diff --git a/libavcodec/jpeg2000.h b/libavcodec/jpeg2000.h
index 4bdc38df7c..93221d90ca 100644
--- a/libavcodec/jpeg2000.h
+++ b/libavcodec/jpeg2000.h
@@ -200,6 +200,8 @@ typedef struct Jpeg2000Cblk {
     /* specific to HT code-blocks */
     int zbp;
     int pass_lengths[2];
+    uint8_t modes; // copy of SPcod/SPcoc field to parse HT-MIXED mode
+    uint8_t ht_plhd; // are we looking for HT placeholder passes?
 } Jpeg2000Cblk; // code block
 
 typedef struct Jpeg2000Prec {
diff --git a/libavcodec/jpeg2000dec.c b/libavcodec/jpeg2000dec.c
index a1f82389fa..00ff9bea6f 100644
--- a/libavcodec/jpeg2000dec.c
+++ b/libavcodec/jpeg2000dec.c
@@ -54,6 +54,15 @@
 #define HAD_COC 0x01
 #define HAD_QCC 0x02
 
+// Values of flag for placeholder passes
+enum HT_PLHD_STATUS {
+    HT_PLHD_OFF,
+    HT_PLHD_ON
+};
+
+#define HT_MIXED 0x80 // bit 7 of SPcod/SPcoc
+
+
 /* get_bits functions for JPEG2000 packet bitstream
  * It is a get_bit function with a bit-stuffing routine. If the value of the
  * byte is 0xFF, the next byte includes an extra zero bit stuffed into the MSB.
@@ -1160,100 +1169,293 @@ static int jpeg2000_decode_packet(Jpeg2000DecoderContext *s, Jpeg2000Tile *tile,
             int incl, newpasses, llen;
             void *tmp;
 
-            if (cblk->npasses)
-                incl = get_bits(s, 1);
-            else
+            if (!cblk->incl) {
+                incl = 0;
+                cblk->modes = codsty->cblk_style;
+                if (cblk->modes >= JPEG2000_CTSY_HTJ2K_F)
+                    cblk->ht_plhd = HT_PLHD_ON;
+                if (layno > 0)
+                    incl = tag_tree_decode(s, prec->cblkincl + cblkno, 0 + 1) == 0;
                 incl = tag_tree_decode(s, prec->cblkincl + cblkno, layno + 1) == layno;
-            if (!incl)
-                continue;
-            else if (incl < 0)
-                return incl;
-
-            if (!cblk->npasses) {
-                int zbp = tag_tree_decode(s, prec->zerobits + cblkno, 100);
-                int v = expn[bandno] + numgbits - 1 - zbp;
 
-                if (v < 0 || v > 30) {
-                    av_log(s->avctx, AV_LOG_ERROR,
-                           "nonzerobits %d invalid or unsupported\n", v);
-                    return AVERROR_INVALIDDATA;
+                if (incl) {
+                    int zbp = tag_tree_decode(s, prec->zerobits + cblkno, 100);
+                    int v = expn[bandno] + numgbits - 1 - (zbp - tile->comp->roi_shift);
+                    if (v < 0 || v > 30) {
+                        av_log(s->avctx, AV_LOG_ERROR,
+                               "nonzerobits %d invalid or unsupported\n", v);
+                        return AVERROR_INVALIDDATA;
+                    }
+                    cblk->incl = 1;
+                    cblk->nonzerobits = v;
+                    cblk->zbp = zbp;
+                    cblk->lblock = 3;
                 }
-                cblk->zbp = zbp;
-                cblk->nonzerobits = v;
-            }
-            if ((newpasses = getnpasses(s)) < 0)
-                return newpasses;
-            av_assert2(newpasses > 0);
-            if (cblk->npasses + newpasses >= JPEG2000_MAX_PASSES) {
-                avpriv_request_sample(s->avctx, "Too many passes");
-                return AVERROR_PATCHWELCOME;
-            }
-            if ((llen = getlblockinc(s)) < 0)
-                return llen;
-            if (cblk->lblock + llen + av_log2(newpasses) > 16) {
-                avpriv_request_sample(s->avctx,
-                                      "Block with length beyond 16 bits");
-                return AVERROR_PATCHWELCOME;
+            } else {
+                incl = get_bits(s, 1);
             }
 
