Decode image sequence with libavcodec

[rick666]

Hi man,
I've successfully encoded a bitmap sequence with libavcodec into a mp4 file.
Now I want to encode every image and then decode to obtain the initial sequence but when I've tryed to encode and decode I've founded that the image decoded wasn't equal to the original, infact I've some images good (I think these are keyframes) and some others appear greyscaled like there are only difference between one frame and the following.
I simply encode and decode, nothing more nothing less, in this way:

/* encode the image */
out_size = avcodec_encode_video(c, video_outbuf, video_outbuf_size, picture);


/*decode the image*/
int got_picture;
long test =avcodec_decode_video(d, pictureDecode,&got_picture,video_outbuf, out_size);


Someone knows the answer for this (I think) stupid question?

[Sharktooth]

mpeg-4 has different kinds of frame types.
what you experienced are techniques used in MPEG-4 standard to rise compression.
If you want to decode pure pictures you'll need to:
1 - Use a MPEG-4 compilant decoder (or write your own)
2 - Encode with Keyframes (aka I-Frames) ONLY.

[GodofaGap]

MPEG4 ASP is inherently lossy, so you cannot obtain the original sequence.

You could use MPEG4 AVC q0 (for example with x264), but your sequence must be downsampled to YV12 then, which is lossy too.

(If you were not talking about compression artifacts, please ignore this post)

[Sharktooth]

he's talking about getting partial frames (probably P or B-Frames).

[rick666]

ok, maybe I'm doing some error here.
The aim of my test is to compress every bitmap image with mpeg4 and then send it to a remote client that must visualize every decoded frame in a sequence like a video. Is it possible to just add the frame difference to the keyframes and so correctly visualize the sequence?And ,if yes, how can I do that?
Thank's and regards.

[Sharktooth]

get a reference decoder and the MPEG documentation and look at how the frames are reconstructed.

[nm]

Libavcodec should do that for him. I'd suggest taking a look at ffplay or some other simple player that uses the library.