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1 /*
2  * Microsoft Screen 4 (aka Microsoft Expression Encoder Screen) decoder
3  * Copyright (c) 2012 Konstantin Shishkov
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 /**
23  * @file
24  * Microsoft Screen 4 (aka Microsoft Titanium Screen 2,
25  * aka Microsoft Expression Encoder Screen) decoder
26  */
27 
28 #include "libavutil/thread.h"
29 #include "libavutil/imgutils.h"
30 
31 #include "avcodec.h"
32 #include "bytestream.h"
33 #include "codec_internal.h"
34 #include "get_bits.h"
35 #include "internal.h"
36 #include "jpegtables.h"
37 #include "mss34dsp.h"
38 #include "unary.h"
39 
40 #define HEADER_SIZE 8
41 
42 enum FrameType {
43     INTRA_FRAME = 0,
44     INTER_FRAME,
45     SKIP_FRAME
46 };
47 
48 enum BlockType {
49     SKIP_BLOCK = 0,
50     DCT_BLOCK,
51     IMAGE_BLOCK,
52 };
53 
54 enum CachePos {
55     LEFT = 0,
56     TOP_LEFT,
57     TOP,
58 };
59 
60 static const uint8_t mss4_dc_vlc_lens[2][16] = {
61     { 0, 1, 5, 1, 1, 1, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0 },
62     { 0, 3, 1, 1, 1, 1, 1, 1, 1, 2, 0, 0, 0, 0, 0, 0 }
63 };
64 
65 static const uint8_t vec_len_syms[2][4] = {
66     { 4, 2, 3, 1 },
67     { 4, 1, 2, 3 }
68 };
69 
70 static const uint8_t mss4_vec_entry_vlc_lens[2][16] = {
71     { 0, 2, 2, 3, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
72     { 0, 1, 5, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
73 };
74 
75 static const uint8_t mss4_vec_entry_vlc_syms[2][9] = {
76     { 0, 7, 6, 5, 8, 4, 3, 1, 2 },
77     { 0, 2, 3, 4, 5, 6, 7, 1, 8 }
78 };
79 
80 #define MAX_ENTRIES  162
81 
82 typedef struct MSS4Context {
83     AVFrame    *pic;
84 
85     int        block[64];
86     uint8_t    imgbuf[3][16 * 16];
87 
88     int        quality;
89     uint16_t   quant_mat[2][64];
90 
91     int        *prev_dc[3];
92     ptrdiff_t  dc_stride[3];
93     int        dc_cache[4][4];
94 
95     int        prev_vec[3][4];
96 } MSS4Context;
97 
98 static VLC dc_vlc[2], ac_vlc[2];
99 static VLC vec_entry_vlc[2];
100 
mss4_init_vlc(VLC * vlc,unsigned * offset,const uint8_t * lens,const uint8_t * syms)101 static av_cold void mss4_init_vlc(VLC *vlc, unsigned *offset,
102                                   const uint8_t *lens, const uint8_t *syms)
103 {
104     static VLCElem vlc_buf[2146];
105     uint8_t  bits[MAX_ENTRIES];
106     int i, j;
107     int idx = 0;
108 
109     for (i = 0; i < 16; i++) {
110         for (j = 0; j < lens[i]; j++) {
111             bits[idx]  = i + 1;
112             idx++;
113         }
114     }
115 
116     vlc->table           = &vlc_buf[*offset];
117     vlc->table_allocated = FF_ARRAY_ELEMS(vlc_buf) - *offset;
118     ff_init_vlc_from_lengths(vlc, FFMIN(bits[idx - 1], 9), idx,
119                              bits, 1, syms, 1, 1,
120                              0, INIT_VLC_STATIC_OVERLONG, NULL);
121     *offset += vlc->table_size;
122 }
123 
mss4_init_vlcs(void)124 static av_cold void mss4_init_vlcs(void)
125 {
126     for (unsigned i = 0, offset = 0; i < 2; i++) {
127         mss4_init_vlc(&dc_vlc[i], &offset, mss4_dc_vlc_lens[i], NULL);
128         mss4_init_vlc(&ac_vlc[i], &offset,
129                       i ? ff_mjpeg_bits_ac_chrominance + 1
130                         : ff_mjpeg_bits_ac_luminance   + 1,
131                       i ? ff_mjpeg_val_ac_chrominance
132                         : ff_mjpeg_val_ac_luminance);
133         mss4_init_vlc(&vec_entry_vlc[i], &offset, mss4_vec_entry_vlc_lens[i],
134                       mss4_vec_entry_vlc_syms[i]);
135     }
136 }
137 
138 /* This function returns values in the range
139  * (-range + 1; -range/2] U [range/2; range - 1)
140  * i.e.