-            cblk->lblock += llen;
-
-            cblk->nb_lengthinc = 0;
-            cblk->nb_terminationsinc = 0;
-            av_free(cblk->lengthinc);
-            cblk->lengthinc = av_calloc(newpasses, sizeof(*cblk->lengthinc));
-            if (!cblk->lengthinc)
-                return AVERROR(ENOMEM);
-            tmp = av_realloc_array(cblk->data_start, cblk->nb_terminations + newpasses + 1, sizeof(*cblk->data_start));
-            if (!tmp)
-                return AVERROR(ENOMEM);
-            cblk->data_start = tmp;
-            do {
-                int newpasses1 = 0;
+            if (incl) {
+                uint8_t bypass_term_threshold = 0;
+                uint8_t bits_to_read = 0;
+                uint32_t segment_bytes = 0;
+                int32_t segment_passes = 0;
+                uint8_t next_segment_passes = 0;
+                int32_t href_passes, pass_bound;
+                uint32_t tmp_length = 0;
+                int32_t newpasses_copy, npasses_copy;
+
+                if ((newpasses = getnpasses(s)) <= 0)
+                    return newpasses;
+                if (cblk->npasses + newpasses >= JPEG2000_MAX_PASSES) {
+                    avpriv_request_sample(s->avctx, "Too many passes");
+                    return AVERROR_PATCHWELCOME;
+                }
+                if ((llen = getlblockinc(s)) < 0)
+                    return llen;
+                if (cblk->lblock + llen + av_log2(newpasses) > 16) {
+                    avpriv_request_sample(s->avctx,
+                                          "Block with length beyond 16 bits");
+                    return AVERROR_PATCHWELCOME;
+                }
+                cblk->nb_lengthinc = 0;
+                cblk->nb_terminationsinc = 0;
+                av_free(cblk->lengthinc);
+                cblk->lengthinc = av_calloc(newpasses, sizeof(*cblk->lengthinc));
+                if (!cblk->lengthinc)
+                    return AVERROR(ENOMEM);
+                tmp = av_realloc_array(cblk->data_start, cblk->nb_terminations + newpasses + 1,
+                                       sizeof(*cblk->data_start));
+                if (!tmp)
+                    return AVERROR(ENOMEM);
+                cblk->data_start = tmp;
+                cblk->lblock += llen;
+
+                // Count number of necessary terminations for non HT code block
+                newpasses_copy = newpasses;
+                npasses_copy = cblk->npasses;
+                if (!(cblk->modes & JPEG2000_CTSY_HTJ2K_F)) {
+                    do {
+                        int newpasses1 = 0;
+
+                        while (newpasses1 < newpasses_copy) {
+                            newpasses1++;
+                            if (needs_termination(codsty->cblk_style, npasses_copy + newpasses1 - 1)) {
+                                cblk->nb_terminationsinc++;
+                                break;
+                            }
+                        }
+                        npasses_copy += newpasses1;
+                        newpasses_copy -= newpasses1;
+                    } while (newpasses_copy);
+                }
 
