Saving frames as JPG with FFMPEG (Visual Studio / C++) - ffmpeg

Question

I am trying to save all frames from a mp4 video in separate JPG files, I have a code that runs and actually saves something to JPG files but files are not recognized as images and nothing is showing.
Below my full code, I am using Visual Studio 2022 in Windows 11 and FFMPEG 5.1. The function that saves the images is save_frame_as_jpeg which is actually an adaption from the code provided here but changing the use of avcodec_encode_video2 for avcodec_send_frame/avcodec_receive_packet as indicated in the documentation.
I am obiously doing something wrong but cannot quite find it, BTW, I know that a simple command (ffmpeg -i input.mp4 -vf fps=1 vid_%d.png) will do this but I am requiring to do it by code.
Any help is appreciated, thanks in advance!
// FfmpegTests.cpp : This file contains the 'main' function. Program execution begins and ends there.
//
#pragma warning(disable : 4996)
extern "C"
{
#include "libavformat/avformat.h"
#include "libavcodec/avcodec.h"
#include "libavfilter/avfilter.h"
#include "libavutil/opt.h"
#include "libavutil/avutil.h"
#include "libavutil/error.h"
#include "libavfilter/buffersrc.h"
#include "libavfilter/buffersink.h"
#include "libswscale/swscale.h"
}
#pragma comment(lib, "avcodec.lib")
#pragma comment(lib, "avformat.lib")
#pragma comment(lib, "avfilter.lib")
#pragma comment(lib, "avutil.lib")
#pragma comment(lib, "swscale.lib")
#include <cstdio>
#include <iostream>
#include <chrono>
#include <thread>
static AVFormatContext* fmt_ctx;
static AVCodecContext* dec_ctx;
AVFilterGraph* filter_graph;
AVFilterContext* buffersrc_ctx;
AVFilterContext* buffersink_ctx;
static int video_stream_index = -1;
const char* filter_descr = "scale=78:24,transpose=cclock";
static int64_t last_pts = AV_NOPTS_VALUE;
static int open_input_file(const char* filename)
{
const 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;
enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_GRAY8, AV_PIX_FMT_NONE };
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. */
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);
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;
}
outputs->name = av_strdup("in");
outputs->filter_ctx = buffersrc_ctx;
outputs->pad_idx = 0;
outputs->next = NULL;
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;
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. */
AVRational timeBaseQ;
timeBaseQ.num = 1;
timeBaseQ.den = AV_TIME_BASE;
delay = av_rescale_q(frame->pts - last_pts, time_base, timeBaseQ);
if (delay > 0 && delay < 1000000)
std::this_thread::sleep_for(std::chrono::microseconds(delay));
}
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);
}
int save_frame_as_jpeg(AVCodecContext* pCodecCtx, AVFrame* pFrame, int FrameNo) {
int ret = 0;
const AVCodec* jpegCodec = avcodec_find_encoder(AV_CODEC_ID_JPEG2000);
if (!jpegCodec) {
return -1;
}
AVCodecContext* jpegContext = avcodec_alloc_context3(jpegCodec);
if (!jpegContext) {
return -1;
}
jpegContext->pix_fmt = pCodecCtx->pix_fmt;
jpegContext->height = pFrame->height;
jpegContext->width = pFrame->width;
jpegContext->time_base = AVRational{ 1,10 };
ret = avcodec_open2(jpegContext, jpegCodec, NULL);
if (ret < 0) {
return ret;
}
FILE* JPEGFile;
char JPEGFName[256];
AVPacket packet;
packet.data = NULL;
packet.size = 0;
av_init_packet(&packet);
int gotFrame;
ret = avcodec_send_frame(jpegContext, pFrame);
if (ret < 0) {
return ret;
}
ret = avcodec_receive_packet(jpegContext, &packet);
if (ret < 0) {
return ret;
}
sprintf(JPEGFName, "c:\\folder\\dvr-%06d.jpg", FrameNo);
JPEGFile = fopen(JPEGFName, "wb");
fwrite(packet.data, 1, packet.size, JPEGFile);
fclose(JPEGFile);
av_packet_unref(&packet);
avcodec_close(jpegContext);
return 0;
}
int main(int argc, char** argv)
{
AVFrame* frame;
AVFrame* filt_frame;
AVPacket* packet;
int ret;
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;
while (true)
{
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);
ret = save_frame_as_jpeg(dec_ctx, frame, dec_ctx->frame_number);
if (ret < 0)
goto end;
}
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) {
char errBuf[AV_ERROR_MAX_STRING_SIZE]{0};
int res = av_strerror(ret, errBuf, AV_ERROR_MAX_STRING_SIZE);
fprintf(stderr, "Error: %s\n", errBuf);
exit(1);
}
exit(0);
}

Answer 1

Well nevermind, I just realized I had an error with the specified codec for JPEG decoding, if somenone's facing this issue you have to use:
*const AVCodec* jpegCodec = avcodec_find_encoder(AV_CODEC_ID_MJPEG);*
instead of:
*const AVCodec* jpegCodec = avcodec_find_encoder(AV_CODEC_ID_JPEG2000);*
and also add this line:
*jpegContext->strict_std_compliance = FF_COMPLIANCE_UNOFFICIAL;*
before the call to avcodec_open2