FFmpeg
nuv.c
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1 /*
2  * NuppelVideo decoder
3  * Copyright (c) 2006 Reimar Doeffinger
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 #include <stddef.h>
23 #include <limits.h>
24 
25 #include "libavutil/common.h"
26 #include "libavutil/intreadwrite.h"
27 #include "libavutil/lzo.h"
28 #include "libavutil/imgutils.h"
29 #include "avcodec.h"
30 #include "codec_internal.h"
31 #include "decode.h"
32 #include "jpegquanttables.h"
33 #include "rtjpeg.h"
34 
35 typedef struct NuvContext {
38  int quality;
39  int width, height;
40  unsigned int decomp_size;
41  unsigned char *decomp_buf;
42  uint32_t lq[64], cq[64];
44 } NuvContext;
45 
46 /**
47  * @brief copy frame data from buffer to AVFrame, handling stride.
48  * @param f destination AVFrame
49  * @param src source buffer, does not use any line-stride
50  * @param width width of the video frame
51  * @param height height of the video frame
52  */
53 static void copy_frame(AVFrame *f, const uint8_t *src, int width, int height)
54 {
55  uint8_t *src_data[4];
56  int src_linesize[4];
57  av_image_fill_arrays(src_data, src_linesize, src,
58  f->format, width, height, 1);
59  av_image_copy2(f->data, f->linesize, src_data, src_linesize,
60  f->format, width, height);
61 }
62 
63 /**
64  * @brief extract quantization tables from codec data into our context
65  */
66 static int get_quant(AVCodecContext *avctx, NuvContext *c, const uint8_t *buf,
67  int size)
68 {
69  int i;
70  if (size < 2 * 64 * 4) {
71  av_log(avctx, AV_LOG_ERROR, "insufficient rtjpeg quant data\n");
72  return AVERROR_INVALIDDATA;
73  }
74  for (i = 0; i < 64; i++, buf += 4)
75  c->lq[i] = AV_RL32(buf);
76  for (i = 0; i < 64; i++, buf += 4)
77  c->cq[i] = AV_RL32(buf);
78  return 0;
79 }
80 
81 /**
82  * @brief set quantization tables from a quality value
83  */
85 {
86  int i;
87  quality = FFMAX(quality, 1);
88  for (i = 0; i < 64; i++) {
89  c->lq[i] = (ff_mjpeg_std_luminance_quant_tbl[i] << 7) / quality;
91  }
92 }
93 
94 static int codec_reinit(AVCodecContext *avctx, int width, int height,
95  int quality)
96 {
97  NuvContext *c = avctx->priv_data;
98  int ret;
99 
100  width = FFALIGN(width, 2);
101  height = FFALIGN(height, 2);
102  if (quality >= 0)
104  if (width != c->width || height != c->height) {
105  // also reserve space for a possible additional header
106  int64_t buf_size = height * (int64_t)width * 3 / 2
109  if (buf_size > INT_MAX/8)
110  return -1;
111  if ((ret = ff_set_dimensions(avctx, width, height)) < 0)
112  return ret;
113  c->width = width;
114  c->height = height;
115  av_fast_malloc(&c->decomp_buf, &c->decomp_size,
116  buf_size);
117  if (!c->decomp_buf) {
118  av_log(avctx, AV_LOG_ERROR,
119  "Can't allocate decompression buffer.\n");
120  return AVERROR(ENOMEM);
121  }
122  ff_rtjpeg_decode_init(&c->rtj, c->width, c->height, c->lq, c->cq);
123  av_frame_unref(c->pic);
124  return 1;
125  } else if (quality != c->quality)
126  ff_rtjpeg_decode_init(&c->rtj, c->width, c->height, c->lq, c->cq);
127 
128  return 0;
129 }
130 
131 static int decode_frame(AVCodecContext *avctx, AVFrame *picture,
132  int *got_frame, AVPacket *avpkt)
133 {
134  const uint8_t *buf = avpkt->data;
135  int buf_size = avpkt->size;
136  NuvContext *c = avctx->priv_data;
137  int orig_size = buf_size;
138  int keyframe, ret;
139  int size_change = 0;
140  int minsize = 0;
141  int flags = 0;
142  int result, init_frame = !avctx->frame_num;
143  enum {
144  NUV_UNCOMPRESSED = '0',
145  NUV_RTJPEG = '1',
146  NUV_RTJPEG_IN_LZO = '2',
147  NUV_LZO = '3',
148  NUV_BLACK = 'N',
149  NUV_COPY_LAST = 'L'
150  } comptype;
151 
152  if (buf_size < 12) {
153  av_log(avctx, AV_LOG_ERROR, "coded frame too small\n");
154  return AVERROR_INVALIDDATA;
155  }
156 
157  // codec data (rtjpeg quant tables)
158  if (buf[0] == 'D' && buf[1] == 'R') {
159  int ret;
160  // Skip the rest of the frame header.