-                while (newpasses1 < newpasses) {
-                    newpasses1 ++;
-                    if (needs_termination(codsty->cblk_style, cblk->npasses + newpasses1 - 1)) {
-                        cblk->nb_terminationsinc ++;
-                        break;
+                if (cblk->ht_plhd) {
+                    href_passes = (cblk->npasses + newpasses - 1) % 3;
+                    segment_passes = newpasses - href_passes;
+                    pass_bound = 2;
+                    bits_to_read = cblk->lblock;
+                    if (segment_passes < 1) {
+                        // No possible HT Cleanup pass here; may have placeholder passes
+                        // or an original J2K block bit-stream (in MIXED mode).
+                        segment_passes = newpasses;
+                        while (pass_bound <= segment_passes) {
+                            bits_to_read++;
+                            pass_bound += pass_bound;
+                        }
+                        segment_bytes = get_bits(s, bits_to_read);
+                        if (segment_bytes) {
+                            if (cblk->modes & HT_MIXED) {
+                                cblk->ht_plhd = HT_PLHD_OFF;
+                                cblk->modes &= (uint8_t) (~(JPEG2000_CTSY_HTJ2K_F));
+                            }
+                            else {
+                                av_log(s->avctx, AV_LOG_WARNING, "Length information for a HT-codeblock is invalid\n");
+                            }
+                        }
+                    } else {
+                        while (pass_bound <= segment_passes) {
+                            bits_to_read++;
+                            pass_bound += pass_bound;
+                        }
+                        segment_bytes = get_bits(s, bits_to_read);
+                        if (segment_bytes) {
+                            // No more placeholder passes
+                            if (!(cblk->modes & HT_MIXED)) {
+                                // Must be the first HT Cleanup pass
+                                if (segment_bytes < 2)
+                                    av_log(s->avctx, AV_LOG_WARNING, "Length information for a HT-codeblock is invalid\n");
+                                next_segment_passes = 2;
+                                cblk->ht_plhd = HT_PLHD_OFF;
+                                // Write length information for HT CleanUp segment
+                                cblk->pass_lengths[0] = segment_bytes;
+                            } else if (cblk->lblock > 3 && segment_bytes > 1
+                                       && (segment_bytes >> (bits_to_read - 1)) == 0) {
+                                // Must be the first HT Cleanup pass, since length MSB is 0
+                                next_segment_passes = 2;
+                                cblk->ht_plhd = HT_PLHD_OFF;
+                                // Write length information for HT CleanUp segment
+                                cblk->pass_lengths[0] = segment_bytes;
+                            } else {
+                                // Must have an original (non-HT) block coding pass
+                                cblk->modes &= (uint8_t) (~(JPEG2000_CTSY_HTJ2K_F));
+                                cblk->ht_plhd = HT_PLHD_OFF;
+                                segment_passes = newpasses;
+                                while (pass_bound <= segment_passes) {
+                                    bits_to_read++;
+                                    pass_bound += pass_bound;
+                                    segment_bytes <<= 1;
+                                    segment_bytes += get_bits(s, 1);
+                                }
+                            }
+                        } else {
+                            // Probably parsing placeholder passes, but we need to read an
+                            // extra length bit to verify this, since prior to the first
+                            // HT Cleanup pass, the number of length bits read for a
+                            // contributing code-block is dependent on the number of passes
+                            // being included, as if it were a non-HT code-block.
+                            segment_passes = newpasses;
+                            if (pass_bound <= segment_passes) {
+                                while (1) {
+                                    bits_to_read++;
+                                    pass_bound += pass_bound;
+                                    segment_bytes <<= 1;
+                                    segment_bytes += get_bits(s, 1);
+                                    if (pass_bound > segment_passes)
+                                        break;
+                                }
+                                if (segment_bytes) {
+                                    if (cblk->modes & HT_MIXED) {
+                                        cblk->modes &= (uint8_t) (~(JPEG2000_CTSY_HTJ2K_F));
+                                        cblk->ht_plhd = HT_PLHD_OFF;
+                                    } else {
+                                        av_log(s->avctx, AV_LOG_WARNING, "Length information for a HT-codeblock is invalid\n");
+                                    }
+                                }
+                            }
+                        }
+                    }
+                } else if (cblk->modes & JPEG2000_CTSY_HTJ2K_F) {
+                    // Quality layer commences with a non-initial HT coding pass
+                    if(bits_to_read != 0)
+                        av_log(s->avctx, AV_LOG_WARNING, "Length information for a HT-codeblock is invalid\n");
+                    segment_passes = cblk->npasses % 3;
+                    if (segment_passes == 0) {
+                        // newpasses is a HT Cleanup pass; next segment has refinement passes
+                        segment_passes = 1;
+                        next_segment_passes = 2;
+                        if (segment_bytes == 1)
+                            av_log(s->avctx, AV_LOG_WARNING, "Length information for a HT-codeblock is invalid\n");
+                    } else {
+                        // newpasses = 1 means npasses is HT SigProp; 2 means newpasses is
+                        // HT MagRef pass
+                        segment_passes = newpasses > 1 ? 3 - segment_passes : 1;
+                        next_segment_passes = 1;
+                        bits_to_read = av_log2(segment_passes);
+                    }
+                    bits_to_read = (uint8_t) (bits_to_read + cblk->lblock);
+                    segment_bytes = get_bits(s, bits_to_read);
+                    // Write length information for HT Refinment segment
+                    cblk->pass_lengths[1] += segment_bytes;
+                } else if (!(cblk->modes & (JPEG2000_CBLK_TERMALL | JPEG2000_CBLK_BYPASS))) {
+                    // Common case for non-HT code-blocks; we have only one segment
+                    bits_to_read = (uint8_t) cblk->lblock + av_log2((uint8_t) newpasses);
+                    segment_bytes = get_bits(s, bits_to_read);
+                    segment_passes = newpasses;
+                } else if (cblk->modes & JPEG2000_CBLK_TERMALL) {