141  * nbits = 0 -> 0
142  * nbits = 1 -> -1, 1
143  * nbits = 2 -> -3, -2, 2, 3
144  */
get_coeff_bits(GetBitContext * gb,int nbits)145 static av_always_inline int get_coeff_bits(GetBitContext *gb, int nbits)
146 {
147     int val;
148 
149     if (!nbits)
150         return 0;
151 
152     val = get_bits(gb, nbits);
153     if (val < (1 << (nbits - 1)))
154         val -= (1 << nbits) - 1;
155 
156     return val;
157 }
158 
get_coeff(GetBitContext * gb,VLC * vlc)159 static inline int get_coeff(GetBitContext *gb, VLC *vlc)
160 {
161     int val = get_vlc2(gb, vlc->table, vlc->bits, 2);
162 
163     return get_coeff_bits(gb, val);
164 }
165 
mss4_decode_dct(GetBitContext * gb,VLC * dc_vlc,VLC * ac_vlc,int * block,int * dc_cache,int bx,int by,uint16_t * quant_mat)166 static int mss4_decode_dct(GetBitContext *gb, VLC *dc_vlc, VLC *ac_vlc,
167                            int *block, int *dc_cache,
168                            int bx, int by, uint16_t *quant_mat)
169 {
170     int skip, val, pos = 1, zz_pos, dc;
171 
172     memset(block, 0, sizeof(*block) * 64);
173 
174     dc = get_coeff(gb, dc_vlc);
175     // DC prediction is the same as in MSS3
176     if (by) {
177         if (bx) {
178             int l, tl, t;
179 
180             l  = dc_cache[LEFT];
181             tl = dc_cache[TOP_LEFT];
182             t  = dc_cache[TOP];
183 
184             if (FFABS(t - tl) <= FFABS(l - tl))
185                 dc += l;
186             else
187                 dc += t;
188         } else {
189             dc += dc_cache[TOP];
190         }
191     } else if (bx) {
192         dc += dc_cache[LEFT];
193     }
194     dc_cache[LEFT] = dc;
195     block[0]       = dc * quant_mat[0];
196 
197     while (pos < 64) {
198         val = get_vlc2(gb, ac_vlc->table, 9, 2);
199         if (!val)
200             return 0;
201         if (val == -1)
202             return -1;
203         if (val == 0xF0) {
204             pos += 16;
205             continue;
206         }
207         skip = val >> 4;
208         val  = get_coeff_bits(gb, val & 0xF);
209         pos += skip;
210         if (pos >= 64)
211             return -1;
212 
213         zz_pos = ff_zigzag_direct[pos];
214         block[zz_pos] = val * quant_mat[zz_pos];
215         pos++;
216     }
217 
218     return pos == 64 ? 0 : -1;
219 }
220 
mss4_decode_dct_block(MSS4Context * c,GetBitContext * gb,uint8_t * dst[3],int mb_x,int mb_y)221 static int mss4_decode_dct_block(MSS4Context *c, GetBitContext *gb,
222                                  uint8_t *dst[3], int mb_x, int mb_y)
223 {
224     int i, j, k, ret;
225     uint8_t *out = dst[0];
226 
227     for (j = 0; j < 2; j++) {
228         for (i = 0; i < 2; i++) {
229             int xpos = mb_x * 2 + i;
230             c->dc_cache[j][TOP_LEFT] = c->dc_cache[j][TOP];
231             c->dc_cache[j][TOP]      = c->prev_dc[0][mb_x * 2 + i];
232             ret = mss4_decode_dct(gb, &dc_vlc[0], &ac_vlc[0], c->block,
233                                   c->dc_cache[j],
234                                   xpos, mb_y * 2 + j, c->quant_mat[0]);
235             if (ret)
236                 return ret;
237             c->prev_dc[0][mb_x * 2 + i] = c->dc_cache[j][LEFT];
238 
239             ff_mss34_dct_put(out + xpos * 8, c->pic->linesize[0],
240                              c->block);
241         }
242         out += 8 * c->pic->linesize[0];
243     }
244 
245     for (i = 1; i < 3; i++) {
246         c->dc_cache[i + 1][TOP_LEFT] = c->dc_cache[i + 1][TOP];
247         c->dc_cache[i + 1][TOP]      = c->prev_dc[i][mb_x];
248         ret = mss4_decode_dct(gb, &dc_vlc[1], &ac_vlc[1],
249                               c->block, c->dc_cache[i + 1], mb_x, mb_y,
250                               c->quant_mat[1]);
251         if (ret)
252             return ret;
253         c->prev_dc[i][mb_x] = c->dc_cache[i + 1][LEFT];
254 
255         ff_mss34_dct_put(c->imgbuf[i], 8, c->block);
256         out = dst[i] + mb_x * 16;
257         // Since the DCT block is coded as YUV420 and the whole frame as YUV444,
258         // we need to scale chroma.