161  buf = &buf[12];
162  buf_size -= 12;
163  ret = get_quant(avctx, c, buf, buf_size);
164  if (ret < 0)
165  return ret;
166  ff_rtjpeg_decode_init(&c->rtj, c->width, c->height, c->lq, c->cq);
167  return orig_size;
168  }
169 
170  if (buf_size < 12 || buf[0] != 'V') {
171  av_log(avctx, AV_LOG_ERROR, "not a nuv video frame\n");
172  return AVERROR_INVALIDDATA;
173  }
174  comptype = buf[1];
175  switch (comptype) {
176  case NUV_RTJPEG_IN_LZO:
177  case NUV_RTJPEG:
178  keyframe = !buf[2];
179  if (c->width < 16 || c->height < 16) {
180  return AVERROR_INVALIDDATA;
181  }
182  break;
183  case NUV_COPY_LAST:
185  keyframe = 0;
186  break;
187  default:
188  keyframe = 1;
189  break;
190  }
191  switch (comptype) {
192  case NUV_UNCOMPRESSED:
193  minsize = c->width * c->height * 3 / 2;
194  break;
195  case NUV_RTJPEG:
196  minsize = c->width/16 * (c->height/16) * 6;
197  break;
198  case NUV_BLACK:
199  case NUV_COPY_LAST:
200  case NUV_LZO:
201  case NUV_RTJPEG_IN_LZO:
202  break;
203  default:
204  av_log(avctx, AV_LOG_ERROR, "unknown compression\n");
205  return AVERROR_INVALIDDATA;
206  }
207  if (buf_size < minsize / 4)
208  return AVERROR_INVALIDDATA;
209 retry:
210  // Skip the rest of the frame header.
211  buf = &buf[12];
212  buf_size -= 12;
213  if (comptype == NUV_RTJPEG_IN_LZO || comptype == NUV_LZO) {
214  int outlen = c->decomp_size - FFMAX(AV_INPUT_BUFFER_PADDING_SIZE, AV_LZO_OUTPUT_PADDING);
215  int inlen = buf_size;
216  if (av_lzo1x_decode(c->decomp_buf, &outlen, buf, &inlen)) {
217  av_log(avctx, AV_LOG_ERROR, "error during lzo decompression\n");
218  return AVERROR_INVALIDDATA;
219  }
220  buf = c->decomp_buf;
221  buf_size = c->decomp_size - FFMAX(AV_INPUT_BUFFER_PADDING_SIZE, AV_LZO_OUTPUT_PADDING) - outlen;
222  memset(c->decomp_buf + buf_size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
223  }
224  if (c->codec_frameheader) {
225  int w, h, q;
226  if (buf_size < RTJPEG_HEADER_SIZE) {
227  av_log(avctx, AV_LOG_ERROR, "Too small NUV video frame\n");
228  return AVERROR_INVALIDDATA;
229  }
230  // There seem to exist two variants of this header: one starts with 'V'
231  // and 5 bytes unknown, the other matches current MythTV and is 4 bytes size,
232  // 1 byte header size (== 12), 1 byte version (== 0)
233  if (buf[0] != 'V' && AV_RL16(&buf[4]) != 0x000c) {
234  av_log(avctx, AV_LOG_ERROR, "Unknown secondary frame header (wrong codec_tag?)\n");
235  return AVERROR_INVALIDDATA;
236  }
237  w = AV_RL16(&buf[6]);
238  h = AV_RL16(&buf[8]);
239  q = buf[10];
240  if ((result = codec_reinit(avctx, w, h, q)) < 0)
241  return result;
242  if (result) {
243  buf = avpkt->data;
244  buf_size = avpkt->size;
245  size_change = 1;
246  goto retry;
247  }
248  buf = &buf[RTJPEG_HEADER_SIZE];
249  buf_size -= RTJPEG_HEADER_SIZE;
250  }
251 
252  if (size_change || keyframe) {
