FFmpeg technical - Revision history

Sesse: Symbol has changed name

2017-01-03T21:44:54Z

Symbol has changed name

← Older revision		Revision as of 14:44, 3 January 2017
Line 98:		Line 98:

	=== How to use bitstream ===		=== How to use bitstream ===
	There are two bitstream reading methods - MSB and 32-bit little-endian word. In order to use the second you need to #define ~~ALT_BITSTREAM_READER_LE~~ before including bitstream.h.		There are two bitstream reading methods - MSB and 32-bit little-endian word. In order to use the second you need to #define BITSTREAM_READER_LE before including bitstream.h.

	Simple bitstream writing:		Simple bitstream writing:

Vitor: /* MDCT */ Suggest using more efficient dsp.vector_fmul_window()

2012-02-26T20:06:01Z

MDCT: Suggest using more efficient dsp.vector_fmul_window()

← Older revision		Revision as of 13:06, 26 February 2012
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	MDCTContext mdct;		MDCTContext mdct;
	DSPUtil dsp;		DSPUtil dsp;
	float window[N];		float window[N];
	float output[2*N];		float output[2*N];
	float saved[2*N];		float saved[2][N];
	float ~~tmp~~[N];		float *prev = saved[0];
			float *curr = saved[1];
	float coefs[N]; // result of MDCT will be stored here		float coefs[N]; // result of MDCT will be stored here

Line 172:		Line 173:
	Backward windowed MDCT:		Backward windowed MDCT:

	~~ff_imdct_calc~~(&mdct, ~~output~~, coefs~~, tmp~~);		mdct->imdct_half(mdct, curr, coefs);
	dsp.~~vector_fmul_add_add~~(out, ~~output~~, window~~, saved, 0.0f~~, N~~, 1~~);		dsp.vector_fmul_window(out, prev, curr, window, N/2);
	~~dsp.vector_fmul_reverse~~(~~saved, output+1024, window~~, N);		FFSWAP(prev, curr);

	[[Category:FFmpeg Tutorials]]		[[Category:FFmpeg Tutorials]]

Kostya: Add flawed MDCT routine

2008-06-08T17:59:48Z

Add flawed MDCT routine

← Older revision		Revision as of 10:59, 8 June 2008
Line 147:		Line 147:

	For simple delta-frame codec look at RPZA (file libavcodec/rpza.c).		For simple delta-frame codec look at RPZA (file libavcodec/rpza.c).

			== MDCT ==
			'''I would like to know how to do that properly''', for now this is the stuff borrowed from wmadec.c and wmaenc.c.

			In order to perform N-point transform you need those variables:

			MDCTContext mdct;
			DSPUtil dsp;
			float window[N];
			float output[2*N];
			float saved[2*N];
			float tmp[N];
			float coefs[N]; // result of MDCT will be stored here

			Forward windowed MDCT:

			memcpy(output, saved, sizeof(float)*N);
			for (i = 0; i < N; i++){
			output[i+N] = audio[i] / (N/2) * window[N - i - 1];
			saved [i] = audio[i] / (N/2) * window[i];
			}
			ff_mdct_calc(&mdct, coefs, output, tmp);

			Backward windowed MDCT:

			ff_imdct_calc(&mdct, output, coefs, tmp);
			dsp.vector_fmul_add_add(out, output, window, saved, 0.0f, N, 1);
			dsp.vector_fmul_reverse(saved, output+1024, window, N);

	[[Category:FFmpeg Tutorials]]		[[Category:FFmpeg Tutorials]]

Suxen drol: cat:ffmpeg tutorials

2008-03-16T02:09:32Z

cat:ffmpeg tutorials

← Older revision		Revision as of 19:09, 15 March 2008
Line 147:		Line 147:

	For simple delta-frame codec look at RPZA (file libavcodec/rpza.c).		For simple delta-frame codec look at RPZA (file libavcodec/rpza.c).