259         for (j = 0; j < 16; j++) {
260             for (k = 0; k < 8; k++)
261                 AV_WN16A(out + k * 2, c->imgbuf[i][k + (j & ~1) * 4] * 0x101);
262             out += c->pic->linesize[i];
263         }
264     }
265 
266     return 0;
267 }
268 
read_vec_pos(GetBitContext * gb,int * vec_pos,int * sel_flag,int * sel_len,int * prev)269 static void read_vec_pos(GetBitContext *gb, int *vec_pos, int *sel_flag,
270                          int *sel_len, int *prev)
271 {
272     int i, y_flag = 0;
273 
274     for (i = 2; i >= 0; i--) {
275         if (!sel_flag[i]) {
276             vec_pos[i] = 0;
277             continue;
278         }
279         if ((!i && !y_flag) || get_bits1(gb)) {
280             if (sel_len[i] > 0) {
281                 int pval = prev[i];
282                 vec_pos[i] = get_bits(gb, sel_len[i]);
283                 if (vec_pos[i] >= pval)
284                     vec_pos[i]++;
285             } else {
286                 vec_pos[i] = !prev[i];
287             }
288             y_flag = 1;
289         } else {
290             vec_pos[i] = prev[i];
291         }
292     }
293 }
294 
get_value_cached(GetBitContext * gb,int vec_pos,uint8_t * vec,int vec_size,int component,int shift,int * prev)295 static int get_value_cached(GetBitContext *gb, int vec_pos, uint8_t *vec,
296                             int vec_size, int component, int shift, int *prev)
297 {
298     if (vec_pos < vec_size)
299         return vec[vec_pos];
300     if (!get_bits1(gb))
301         return prev[component];
302     prev[component] = get_bits(gb, 8 - shift) << shift;
303     return prev[component];
304 }
305 
306 #define MKVAL(vals)  ((vals)[0] | ((vals)[1] << 3) | ((vals)[2] << 6))
307 
308 /* Image mode - the hardest to comprehend MSS4 coding mode.
309  *
310  * In this mode all three 16x16 blocks are coded together with a method
311  * remotely similar to the methods employed in MSS1-MSS3.
312  * The idea is that every component has a vector of 1-4 most common symbols
313  * and an escape mode for reading new value from the bitstream. Decoding
314  * consists of retrieving pixel values from the vector or reading new ones
315  * from the bitstream; depending on flags read from the bitstream, these vector
316  * positions can be updated or reused from the state of the previous line
317  * or previous pixel.