253  av_frame_unref(c->pic);
254  init_frame = 1;
255  }
256 
257  if ((result = ff_reget_buffer(avctx, c->pic, flags)) < 0)
258  return result;
259  if (init_frame) {
260  memset(c->pic->data[0], 0, avctx->height * c->pic->linesize[0]);
261  memset(c->pic->data[1], 0x80, avctx->height * c->pic->linesize[1] / 2);
262  memset(c->pic->data[2], 0x80, avctx->height * c->pic->linesize[2] / 2);
263  }
264 
265  c->pic->pict_type = keyframe ? AV_PICTURE_TYPE_I : AV_PICTURE_TYPE_P;
266  if (keyframe)
267  c->pic->flags |= AV_FRAME_FLAG_KEY;
268  else
269  c->pic->flags &= ~AV_FRAME_FLAG_KEY;
270  // decompress/copy/whatever data
271  switch (comptype) {
272  case NUV_LZO:
273  case NUV_UNCOMPRESSED: {
274  int height = c->height;
275  if (buf_size < c->width * height * 3 / 2) {
276  av_log(avctx, AV_LOG_ERROR, "uncompressed frame too short\n");
277  height = buf_size / c->width / 3 * 2;
278  }
279  if(height > 0)
280  copy_frame(c->pic, buf, c->width, height);
281  break;
282  }
283  case NUV_RTJPEG_IN_LZO:
284  case NUV_RTJPEG:
285  ret = ff_rtjpeg_decode_frame_yuv420(&c->rtj, c->pic, buf, buf_size);
286  if (ret < 0)
287  return ret;
288  break;
289  case NUV_BLACK:
290  memset(c->pic->data[0], 0, c->width * c->height);
291  memset(c->pic->data[1], 128, c->width * c->height / 4);
292  memset(c->pic->data[2], 128, c->width * c->height / 4);
293  break;
294  case NUV_COPY_LAST:
295  /* nothing more to do here */
296  break;
297  }
298 
299  if ((result = av_frame_ref(picture, c->pic)) < 0)
300  return result;
301 
302  *got_frame = 1;
303  return orig_size;
304 }
305 
307 {
308  NuvContext *c = avctx->priv_data;
309  int ret;
310 
311  c->pic = av_frame_alloc();
312  if (!c->pic)
313  return AVERROR(ENOMEM);
314 
315  avctx->pix_fmt = AV_PIX_FMT_YUV420P;
316  c->decomp_buf = NULL;
317  c->quality = -1;
318  c->width = 0;
319  c->height = 0;
320 
321  c->codec_frameheader = avctx->codec_tag == MKTAG('R', 'J', 'P', 'G');
322 
323  if (avctx->extradata_size)
324  get_quant(avctx, c, avctx->extradata, avctx->extradata_size);
325 
326  ff_rtjpeg_init(&c->rtj, avctx);
327 
328  if ((ret = codec_reinit(avctx, avctx->width, avctx->height, -1)) < 0)
329  return ret;
330 
331  return 0;
332 }
333 
335 {
336  NuvContext *c = avctx->priv_data;
337 
338  av_freep(&c->decomp_buf);
339  av_frame_free(&c->pic);
340 
341  return 0;
342 }
343 
345  .p.name = "nuv",
346  CODEC_LONG_NAME("NuppelVideo/RTJPEG"),
347  .p.type = AVMEDIA_TYPE_VIDEO,
348  .p.id = AV_CODEC_ID_NUV,
349  .priv_data_size = sizeof(NuvContext),
350  .init = decode_init,
351  .close = decode_end,
353  .p.capabilities = AV_CODEC_CAP_DR1,
354  .caps_internal = FF_CODEC_CAP_INIT_CLEANUP,
355 };
FF_CODEC_CAP_INIT_CLEANUP
#define FF_CODEC_CAP_INIT_CLEANUP
The codec allows calling the close function for deallocation even if the init function returned a fai...