			[[Category:FFmpeg Tutorials]]

Kostya: /* Decoder stuff */ Some tips on writing lavc decoder

2007-03-08T18:12:46Z

Decoder stuff: Some tips on writing lavc decoder

← Older revision		Revision as of 11:12, 8 March 2007
Line 77:		Line 77:

	== Decoder stuff ==		== Decoder stuff ==

			=== How to use delta frames ===
			There are two ways: either use your own buffer for frame data or reget old frame and modify it.
			This can be done by calling reget_buffer() instead of get_buffer().
			Usual code:

			if(avctx->get_buffer(avctx, &c->pic) < 0){
			av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
			return -1;
			}

			New code:

			c->pic.reference = 1;
			c->pic.buffer_hints = FF_BUFFER_HINTS_VALID \| FF_BUFFER_HINTS_PRESERVE \| FF_BUFFER_HINTS_REUSABLE;
			if(avctx->reget_buffer(avctx, &c->pic) < 0){
			av_log(avctx, AV_LOG_ERROR, "reget_buffer() failed\n");
			return -1;
			}

			=== How to use bitstream ===
			There are two bitstream reading methods - MSB and 32-bit little-endian word. In order to use the second you need to #define ALT_BITSTREAM_READER_LE before including bitstream.h.

			Simple bitstream writing:

			PutBitContext pb;
			init_put_bits(&pb, buffer, max_bytes); // max_bytes is buffer size to avoid out-of-bounds write
			put_bits(&pb, 13, value); // Note that the second argument is value size in bits, not actual value
			size = put_bits_count(&pb); // get the number of bits written so far
			flush_put_bits(&pb); // call this to finish bit writing so bits won't be lost in buffer

			Simple bitstream reading:

			GetBitContext gb;
			init_get_bits(&gb, buffer, buffer_size * 8); // init_get_bits() expects buffer size in _bits_
			a = get_bits(&gb, 16); // read some bits from stream
			b = get_sbits(&gb, 15); // read some bits from stream as signed integer
			c = get_bits_long(&gb, 18); // get_bits() is guaranteed to work only for bits <= 17, for greater values use get_bits_long()
			d = show_bits(&gb, 5); // peek bits but don't change position in bitstream, also has show_bits_long() counterpart

			Variable-length codes reading is also quite simple.

			VLC vlc;
			init_vlc(&vlc, MAX_BITS, codes_size,
			bits, bits_skip, sizeof(bits[0]),
			codes, codes_skip, sizeof(codes[0]), flags);
			value = get_vlc2(&gb, vlc.table, MAX_BITS, wrap);
			free_vlc(&vlc);

			Notes on VLC reading:
			* MAX_BITS is the maximum code length but not greater than 9 (i.e. if max codeword length is 5 then MAX_BITS=5 but if max codeword length is 15 then MAX_BITS=6)
			* _skip should be either equal to sizeof() if bits and codes are stored in separate tables or = ((uint8_t)bits[1])-((uint8_t*)bits[0]). For example if you store all in struct { char bits; short value;} codes[100] you should call

			init_vlc(&vlc, MAX_BITS, 100, &codes[0].bits, sizeof(codes[0]), 1, &codes[0].value, sizeof(codes[0]), 2, 0);

			* flags may be:
			** INIT_VLC_USE_STATIC - init codes statically ( free_vlc() is not needed)
			** INIT_VLC_LE - use alternative VLC reading mode

			* wrap = ceil(REAL_MAX_BITS/MAX_BITS) and should not be greater than 3. For example is maximum codeword size is 15 then wrap = ceil(15/9) = 2

			=== How to use frame reordering ===
			The simpliest way is to adapt decode_frame() from h263dec.c to your needs (that will also require to include MpegEncContext into your codec context).

			=== Some example decoder ===
			The simpliest decoder that does not even use bitstream reading is ATI VCR1 decoder, file libavcodec/vcr1.c.

			For simple codec with VLC reading look at WNV1 (file libavcodec/wnv1.c).

			For simple delta-frame codec look at RPZA (file libavcodec/rpza.c).