+                    // RESTART MODE
+                    bits_to_read = cblk->lblock;
+                    segment_bytes = get_bits(s, bits_to_read);
+                    segment_passes = 1;
+                    next_segment_passes = 1;
+                } else {
+                    // BYPASS MODE
+                    bypass_term_threshold = 10;
+                    if(bits_to_read != 0)
+                        av_log(s->avctx, AV_LOG_WARNING, "Length information for a codeblock is invalid\n");
+                    if (cblk->npasses < bypass_term_threshold) {
+                        // May have from 1 to 10 uninterrupted passes before 1st RAW SigProp
+                        segment_passes = bypass_term_threshold - cblk->npasses;
+                        if (segment_passes > newpasses)
+                            segment_passes = newpasses;
+                        while ((2 << bits_to_read) <= segment_passes)
+                            bits_to_read++;
+                        next_segment_passes = 2;
+                    } else if ((cblk->npasses - bypass_term_threshold) % 3 < 2) {
+                        // 0 means newpasses is a RAW SigProp; 1 means newpasses is a RAW MagRef pass
+                        segment_passes = newpasses > 1 ? 2 - (cblk->npasses - bypass_term_threshold) % 3 : 1;
+                        bits_to_read = av_log2(segment_passes);
+                        next_segment_passes = 1;
+                    } else {
+                        // newpasses is an isolated Cleanup pass that precedes a RAW SigProp pass
+                        segment_passes = 1;
+                        next_segment_passes = 2;
                     }
+                    bits_to_read = (uint8_t) (bits_to_read + cblk->lblock);
+                    segment_bytes = get_bits(s, bits_to_read);
                 }
-
-                if (newpasses > 1 && (codsty->cblk_style & JPEG2000_CTSY_HTJ2K_F)) {
-                    // Retrieve pass lengths for each pass
-                    int href_passes =  (cblk->npasses + newpasses - 1) % 3;
-                    int eb = av_log2(newpasses - href_passes);
-                    int extra_bit = newpasses > 2 ? 1 : 0;
-                    if ((ret = get_bits(s, llen + eb + 3)) < 0)
-                        return ret;
-                    cblk->pass_lengths[0] = ret;
-                    if ((ret = get_bits(s, llen + 3 + extra_bit)) < 0)
-                        return ret;
-                    cblk->pass_lengths[1] = ret;
-                    ret = cblk->pass_lengths[0] + cblk->pass_lengths[1];
+                // Update cblk->npasses and write length information
+                cblk->npasses = (uint8_t) (cblk->npasses + segment_passes);
+                cblk->lengthinc[cblk->nb_lengthinc++] = segment_bytes;
+
+                if ((cblk->modes & JPEG2000_CTSY_HTJ2K_F) && cblk->ht_plhd == HT_PLHD_OFF) {
+                    newpasses -= (uint8_t) segment_passes;
+                    while (newpasses > 0) {
+                        segment_passes = newpasses > 1 ? next_segment_passes : 1;
+                        next_segment_passes = (uint8_t) (3 - next_segment_passes);
+                        bits_to_read = (uint8_t) (cblk->lblock + av_log2(segment_passes));
+                        segment_bytes = get_bits(s, bits_to_read);
+                        newpasses -= (uint8_t) (segment_passes);
+                        // This is a FAST Refinement pass
+                        // Write length information for HT Refinement segment
+                        cblk->pass_lengths[1] += segment_bytes;
+                        // Update cblk->npasses and write length information
+                        cblk->npasses = (uint8_t) (cblk->npasses + segment_passes);
+                        cblk->lengthinc[cblk->nb_lengthinc++] = segment_bytes;
+                    }
                 } else {
-                    if ((ret = get_bits(s, av_log2(newpasses1) + cblk->lblock)) < 0)
-                        return ret;
-                    cblk->pass_lengths[0] = ret;
+                    newpasses -= (uint8_t) (segment_passes);
+                    while (newpasses > 0) {
+                        if (bypass_term_threshold != 0) {
+                            segment_passes = newpasses > 1 ? next_segment_passes : 1;
+                            next_segment_passes = (uint8_t) (3 - next_segment_passes);
+                            bits_to_read = (uint8_t) (cblk->lblock + av_log2(segment_passes));
+                        } else {
+                            if ((cblk->modes & JPEG2000_CBLK_TERMALL) == 0)
+                                av_log(s->avctx, AV_LOG_WARNING, "Corrupted packet header is found.\n");
+                            segment_passes = 1;
+                            bits_to_read = cblk->lblock;
+                        }
+                        segment_bytes = get_bits(s, bits_to_read);
+                        newpasses -= (uint8_t) (segment_passes);
+
+                        // Update cblk->npasses and write length information
+                        cblk->npasses = (uint8_t) (cblk->npasses + segment_passes);
+                        cblk->lengthinc[cblk->nb_lengthinc++] = segment_bytes;
+                    }
                 }
-                if (ret > cblk->data_allocated) {
-                    size_t new_size = FFMAX(2*cblk->data_allocated, ret);
+
+                for (int i = 0; i < cblk->nb_lengthinc; ++i)
+                    tmp_length = (tmp_length < cblk->lengthinc[i]) ? cblk->lengthinc[i] : tmp_length;
+
+                if (tmp_length > cblk->data_allocated) {
+                    size_t new_size = FFMAX(2 * cblk->data_allocated, tmp_length);
                     void *new = av_realloc(cblk->data, new_size);
                     if (new) {
                         cblk->data = new;
                         cblk->data_allocated = new_size;
                     }
                 }
-                if (ret > cblk->data_allocated) {
+                if (tmp_length > cblk->data_allocated) {
                     avpriv_request_sample(s->avctx,
                                         "Block with lengthinc greater than %"SIZE_SPECIFIER"",
                                         cblk->data_allocated);
                     return AVERROR_PATCHWELCOME;
                 }
-                cblk->lengthinc[cblk->nb_lengthinc++] = ret;
-                cblk->npasses  += newpasses1;
-                newpasses -= newpasses1;
-            } while(newpasses);
+            } else {
+                // This codeblock has no contribution to the current packet
+                continue;
+            }
         }
     }
     jpeg2000_flush(s);
@@ -1788,7 +1990,7 @@ static int decode_cblk(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *cod
                        Jpeg2000T1Context *t1, Jpeg2000Cblk *cblk,
                        int width, int height, int bandpos, uint8_t roi_shift)
 {
-    int passno = cblk->npasses, pass_t = 2, bpno = cblk->nonzerobits - 1 + roi_shift;
+    int passno = cblk->npasses, pass_t = 2, bpno = cblk->nonzerobits - 1;
     int pass_cnt = 0;
     int vert_causal_ctx_csty_symbol = codsty->cblk_style & JPEG2000_CBLK_VSC;
     int term_cnt = 0;
@@ -2028,7 +2230,7 @@ static inline int tile_codeblocks(const Jpeg2000DecoderContext *s, Jpeg2000Tile
 