Definition: codec_internal.h:42
AVERROR
Filter the word “frame” indicates either a video frame or a group of audio as stored in an AVFrame structure Format for each input and each output the list of supported formats For video that means pixel format For audio that means channel sample they are references to shared objects When the negotiation mechanism computes the intersection of the formats supported at each end of a all references to both lists are replaced with a reference to the intersection And when a single format is eventually chosen for a link amongst the remaining all references to the list are updated That means that if a filter requires that its input and output have the same format amongst a supported all it has to do is use a reference to the same list of formats query_formats can leave some formats unset and return AVERROR(EAGAIN) to cause the negotiation mechanism toagain later. That can be used by filters with complex requirements to use the format negotiated on one link to set the formats supported on another. Frame references ownership and permissions
NuvContext::codec_frameheader
int codec_frameheader
Definition: nuv.c:37
NuvContext::cq
uint32_t cq[64]
Definition: nuv.c:42
av_frame_free
void av_frame_free(AVFrame **frame)
Free the frame and any dynamically allocated objects in it, e.g.
Definition: frame.c:100
copy_frame
static void copy_frame(AVFrame *f, const uint8_t *src, int width, int height)
copy frame data from buffer to AVFrame, handling stride.
Definition: nuv.c:53
AVFrame
This structure describes decoded (raw) audio or video data.
Definition: frame.h:340
w
uint8_t w
Definition: llviddspenc.c:38
NuvContext::width
int width
Definition: nuv.c:39
AVPacket::data
uint8_t * data
Definition: packet.h:491
codec_reinit
static int codec_reinit(AVCodecContext *avctx, int width, int height, int quality)
Definition: nuv.c:94
FFCodec
Definition: codec_internal.h:127
FFMAX
#define FFMAX(a, b)
Definition: macros.h:47
ff_set_dimensions
int ff_set_dimensions(AVCodecContext *s, int width, int height)
Check that the provided frame dimensions are valid and set them on the codec context.
Definition: utils.c:94
quality
trying all byte sequences megabyte in length and selecting the best looking sequence will yield cases to try But a word about quality
Definition: rate_distortion.txt:12
RTJPEG_HEADER_SIZE
#define RTJPEG_HEADER_SIZE
Definition: rtjpeg.h:32
FFCodec::p
AVCodec p
The public AVCodec.
Definition: codec_internal.h:131
NuvContext::quality
int quality
Definition: nuv.c:38
decode_frame
static int decode_frame(AVCodecContext *avctx, AVFrame *picture, int *got_frame, AVPacket *avpkt)
Definition: nuv.c:131
av_frame_alloc
AVFrame * av_frame_alloc(void)
Allocate an AVFrame and set its fields to default values.
Definition: frame.c:88
ff_rtjpeg_decode_init
void ff_rtjpeg_decode_init(RTJpegContext *c, int width, int height, const uint32_t *lquant, const uint32_t *cquant)
initialize an RTJpegContext, may be called multiple times
Definition: rtjpeg.c:158
AV_LOG_ERROR
#define AV_LOG_ERROR
Something went wrong and cannot losslessly be recovered.
Definition: log.h:180
av_cold
#define av_cold
Definition: attributes.h:90
AV_FRAME_FLAG_KEY
#define AV_FRAME_FLAG_KEY
A flag to mark frames that are keyframes.
