[FFmpeg-devel] [PATCH v4] libavcodec/flacenc: add backward-compatible 32 bit-per-sample capability

[Martijn van Beurden <mvanb1@gmail.com>]

Enables creation of FLAC files with up to 32 bits-per-sample, up from the
previous limit of 24 bit. This is a feature requested for RAWcooked, the
archiving community has a need for storing files with 32-bit integer audio
samples. See https://github.com/MediaArea/RAWcooked/issues/356

Restrictions to the encoder are added so created files are compatible with
existing decoders. Stereo decorrelation is disabled on 32 bit-per-sample,
because a side channel would need 33 bit-per-sample, causing problems in
existing 32 bit datapaths. Also only LPC encoding is enabled, because
decoders capable of processing 24-bit files already use 64-bit processing
for LPC, but not for fixed subframes.

Furthermore, predictions and residuals are checked for causing integer
overflow, reverting to a verbatim (store) subframe in case no LPC coeffs
can be found that do not cause overflow.

ffmpeg's FLAC decoder has been forward-compatible with this change since
commit c720b9ce98 (May 2015). libFLAC is forward-compatible since release
1.2.1 (September 2007), the flac command line tool however blocks 32-bit
files out of caution, it having been untested until now.

To create 32 bit files, -bits_per_raw_sample 32 must be specified because
of trac ticket 9563
---
 libavcodec/flacenc.c | 138 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 123 insertions(+), 15 deletions(-)

diff --git a/libavcodec/flacenc.c b/libavcodec/flacenc.c
index 9f6f449323..bec3bf2ca5 100644
--- a/libavcodec/flacenc.c
+++ b/libavcodec/flacenc.c
@@ -49,6 +49,9 @@
 #define MIN_LPC_SHIFT       0
 #define MAX_LPC_SHIFT      15
 
+#define ZIGZAG_32BIT_MAX  0x3FFFFFFF
+#define ZIGZAG_32BIT_MIN -0x3FFFFFFF
+
 enum CodingMode {
     CODING_MODE_RICE  = 4,
     CODING_MODE_RICE2 = 5,
@@ -254,10 +257,30 @@ static av_cold int flac_encode_init(AVCodecContext *avctx)
         s->bps_code                = 4;
         break;
     case AV_SAMPLE_FMT_S32:
-        if (avctx->bits_per_raw_sample != 24)
-            av_log(avctx, AV_LOG_WARNING, "encoding as 24 bits-per-sample\n");
-        avctx->bits_per_raw_sample = 24;
-        s->bps_code                = 6;
+        if (avctx->bits_per_raw_sample <= 24) {
+            if (avctx->bits_per_raw_sample < 24)
+                av_log(avctx, AV_LOG_WARNING, "encoding as 24 bits-per-sample\n");
+            avctx->bits_per_raw_sample = 24;
+            s->bps_code                = 6;
+        } else {
+            av_log(avctx, AV_LOG_WARNING, "non-streamable bits-per-sample\n");
+            s->bps_code = 0;
+            if (avctx->bits_per_raw_sample == 0)
+                avctx->bits_per_raw_sample = 32;
+            if (s->options.lpc_type != FF_LPC_TYPE_LEVINSON &&
+                s->options.lpc_type != FF_LPC_TYPE_CHOLESKY) {
+                av_log(avctx, AV_LOG_WARNING, "forcing lpc_type levinson, lpc_type fixed or none not supported with >24 bits-per-sample FLAC\n");
+                s->options.lpc_type = FF_LPC_TYPE_LEVINSON;
+            }
+            if (avctx->bits_per_raw_sample == 32) {
+                /* Because stereo decorrelation can raise the bitdepth of
+                 * a subframe to 33 bits, we disable it */
+                if (s->options.ch_mode != FLAC_CHMODE_INDEPENDENT) {
+                    av_log(avctx, AV_LOG_WARNING, "disabling stereo decorrelation, not supported with 32 bits-per-sample FLAC\n");
+                    s->options.ch_mode = FLAC_CHMODE_INDEPENDENT;
+                }
+            }
+        }
         break;
     }
 