                         Jpeg2000Cblk *cblk = prec->cblk + cblkno;
 
-                        if (codsty->cblk_style & JPEG2000_CTSY_HTJ2K_F)
+                        if (cblk->modes & JPEG2000_CTSY_HTJ2K_F)
                             ret = ff_jpeg2000_decode_htj2k(s, codsty, &t1, cblk,
                                                            cblk->coord[0][1] - cblk->coord[0][0],
                                                            cblk->coord[1][1] - cblk->coord[1][0],
diff --git a/libavcodec/jpeg2000htdec.c b/libavcodec/jpeg2000htdec.c
index eba0936089..9b473e11d3 100644
--- a/libavcodec/jpeg2000htdec.c
+++ b/libavcodec/jpeg2000htdec.c
@@ -1032,25 +1032,31 @@ static void jpeg2000_calc_mbr(uint8_t *mbr, const uint16_t i, const uint16_t j,
 static void jpeg2000_process_stripes_block(StateVars *sig_prop, int i_s, int j_s,
                                            int width, int height, int stride, int pLSB,
                                            int32_t *sample_buf, uint8_t *block_states,
-                                           uint8_t *magref_segment, uint32_t magref_length)
+                                           uint8_t *magref_segment, uint32_t magref_length,
+                                           uint8_t is_causal)
 {
     for (int j = j_s; j < j_s + width; j++) {
         uint32_t  mbr_info = 0;
         for (int i = i_s; i < i_s + height; i++) {
-            int modify_state, cond;
+            int modify_state;
             uint8_t bit;
-            uint8_t causal_cond = i != (i_s + height - 1);
+            uint8_t causal_cond = (is_causal == 0) || (i != (i_s + height - 1));
             int32_t *sp = &sample_buf[j + (i * (stride))];
             uint8_t mbr = 0;
 