Definition: frame.h:628
AVCodecContext::extradata_size
int extradata_size
Definition: avcodec.h:543
width
#define width
FF_CODEC_DECODE_CB
#define FF_CODEC_DECODE_CB(func)
Definition: codec_internal.h:306
intreadwrite.h
AV_CODEC_ID_NUV
@ AV_CODEC_ID_NUV
Definition: codec_id.h:136
NuvContext
Definition: nuv.c:35
init
int(* init)(AVBSFContext *ctx)
Definition: dts2pts_bsf.c:365
get_quant
static int get_quant(AVCodecContext *avctx, NuvContext *c, const uint8_t *buf, int size)
extract quantization tables from codec data into our context
Definition: nuv.c:66
decode.h
limits.h
AV_RL16
uint64_t_TMPL AV_WL64 unsigned int_TMPL AV_WL32 unsigned int_TMPL AV_WL24 unsigned int_TMPL AV_RL16
Definition: bytestream.h:94
NuvContext::decomp_buf
unsigned char * decomp_buf
Definition: nuv.c:41
AV_PIX_FMT_YUV420P
@ AV_PIX_FMT_YUV420P
planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
Definition: pixfmt.h:66
CODEC_LONG_NAME
#define CODEC_LONG_NAME(str)
Definition: codec_internal.h:272
result
and forward the result(frame or status change) to the corresponding input. If nothing is possible
NULL
#define NULL
Definition: coverity.c:32
ff_nuv_decoder
const FFCodec ff_nuv_decoder
Definition: nuv.c:344
AV_PICTURE_TYPE_I
@ AV_PICTURE_TYPE_I
Intra.
Definition: avutil.h:279
ff_rtjpeg_decode_frame_yuv420
int ff_rtjpeg_decode_frame_yuv420(RTJpegContext *c, AVFrame *f, const uint8_t *buf, int buf_size)
decode one rtjpeg YUV420 frame
Definition: rtjpeg.c:106
jpegquanttables.h
c
Undefined Behavior In the C some operations are like signed integer dereferencing freed accessing outside allocated Undefined Behavior must not occur in a C it is not safe even if the output of undefined operations is unused The unsafety may seem nit picking but Optimizing compilers have in fact optimized code on the assumption that no undefined Behavior occurs Optimizing code based on wrong assumptions can and has in some cases lead to effects beyond the output of computations The signed integer overflow problem in speed critical code Code which is highly optimized and works with signed integers sometimes has the problem that often the output of the computation does not c
Definition: undefined.txt:32
decode_end
static av_cold int decode_end(AVCodecContext *avctx)
Definition: nuv.c:334
f
f
Definition: af_crystalizer.c:121
av_lzo1x_decode
int av_lzo1x_decode(void *out, int *outlen, const void *in, int *inlen)
Decodes LZO 1x compressed data.
Definition: lzo.c:136
AV_CODEC_CAP_DR1
#define AV_CODEC_CAP_DR1
Codec uses get_buffer() or get_encode_buffer() for allocating buffers and supports custom allocators.
Definition: codec.h:52
AVPacket::size
int size
Definition: packet.h:492
av_image_fill_arrays
int av_image_fill_arrays(uint8_t *dst_data[4], int dst_linesize[4], const uint8_t *src, enum AVPixelFormat pix_fmt, int width, int height, int align)
Setup the data pointers and linesizes based on the specified image parameters and the provided array.
Definition: imgutils.c:446
av_frame_ref
int av_frame_ref(AVFrame *dst, const AVFrame *src)
Set up a new reference to the data described by the source frame.
Definition: frame.c:361
codec_internal.h
size
int size
Definition: twinvq_data.h:10344
get_quant_quality
static void get_quant_quality(NuvContext *c, int quality)
set quantization tables from a quality value
Definition: nuv.c:84
height
#define height
NuvContext::decomp_size
unsigned int decomp_size
Definition: nuv.c:40
i
#define i(width, name, range_min, range_max)
Definition: cbs_h2645.c:255
AV_LZO_OUTPUT_PADDING
#define AV_LZO_OUTPUT_PADDING
Definition: lzo.h:47
AVCodecContext::extradata
uint8_t * extradata
some codecs need / can use extradata like Huffman tables.
Definition: avcodec.h:542
common.h
ff_mjpeg_std_chrominance_quant_tbl
const uint8_t ff_mjpeg_std_chrominance_quant_tbl[64]
Definition: jpegquanttables.c:45
av_frame_unref
void av_frame_unref(AVFrame *frame)
Unreference all the buffers referenced by frame and reset the frame fields.