318  */
mss4_decode_image_block(MSS4Context * ctx,GetBitContext * gb,uint8_t * picdst[3],int mb_x,int mb_y)319 static int mss4_decode_image_block(MSS4Context *ctx, GetBitContext *gb,
320                                    uint8_t *picdst[3], int mb_x, int mb_y)
321 {
322     uint8_t vec[3][4];
323     int     vec_len[3];
324     int     sel_len[3], sel_flag[3];
325     int     i, j, k, mode, split;
326     int     prev_vec1 = 0, prev_split = 0;
327     int     vals[3] = { 0 };
328     int     prev_pix[3] = { 0 };
329     int     prev_mode[16] = { 0 };
330     uint8_t *dst[3];
331 
332     const int val_shift = ctx->quality == 100 ? 0 : 2;
333 
334     for (i = 0; i < 3; i++)
335         dst[i] = ctx->imgbuf[i];
336 
337     for (i = 0; i < 3; i++) {
338         vec_len[i] = vec_len_syms[!!i][get_unary(gb, 0, 3)];
339         for (j = 0; j < vec_len[i]; j++) {
340             vec[i][j]  = get_coeff(gb, &vec_entry_vlc[!!i]);
341             vec[i][j] += ctx->prev_vec[i][j];
342             ctx->prev_vec[i][j] = vec[i][j];
343         }
344         sel_flag[i] = vec_len[i] > 1;
345         sel_len[i]  = vec_len[i] > 2 ? vec_len[i] - 2 : 0;
346     }
347 
348     for (j = 0; j < 16; j++) {
349         if (get_bits1(gb)) {
350             split = 0;
351             if (get_bits1(gb)) {
352                 prev_mode[0] = 0;
353                 vals[0] = vals[1] = vals[2] = 0;
354                 mode = 2;
355             } else {
356                 mode = get_bits1(gb);
357                 if (mode)
358                     split = get_bits(gb, 4);
359             }
360             for (i = 0; i < 16; i++) {
361                 if (mode <= 1) {
362                     vals[0] =  prev_mode[i]       & 7;
363                     vals[1] = (prev_mode[i] >> 3) & 7;
364                     vals[2] =  prev_mode[i] >> 6;
365                     if (mode == 1 && i == split) {
366                         read_vec_pos(gb, vals, sel_flag, sel_len, vals);
367                     }
368                 } else if (mode == 2) {
369                     if (get_bits1(gb))
370                         read_vec_pos(gb, vals, sel_flag, sel_len, vals);
371                 }
372                 for (k = 0; k < 3; k++)
373                     *dst[k]++ = get_value_cached(gb, vals[k], vec[k],
374                                                  vec_len[k], k,
375                                                  val_shift, prev_pix);
376                 prev_mode[i] = MKVAL(vals);
377             }
378         } else {
379             if (get_bits1(gb)) {
380                 split = get_bits(gb, 4);
381                 if (split >= prev_split)
382                     split++;
383                 prev_split = split;
384             } else {
385                 split = prev_split;
386             }
387             if (split) {
388                 vals[0] =  prev_mode[0]       & 7;
389                 vals[1] = (prev_mode[0] >> 3) & 7;
390                 vals[2] =  prev_mode[0] >> 6;
391                 for (i = 0; i < 3; i++) {
392                     for (k = 0; k < split; k++) {
393                         *dst[i]++ = get_value_cached(gb, vals[i], vec[i],
394                                                      vec_len[i], i, val_shift,
395                                                      prev_pix);
396                         prev_mode[k] = MKVAL(vals);
397                     }
398                 }
399             }
400 
401             if (split != 16) {
402                 vals[0] =  prev_vec1       & 7;
403                 vals[1] = (prev_vec1 >> 3) & 7;
404                 vals[2] =  prev_vec1 >> 6;
405                 if (get_bits1(gb)) {
406                     read_vec_pos(gb, vals, sel_flag, sel_len, vals);
407                     prev_vec1 = MKVAL(vals);
408                 }
409                 for (i = 0; i < 3; i++) {
410                     for (k = 0; k < 16 - split; k++) {