-            if (jpeg2000_get_state(i, j, stride - 2, HT_SHIFT_SIGMA, block_states) == 0)
+            if (jpeg2000_get_state(i, j, stride, HT_SHIFT_SIGMA, block_states) == 0)
                 jpeg2000_calc_mbr(&mbr, i, j, mbr_info & 0x1EF, causal_cond, block_states, stride);
             mbr_info >>= 3;
-            cond = mbr != 0;
-            bit = jpeg2000_peek_bit(sig_prop, magref_segment, magref_length);
-            *sp |= (bit * cond) << pLSB;
-            sig_prop->bits -= cond;
-            modify_state = (((1 << HT_SHIFT_REF_IND) | (1 << HT_SHIFT_REF)) * cond) | 1 << HT_SHIFT_SCAN;
+
+            modify_state = block_states[(i + 1) * stride + (j + 1)];
+            modify_state |= 1 << HT_SHIFT_SCAN;
+            if (mbr != 0) {
+                modify_state |= 1 << HT_SHIFT_REF_IND; 
+                bit = jpeg2000_peek_bit(sig_prop, magref_segment, magref_length);
+                modify_state |= bit << HT_SHIFT_REF; 
+                *sp |= bit << pLSB; 
+                *sp |= bit << (pLSB - 1); // Add 0.5 (reconstruction parameter = 1/2)
+            }
             jpeg2000_modify_state(i, j, stride, modify_state, block_states);
         }
     }
@@ -1074,6 +1080,7 @@ static void jpeg2000_decode_sigprop_segment(Jpeg2000Cblk *cblk, uint16_t width,
 
