if ((ret = avfilter_graph_parse_ptr(filter_graph, filters_descr,
        &inputs, &outputs, NULL)) < 0)
        goto end;

    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
        goto end;

end:
    avfilter_inout_free(&inputs);
    avfilter_inout_free(&outputs);

    return ret;
}

static void display_frame(const AVFrame* frame, AVRational time_base)
{
    //int x, y;
    //uint8_t* p0, * p;
    int64_t delay;

    const AVRational av_time_base_q = {1, AV_TIME_BASE};

    if (frame->pts != AV_NOPTS_VALUE) {
        if (last_pts != AV_NOPTS_VALUE) {
            /* sleep roughly the right amount of time;
             * usleep is in microseconds, just like AV_TIME_BASE. */
            delay = av_rescale_q(frame->pts - last_pts,
                time_base, av_time_base_q);
            if (delay > 0 && delay < 1000000)
#ifdef __GNUC__
                usleep(delay);
#else
                //Windows
                //https://stackoverflow.com/questions/4184468/sleep-for-milliseconds
                std::this_thread::sleep_for(std::chrono::microseconds(delay));
#endif
        }
        last_pts = frame->pts;
    }

    /* Trivial ASCII grayscale display. */
    //p0 = frame->data[0];
    //puts("\033c");
    //for (y = 0; y < frame->height; y++) {
    //    p = p0;
    //    for (x = 0; x < frame->width; x++)
    //        putchar(" .-+#"[*(p++) / 52]);
    //    putchar('\n');
    //    p0 += frame->linesize[0];
    //}
    //fflush(stdout);

    //Use OpenCV for display
#ifdef DO_TEST_PIX_FMT_BGR24
    cv::Mat img = cv::Mat(frame->height, frame->width, CV_8UC3, frame->data[0], frame->linesize[0]);    //3 Color channels (BGR)
#else
    cv::Mat img = cv::Mat(frame->height, frame->width, CV_8UC1, frame->data[0], frame->linesize[0]);    //1 Color channels (Grayscale)
#endif
    cv::imshow("img", img);
    cv::waitKey(1);
}

int main(int argc, char** argv)
{
    int ret;
    AVPacket* packet;
    AVFrame* frame;
    AVFrame* filt_frame;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s file\n", argv[0]);
        exit(1);
    }

    frame = av_frame_alloc();
    filt_frame = av_frame_alloc();
    packet = av_packet_alloc();
    if (!frame || !filt_frame || !packet) {
        fprintf(stderr, "Could not allocate frame or packet\n");
        exit(1);
    }

    if ((ret = open_input_file(argv[1])) < 0)
        goto end;
    if ((ret = init_filters(filter_descr)) < 0)
        goto end;

    /* 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;
                }

                frame->pts = frame->best_effort_timestamp;

                /* push the decoded frame into the filtergraph */
                if (av_buffersrc_add_frame_flags(buffersrc_ctx, frame, AV_BUFFERSRC_FLAG_KEEP_REF) < 0) {
                    av_log(NULL, AV_LOG_ERROR, "Error while feeding the filtergraph\n");
                    break;
                }

                /* pull filtered frames from the filtergraph */
                while (1) {
                    ret = av_buffersink_get_frame(buffersink_ctx, filt_frame);
                    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                        break;
                    if (ret < 0)
                        goto end;
                    display_frame(filt_frame, buffersink_ctx->inputs[0]->time_base);
                    av_frame_unref(filt_frame);
                }
                av_frame_unref(frame);
            }
        }
        av_packet_unref(packet);
    }
end:
    avfilter_graph_free(&filter_graph);
    avcodec_free_context(&dec_ctx);
    avformat_close_input(&fmt_ctx);
    av_frame_free(&frame);
    av_frame_free(&filt_frame);
    av_packet_free(&packet);

    if (ret < 0 && ret != AVERROR_EOF) {
        //fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
        char errbuf[AV_ERROR_MAX_STRING_SIZE];
        av_make_error_string(errbuf, AV_ERROR_MAX_STRING_SIZE, ret);    
        fprintf(stderr, "Error occurred: %s\n", errbuf);
        exit(1);
    }

    cv::destroyAllWindows();

    exit(0);
}