Definition: frame.c:622
ff_rtjpeg_init
void ff_rtjpeg_init(RTJpegContext *c, struct AVCodecContext *avctx)
Definition: rtjpeg.c:170
AVCodec::name
const char * name
Name of the codec implementation.
Definition: codec.h:194
AVCodecContext::height
int height
Definition: avcodec.h:621
AVCodecContext::pix_fmt
enum AVPixelFormat pix_fmt
Pixel format, see AV_PIX_FMT_xxx.
Definition: avcodec.h:658
NuvContext::lq
uint32_t lq[64]
Definition: nuv.c:42
avcodec.h
AVCodecContext::frame_num
int64_t frame_num
Frame counter, set by libavcodec.
Definition: avcodec.h:2118
ff_reget_buffer
int ff_reget_buffer(AVCodecContext *avctx, AVFrame *frame, int flags)
Identical in function to ff_get_buffer(), except it reuses the existing buffer if available.
Definition: decode.c:1735
ret
ret
Definition: filter_design.txt:187
NuvContext::pic
AVFrame * pic
Definition: nuv.c:36
lzo.h
AV_INPUT_BUFFER_PADDING_SIZE
#define AV_INPUT_BUFFER_PADDING_SIZE
Definition: defs.h:40
AV_RL32
uint64_t_TMPL AV_WL64 unsigned int_TMPL AV_RL32
Definition: bytestream.h:92
ff_mjpeg_std_luminance_quant_tbl
const uint8_t ff_mjpeg_std_luminance_quant_tbl[64]
Definition: jpegquanttables.c:35
AVCodecContext
main external API structure.
Definition: avcodec.h:441
av_image_copy2
static void av_image_copy2(uint8_t *const dst_data[4], const int dst_linesizes[4], uint8_t *const src_data[4], const int src_linesizes[4], enum AVPixelFormat pix_fmt, int width, int height)
Wrapper around av_image_copy() to workaround the limitation that the conversion from uint8_t * const ...
Definition: imgutils.h:184
rtjpeg.h
NuvContext::rtj
RTJpegContext rtj
Definition: nuv.c:43
init_frame
static void init_frame(FlacEncodeContext *s, int nb_samples)
Definition: flacenc.c:478
AV_PICTURE_TYPE_P
@ AV_PICTURE_TYPE_P
Predicted.
Definition: avutil.h:280
AVMEDIA_TYPE_VIDEO
@ AVMEDIA_TYPE_VIDEO
Definition: avutil.h:201
AVCodecContext::codec_tag
unsigned int codec_tag
fourcc (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
Definition: avcodec.h:466
FFALIGN
#define FFALIGN(x, a)
Definition: macros.h:78
AVPacket
This structure stores compressed data.
Definition: packet.h:468
AVCodecContext::priv_data
void * priv_data
Definition: avcodec.h:468
av_freep
#define av_freep(p)
Definition: tableprint_vlc.h:34
av_fast_malloc
void av_fast_malloc(void *ptr, unsigned int *size, size_t min_size)
Allocate a buffer, reusing the given one if large enough.
Definition: mem.c:555
src
INIT_CLIP pixel * src
Definition: h264pred_template.c:418
decode_init
static av_cold int decode_init(AVCodecContext *avctx)
Definition: nuv.c:306
AVCodecContext::width
int width
picture width / height.
Definition: avcodec.h:621
imgutils.h
flags
#define flags(name, subs,...)
Definition: cbs_av1.c:474
av_log
#define av_log(a,...)
Definition: tableprint_vlc.h:27
NuvContext::height
int height
Definition: nuv.c:39
AVERROR_INVALIDDATA
#define AVERROR_INVALIDDATA
Invalid data found when processing input.
Definition: error.h:61
MKTAG
#define MKTAG(a, b, c, d)
Definition: macros.h:55
h
h
Definition: vp9dsp_template.c:2038
RTJpegContext
Definition: rtjpeg.h:34
FF_REGET_BUFFER_FLAG_READONLY
#define FF_REGET_BUFFER_FLAG_READONLY
the returned buffer does not need to be writable
Definition: decode.h:128