411                         *dst[i]++ = get_value_cached(gb, vals[i], vec[i],
412                                                      vec_len[i], i, val_shift,
413                                                      prev_pix);
414                         prev_mode[split + k] = MKVAL(vals);
415                     }
416                 }
417             }
418         }
419     }
420 
421     for (i = 0; i < 3; i++)
422         for (j = 0; j < 16; j++)
423             memcpy(picdst[i] + mb_x * 16 + j * ctx->pic->linesize[i],
424                    ctx->imgbuf[i] + j * 16, 16);
425 
426     return 0;
427 }
428 
mss4_update_dc_cache(MSS4Context * c,int mb_x)429 static inline void mss4_update_dc_cache(MSS4Context *c, int mb_x)
430 {
431     int i;
432 
433     c->dc_cache[0][TOP]  = c->prev_dc[0][mb_x * 2 + 1];
434     c->dc_cache[0][LEFT] = 0;
435     c->dc_cache[1][TOP]  = 0;
436     c->dc_cache[1][LEFT] = 0;
437 
438     for (i = 0; i < 2; i++)
439         c->prev_dc[0][mb_x * 2 + i] = 0;
440 
441     for (i = 1; i < 3; i++) {
442         c->dc_cache[i + 1][TOP]  = c->prev_dc[i][mb_x];
443         c->dc_cache[i + 1][LEFT] = 0;
444         c->prev_dc[i][mb_x]      = 0;
445     }
446 }
447 
mss4_decode_frame(AVCodecContext * avctx,AVFrame * rframe,int * got_frame,AVPacket * avpkt)448 static int mss4_decode_frame(AVCodecContext *avctx, AVFrame *rframe,
449                              int *got_frame, AVPacket *avpkt)
450 {
451     const uint8_t *buf = avpkt->data;
452     int buf_size = avpkt->size;
453     MSS4Context *c = avctx->priv_data;
454     GetBitContext gb;
455     GetByteContext bc;
456     uint8_t *dst[3];
457     int width, height, quality, frame_type;
458     int x, y, i, mb_width, mb_height, blk_type;
459     int ret;
460 
461     if (buf_size < HEADER_SIZE) {
462         av_log(avctx, AV_LOG_ERROR,
463                "Frame should have at least %d bytes, got %d instead\n",
464                HEADER_SIZE, buf_size);
465         return AVERROR_INVALIDDATA;
466     }
467 
468     bytestream2_init(&bc, buf, buf_size);
469     width      = bytestream2_get_be16(&bc);
470     height     = bytestream2_get_be16(&bc);
471     bytestream2_skip(&bc, 2);
472     quality    = bytestream2_get_byte(&bc);
473     frame_type = bytestream2_get_byte(&bc);
474 
475     if (width > avctx->width ||
476         height != avctx->height) {
477         av_log(avctx, AV_LOG_ERROR, "Invalid frame dimensions %dx%d\n",
478                width, height);
479         return AVERROR_INVALIDDATA;
480     }
481     if (av_image_check_size2(width, height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0)
482         return AVERROR_INVALIDDATA;
483 
484     if (quality < 1 || quality > 100) {
485         av_log(avctx, AV_LOG_ERROR, "Invalid quality setting %d\n", quality);
486         return AVERROR_INVALIDDATA;
487     }
488     if ((frame_type & ~3) || frame_type == 3) {
489         av_log(avctx, AV_LOG_ERROR, "Invalid frame type %d\n", frame_type);
490         return AVERROR_INVALIDDATA;
491     }
492 
493     if (frame_type != SKIP_FRAME && !bytestream2_get_bytes_left(&bc)) {
494         av_log(avctx, AV_LOG_ERROR,
495                "Empty frame found but it is not a skip frame.\n");
496         return AVERROR_INVALIDDATA;
497     }
498     mb_width  = FFALIGN(width,  16) >> 4;
499     mb_height = FFALIGN(height, 16) >> 4;
500 
501     if (frame_type != SKIP_FRAME && 8*buf_size < 8*HEADER_SIZE + mb_width*mb_height)
502         return AVERROR_INVALIDDATA;
503 
504     if ((ret = ff_reget_buffer(avctx, c->pic, 0)) < 0)
505         return ret;
506     c->pic->key_frame = (frame_type == INTRA_FRAME);
507     c->pic->pict_type = (frame_type == INTRA_FRAME) ? AV_PICTURE_TYPE_I
508                                                    : AV_PICTURE_TYPE_P;
509     if (frame_type == SKIP_FRAME) {
510         *got_frame      = 1;
511         if ((ret = av_frame_ref(rframe, c->pic)) < 0)