     int last_width;
     uint16_t i = 0, j = 0;
+    uint8_t is_causal = cblk->modes & JPEG2000_CBLK_VSC;
 
     jpeg2000_init_zero(&sp_dec);
 
@@ -1082,14 +1089,14 @@ static void jpeg2000_decode_sigprop_segment(Jpeg2000Cblk *cblk, uint16_t width,
         for (int n2 = 0; n2 < num_h_stripe; n2++) {
             jpeg2000_process_stripes_block(&sp_dec, i, j, b_width, b_height, stride,
                                            pLSB, sample_buf, block_states, magref_segment,
-                                           magref_length);
+                                           magref_length, is_causal);
             j += 4;
         }
         last_width = width % 4;
         if (last_width)
             jpeg2000_process_stripes_block(&sp_dec, i, j, last_width, b_height, stride,
                                            pLSB, sample_buf, block_states, magref_segment,
-                                           magref_length);
+                                           magref_length, is_causal);
         i += 4;
     }
 
@@ -1099,20 +1106,20 @@ static void jpeg2000_decode_sigprop_segment(Jpeg2000Cblk *cblk, uint16_t width,
     for (int n2 = 0; n2 < num_h_stripe; n2++) {
         jpeg2000_process_stripes_block(&sp_dec, i, j, b_width, b_height, stride,
                                        pLSB, sample_buf, block_states, magref_segment,
-                                       magref_length);
+                                       magref_length, is_causal);
         j += 4;
     }
     last_width = width % 4;
     if (last_width)
         jpeg2000_process_stripes_block(&sp_dec, i, j, last_width, b_height, stride,
                                        pLSB, sample_buf, block_states, magref_segment,
-                                       magref_length);
+                                       magref_length, is_causal);
 }
 
 /**
  * See procedure decodeSigPropMag at Rec. ITU-T T.814, 7.5.
 */
-static int
+static void
 jpeg2000_decode_magref_segment( uint16_t width, uint16_t block_height, const int stride,
                                 uint8_t *magref_segment,uint32_t magref_length,
                                 uint8_t pLSB, int32_t *sample_buf, uint8_t *block_states)
@@ -1152,15 +1159,14 @@ jpeg2000_decode_magref_segment( uint16_t width, uint16_t block_height, const int
             }
         }
     }
-    return 1;
 }
 
 
 int
 ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *codsty, Jpeg2000T1Context *t1, Jpeg2000Cblk *cblk,
-                         int width, int height, int magp, uint8_t roi_shift)
+                         int width, int height, int M_b, uint8_t roi_shift)
 {
-    uint8_t p0 = 0;             // Number of placeholder passes
+    uint8_t p0 = 0;             // 3 * p0 = Number of placeholder passes
     uint32_t Lcup;              // Length of HT cleanup segment
     uint32_t Lref;              // Length of Refinement segment
     uint32_t Scup;              // HT cleanup segment suffix length
@@ -1174,7 +1180,7 @@ ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
 
     int z_blk;                  // Number of ht coding pass
 
-    uint8_t empty_passes;
+    uint8_t num_plhd_passes;    // Number of placeholder passes
 