Kostya: /* Demuxer stuff */

2007-02-20T05:37:42Z

Demuxer stuff

← Older revision		Revision as of 22:37, 19 February 2007
Line 32:		Line 32:
	for(i=0; i<buf_size; i++){		for(i=0; i<buf_size; i++){
	state= (state<<8) \| buf[i];		state= (state<<8) \| buf[i];
	if(state == MARKER){		if(state == MARKER){
	start_found=1;		start_found=1;
	break;		break;
Line 61:		Line 61:
	ParseContext *pc = &pc1->pc;		ParseContext *pc = &pc1->pc;
	int next;		int next;

	if(s->flags & PARSER_FLAG_COMPLETE_FRAMES){		if(s->flags & PARSER_FLAG_COMPLETE_FRAMES){
	next= buf_size;		next= buf_size;

Kostya: /* Demuxer stuff */

2007-02-20T05:36:50Z

Demuxer stuff

← Older revision		Revision as of 22:36, 19 February 2007
Line 26:		Line 26:
	uint32_t state;		uint32_t state;
	ParseContext *pc = &pc1->pc;		ParseContext *pc = &pc1->pc;

	start_found= pc->frame_start_found;		start_found= pc->frame_start_found;
	state= pc->state;		state= pc->state;

	i=0;		i=0;
	if(!start_found){		if(!start_found){
Line 68:		Line 66:
	}else{		}else{
	next= xxx_find_frame_end(pc1, buf, buf_size);		next= xxx_find_frame_end(pc1, buf, buf_size);

	if (ff_combine_frame(pc, next, (uint8_t **)&buf, &buf_size) < 0) {		if (ff_combine_frame(pc, next, (uint8_t **)&buf, &buf_size) < 0) {
	*poutbuf = NULL;		*poutbuf = NULL;

Kostya at 05:13, 20 February 2007

2007-02-20T05:13:48Z

New page

== Demuxer stuff ==
=== How to use demuxer with raw data ===
Write a parser.
There are two main function - parse and split. '''parse''' is used to reconstruct precisely one frame from raw packets, '''split''' is used for extracting extradata from the same stream.

Parser declarations will look like this:

typedef struct MyParseContext{
ParseContext pc; /* always include this first */
another data
};
AVParser some_parser = {
{ CODEC_ID_1 [CODEC_ID_2, ...] },
sizeof (MyParseContext),
NULL, /* usually there is no need in parser_open */
my_parse,
ff_parse_close, /* again, use standard close function */
my_split,
};

And here is the code for some parser which splits frame by some markers:

static int xxx_find_frame_end(XXXParseContext *pc1, const uint8_t *buf,
int buf_size) {
int start_found, i;
uint32_t state;
ParseContext *pc = &pc1->pc;

start_found= pc->frame_start_found;
state= pc->state;

i=0;
if(!start_found){
for(i=0; i<buf_size; i++){
state= (state<<8) | buf[i];
if(state == MARKER){
start_found=1;
break;
}
}
}
if(start_found){
for(; i<buf_size; i++){
state= (state<<8) | buf[i];
if(state == MARKER){
pc->frame_start_found= 0;
pc->state= -1;
return i-3;
}
}
}
pc->frame_start_found= start_found;
pc->state= state;
return END_NOT_FOUND;
}

static int xxx_parse(AVCodecParserContext *s,
AVCodecContext *avctx,
uint8_t **poutbuf, int *poutbuf_size,
const uint8_t *buf, int buf_size)
{
XXXParseContext *pc1 = s->priv_data;
ParseContext *pc = &pc1->pc;
int next;

if(s->flags & PARSER_FLAG_COMPLETE_FRAMES){
next= buf_size;
}else{
next= xxx_find_frame_end(pc1, buf, buf_size);

if (ff_combine_frame(pc, next, (uint8_t **)&buf, &buf_size) < 0) {
*poutbuf = NULL;
*poutbuf_size = 0;
return buf_size;
}
}
*poutbuf = (uint8_t *)buf;
*poutbuf_size = buf_size;
return next;
}

== Decoder stuff ==