Thanks for the quick reply Luca,
> The code doesn't seem attached
I've copied and pasted the relevant code (in its original form) below from
api-example.c for your convenience.
> you might read about the img2 muxer and use AV_CODEC_ID_PPM as encoder.
Are you implying that after decoding the .mpg video using
AV_CODEC_ID_MPEG1VIDEO, it needs to be re-encoded using AV_CODEC_ID_PPM? My
end-goal is to adapt this api-example example to decode MJPEG video (captured
from a USB webcam) in real-time and write the frames to cv::Mat objects that
OpenCV can understand. In other words, if I know how avcodec_decode_video2()
arranges the RGB data into the AVFrame->data buffer, I should be able to create
the cv::Mat object myself.
>> I believe this is because the call to len = avcodec_decode_video2(c,
>> picture, &got_picture, &avpkt) is only storing the gray image data.
> Hardly possible, check all the pointers in the AVFrame data =)
Do you mean that AVFrame->data[i] points to the ith channel? (It appears that
data is an array of pointers with size=AV_NUM_DATA_POINTERS=8, each pointing to
a different channel of image data)
i.e. something like:
AVFrame->data[0] = RED
AVFrame->data[1] = GREEN
AVFrame->data[2] = BLUE
I tried replacing the call to pgm_save(picture->data[i], picture->linesize[i],
c->width, c->height, buf) in the api-example with i=1,2,3 instead of i=0, but
end up with a garbled last frame rather than a gray representation of either
the R, G, or B channel. If AVFrame->data somehow contains RGB data, how do I
access it?
Sorry for the confusion--my thesis has taken me on quite a detour outside of my
wheelhouse. For future reference, Is there a better place for me to find help
besides the handfull of examples, libav mailing list, and the source code
itself? (Is there any documentation where I can read about things like the
"img2 muxer", AV_CODEC_IDs, etc?)
Thanks a ton! =)
-Mike
static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,
char *filename)
{
FILE *f;
int i;
f=fopen(filename,"w");
fprintf(f,"P5\n%d %d\n%d\n",xsize,ysize,255);
for(i=0;i<ysize;i++)
fwrite(buf + i * wrap,1,xsize,f);
fclose(f);
}
static void video_decode_example(const char *outfilename, const char *filename)
{
AVCodec *codec;
AVCodecContext *c= NULL;
int frame, got_picture, len;
FILE *f;
AVFrame *picture;
uint8_t inbuf[INBUF_SIZE + FF_INPUT_BUFFER_PADDING_SIZE];
char buf[1024];
AVPacket avpkt;
av_init_packet(&avpkt);
/* set end of buffer to 0 (this ensures that no overreading happens for
damaged mpeg streams) */
memset(inbuf + INBUF_SIZE, 0, FF_INPUT_BUFFER_PADDING_SIZE);
printf("Video decoding\n");
/* find the mpeg1 video decoder */
codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);
if (!codec) {
fprintf(stderr, "codec not found\n");
exit(1);
}
c = avcodec_alloc_context3(codec);
picture= avcodec_alloc_frame();
if(codec->capabilities&CODEC_CAP_TRUNCATED)
c->flags|= CODEC_FLAG_TRUNCATED; /* we do not send complete frames */
/* For some codecs, such as msmpeg4 and mpeg4, width and height
MUST be initialized there because this information is not
available in the bitstream. */
/* open it */
if (avcodec_open2(c, codec, NULL) < 0) {
fprintf(stderr, "could not open codec\n");
exit(1);
}
/* the codec gives us the frame size, in samples */
f = fopen(filename, "rb");
if (!f) {
fprintf(stderr, "could not open %s\n", filename);
exit(1);
}
frame = 0;
for(;;) {
avpkt.size = fread(inbuf, 1, INBUF_SIZE, f);
if (avpkt.size == 0)
break;
/* NOTE1: some codecs are stream based (mpegvideo, mpegaudio)
and this is the only method to use them because you cannot
know the compressed data size before analysing it.
BUT some other codecs (msmpeg4, mpeg4) are inherently frame
based, so you must call them with all the data for one
frame exactly. You must also initialize 'width' and
'height' before initializing them. */
/* NOTE2: some codecs allow the raw parameters (frame size,
sample rate) to be changed at any frame. We handle this, so
you should also take care of it */
/* here, we use a stream based decoder (mpeg1video), so we
feed decoder and see if it could decode a frame */
avpkt.data = inbuf;
while (avpkt.size > 0) {
len = avcodec_decode_video2(c, picture, &got_picture, &avpkt);
if (len < 0) {
fprintf(stderr, "Error while decoding frame %d\n", frame);
exit(1);
}
if (got_picture) {
printf("saving frame %3d\n", frame);
fflush(stdout);
/* the picture is allocated by the decoder. no need to
free it */
snprintf(buf, sizeof(buf), outfilename, frame);
pgm_save(picture->data[0], picture->linesize[0],
c->width, c->height, buf);
frame++;
}
avpkt.size -= len;
avpkt.data += len;
}
}
/* some codecs, such as MPEG, transmit the I and P frame with a
latency of one frame. You must do the following to have a
chance to get the last frame of the video */
avpkt.data = NULL;
avpkt.size = 0;
len = avcodec_decode_video2(c, picture, &got_picture, &avpkt);
if (got_picture) {
printf("saving last frame %3d\n", frame);
fflush(stdout);
/* the picture is allocated by the decoder. no need to
free it */
snprintf(buf, sizeof(buf), outfilename, frame);
pgm_save(picture->data[0], picture->linesize[0],
c->width, c->height, buf);
frame++;
}
fclose(f);
avcodec_close(c);
av_free(c);
avcodec_free_frame(&picture);
printf("\n");
}
int main(int argc, char **argv)
{
const char *filename;
/* register all the codecs */
avcodec_register_all();
if (argc <= 1) {
//audio_encode_example("/tmp/test.mp2");
//audio_decode_example("/tmp/test.sw", "/tmp/test.mp2");
video_encode_example("/tmp/test.mpg");
filename = "/tmp/test.mpg";
} else {
filename = argv[1];
}
// audio_decode_example("/tmp/test.sw", filename);
video_decode_example("/tmp/test%d.pgm", filename);
return 0;
}
----- Original Message -----
From: "Luca Barbato" <[email protected]>
To: [email protected]
Sent: Tuesday, September 17, 2013 3:21:17 AM
Subject: Re: [libav-api] How to output color (.ppm) images using
api-exmaple.c
On 17/09/13 04:27, Michael B. Darling wrote:
> I have read about the format of .pgm and .ppm files from
> (http://paulbourke.net/dataformats/ppm/), but have not been able to
> generate .ppm images. I have tried creating a new function
> ppm_save() (modified code attached) and replacing all calls to
> pgm_save() with my new function, but I essentially get three
> subsequent frames tiled in the same image file.
The code doesn't seem attached, you might read about the img2 muxer and
use AV_CODEC_ID_PPM as encoder.
> I believe this is because the call to len = avcodec_decode_video2(c,
> picture, &got_picture, &avpkt) is only storing the gray image data.
Hardly possible, check all the pointers in the AVFrame data =)
lu
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