512             return ret;
513 
514         return buf_size;
515     }
516 
517     if (c->quality != quality) {
518         c->quality = quality;
519         for (i = 0; i < 2; i++)
520             ff_mss34_gen_quant_mat(c->quant_mat[i], quality, !i);
521     }
522 
523     if ((ret = init_get_bits8(&gb, buf + HEADER_SIZE, buf_size - HEADER_SIZE)) < 0)
524         return ret;
525     dst[0] = c->pic->data[0];
526     dst[1] = c->pic->data[1];
527     dst[2] = c->pic->data[2];
528 
529     memset(c->prev_vec, 0, sizeof(c->prev_vec));
530     for (y = 0; y < mb_height; y++) {
531         memset(c->dc_cache, 0, sizeof(c->dc_cache));
532         for (x = 0; x < mb_width; x++) {
533             blk_type = decode012(&gb);
534             switch (blk_type) {
535             case DCT_BLOCK:
536                 if (mss4_decode_dct_block(c, &gb, dst, x, y) < 0) {
537                     av_log(avctx, AV_LOG_ERROR,
538                            "Error decoding DCT block %d,%d\n",
539                            x, y);
540                     return AVERROR_INVALIDDATA;
541                 }
542                 break;
543             case IMAGE_BLOCK:
544                 if (mss4_decode_image_block(c, &gb, dst, x, y) < 0) {
545                     av_log(avctx, AV_LOG_ERROR,
546                            "Error decoding VQ block %d,%d\n",
547                            x, y);
548                     return AVERROR_INVALIDDATA;
549                 }
550                 break;
551             case SKIP_BLOCK:
552                 if (frame_type == INTRA_FRAME) {
553                     av_log(avctx, AV_LOG_ERROR, "Skip block in intra frame\n");
554                     return AVERROR_INVALIDDATA;
555                 }
556                 break;
557             }
558             if (blk_type != DCT_BLOCK)
559                 mss4_update_dc_cache(c, x);
560         }
561         dst[0] += c->pic->linesize[0] * 16;
562         dst[1] += c->pic->linesize[1] * 16;
563         dst[2] += c->pic->linesize[2] * 16;
564     }
565 
566     if ((ret = av_frame_ref(rframe, c->pic)) < 0)
567         return ret;
568 
569     *got_frame      = 1;
570 
571     return buf_size;
572 }
573 
mss4_decode_end(AVCodecContext * avctx)574 static av_cold int mss4_decode_end(AVCodecContext *avctx)
575 {
576     MSS4Context * const c = avctx->priv_data;
577     int i;
578 
579     av_frame_free(&c->pic);
580     for (i = 0; i < 3; i++)
581         av_freep(&c->prev_dc[i]);
582 
583     return 0;
584 }
585 
mss4_decode_init(AVCodecContext * avctx)586 static av_cold int mss4_decode_init(AVCodecContext *avctx)
587 {
588     static AVOnce init_static_once = AV_ONCE_INIT;
589     MSS4Context * const c = avctx->priv_data;
590     int i;
591 
592     for (i = 0; i < 3; i++) {
593         c->dc_stride[i] = FFALIGN(avctx->width, 16) >> (2 + !!i);
594         c->prev_dc[i]   = av_malloc_array(c->dc_stride[i], sizeof(**c->prev_dc));
595         if (!c->prev_dc[i]) {
596             av_log(avctx, AV_LOG_ERROR, "Cannot allocate buffer\n");
597             return AVERROR(ENOMEM);
598         }
599     }
600 
601     c->pic = av_frame_alloc();
602     if (!c->pic)
603         return AVERROR(ENOMEM);
604 
605     avctx->pix_fmt     = AV_PIX_FMT_YUV444P;
606 
607     ff_thread_once(&init_static_once, mss4_init_vlcs);
608 
609     return 0;
610 }
611 
612 const FFCodec ff_mts2_decoder = {
613     .p.name         = "mts2",
614     .p.long_name    = NULL_IF_CONFIG_SMALL("MS Expression Encoder Screen"),
615     .p.type         = AVMEDIA_TYPE_VIDEO,
616     .p.id           = AV_CODEC_ID_MTS2,
617     .priv_data_size = sizeof(MSS4Context),
618     .init           = mss4_decode_init,
619     .close          = mss4_decode_end,
620     FF_CODEC_DECODE_CB(mss4_decode_frame),
621     .p.capabilities = AV_CODEC_CAP_DR1,
622     .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP | FF_CODEC_CAP_INIT_THREADSAFE,
623 };
624