     StateVars mag_sgn;          // Magnitude and Sign
     StateVars mel;              // Adaptive run-length coding
@@ -1190,8 +1196,9 @@ ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
     uint8_t *block_states = NULL;
 
     int32_t n, val;             // Post-processing
-
-    int32_t M_b = magp;
+    const uint32_t mask  = UINT32_MAX >> (M_b + 1); // bit mask for ROI detection
+    
+    uint8_t num_rempass;
 
     const int quad_buf_width = width + 4;
     const int quad_buf_height = height + 4;
@@ -1201,22 +1208,17 @@ ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
     av_assert0(width * height <= 4096);
     av_assert0(width * height > 0);
 
-    if (roi_shift)
-        avpriv_report_missing_feature(s->avctx, "ROI shift");
-
     memset(t1->data, 0, t1->stride * height * sizeof(*t1->data));
     memset(t1->flags, 0, t1->stride * (height + 2) * sizeof(*t1->flags));
 
     if (cblk->npasses == 0)
         return 0;
 
-    if (cblk->npasses > 3)
-        p0 = 0;
-    else if (cblk->length == 0)
-        p0 = 1;
-
-    empty_passes = p0 * 3;
-    z_blk = cblk->npasses - empty_passes;
+    num_rempass = cblk->npasses % 3;  // Number of remainder passes
+    num_plhd_passes = num_rempass ? cblk->npasses - num_rempass : cblk->npasses - 3;
+    av_assert0(num_plhd_passes % 3 == 0);
+    p0 = num_plhd_passes / 3;
+    z_blk = cblk->npasses - num_plhd_passes;
 
     if (z_blk <= 0)
         return 0; // No passes within this set, continue
@@ -1232,6 +1234,7 @@ ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
     Dcup = cblk->data;
     Dref  = cblk->data + Lcup; // Dref comes after the refinement segment
     S_blk = p0 + cblk->zbp;
+    cblk->zbp = S_blk - 1;
     pLSB  = 30 - S_blk;
 
     Scup = (Dcup[Lcup - 1] << 4) + (Dcup[Lcup - 2] & 0x0F);
@@ -1277,31 +1280,32 @@ ff_jpeg2000_decode_htj2k(const Jpeg2000DecoderContext *s, Jpeg2000CodingStyle *c
         goto free;
     }
 
-    if (cblk->npasses > 1)
+    if (z_blk > 1)
         jpeg2000_decode_sigprop_segment(cblk, width, height, quad_buf_width, Dref, Lref,
                                         pLSB - 1, sample_buf, block_states);
 
-    if (cblk->npasses > 2) {
-
-        if (Lref < 2){
-            av_log(s->avctx,AV_LOG_ERROR,"Invalid magnitude refinement length\n");
-            ret = AVERROR_INVALIDDATA;
-            goto free;
-        }
-        if ((ret = jpeg2000_decode_magref_segment(width, height, quad_buf_width, Dref, Lref,
-                                                  pLSB - 1, sample_buf, block_states)) < 0)
-            goto free;
-    }
+    if (z_blk > 2)
+        jpeg2000_decode_magref_segment(width, height, quad_buf_width, Dref, Lref,
+                                       pLSB - 1, sample_buf, block_states);
 
     pLSB = 31 - M_b;
 
     /* Reconstruct the sample values */
     for (int y = 0; y < height; y++) {
         for (int x = 0; x < width; x++) {
+            int32_t sign;
+
             n = x + (y * t1->stride);
             val = sample_buf[x + (y * quad_buf_width)];
+            sign = val & INT32_MIN;
+            val &= INT32_MAX;
+            /* ROI shift, if necessary */
+            if (roi_shift && (((uint32_t)val & ~mask) == 0))
+                val <<= roi_shift;
             /* Convert sign-magnitude to two's complement. */
-            val = val >> 31 ? 0x80000000 - val : val;
+            if (sign)
+                val = -val;
+            /* Shift down to 1 bit upper from decimal point for reconstruction value (= 0.5) */
             val >>= (pLSB - 1);
             t1->data[n] = val;
         }