@@ -686,7 +709,7 @@ static uint64_t calc_rice_params(RiceContext *rc,
 
     tmp_rc.coding_mode = rc->coding_mode;
 
-    for (i = 0; i < n; i++)
+    for (i = pred_order; i < n; i++)
         udata[i] = (2 * data[i]) ^ (data[i] >> 31);
 
     calc_sum_top(pmax, exact ? kmax : 0, udata, n, pred_order, sums);
@@ -784,6 +807,47 @@ static void encode_residual_fixed(int32_t *res, const int32_t *smp, int n,
     }
 }
 
+static int lpc_encode_with_overflow_detect(int32_t *res, const int32_t *smp, int len,
+                                               int order, int32_t *coefs, int shift)
+{
+    /* This function checks for every prediction and every residual
+     * whether they cause integer overflow in existing decoders. In
+     * case the prediction exceeds int32_t limits, prediction
+     * coefficients are lowered accordingly. If the residual exceeds
+     * ZIGZAG_32BIT_MAX and _MIN or coefficients have been lowered
+     * twice but the prediction still overflows, give up */
+    int lpc_reduction_tries = 0;
+    int64_t pmax;
+    for (int i = 0; i < order; i++)
+        res[i] = smp[i];
+    do {
+        pmax = 0;
+        for (int i = order; i < len; i++) {
+            int64_t p = 0, tmp;
+            for (int j = 0; j < order; j++)
+                p += (int64_t)coefs[j]*smp[(i-1)-j];
+            p >>= shift;
+            tmp = smp[i] - p;
+            if (p > INT32_MAX && p > pmax)
+                pmax = p;
+            else if (p < INT32_MIN && (p * -1) > pmax)
+                pmax = p * -1;
+            if (tmp > ZIGZAG_32BIT_MAX || tmp < ZIGZAG_32BIT_MIN)
+                return 1;
+            res[i] = tmp;
+        }
+
+        if (pmax > 0) {
+            if (lpc_reduction_tries >= 2)
+                return 1;
+            lpc_reduction_tries++;
+            for (int i = 0; i < order; i++)
+                coefs[i] = ((int64_t)coefs[i] * INT32_MAX) / pmax;
+        }
+    } while (pmax > 0);
+    return 0;
+}
+
 
 static int encode_residual_ch(FlacEncodeContext *s, int ch)
 {
@@ -868,7 +932,11 @@ static int encode_residual_ch(FlacEncodeContext *s, int ch)
             order = av_clip(order, min_order - 1, max_order - 1);
             if (order == last_order)
                 continue;
-            if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(order) <= 32) {
+            if (s->avctx->bits_per_raw_sample > 24) {
+                if (lpc_encode_with_overflow_detect(res, smp, n, order+1,
+                                                    coefs[order], shift[order]))
+                    continue;
+            } else if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(order) <= 32) {
                 s->flac_dsp.lpc16_encode(res, smp, n, order+1, coefs[order],
                                          shift[order]);
             } else {
@@ -888,7 +956,11 @@ static int encode_residual_ch(FlacEncodeContext *s, int ch)
         opt_order = 0;
         bits[0]   = UINT32_MAX;
         for (i = min_order-1; i < max_order; i++) {
-            if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(i) <= 32) {
+            if (s->avctx->bits_per_raw_sample > 24) {
+                if (lpc_encode_with_overflow_detect(res, smp, n, i+1,
+                                                    coefs[i], shift[i]))
+                    continue;
+            } else if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(i) <= 32) {
                 s->flac_dsp.lpc16_encode(res, smp, n, i+1, coefs[i], shift[i]);
             } else {
                 s->flac_dsp.lpc32_encode(res, smp, n, i+1, coefs[i], shift[i]);
@@ -910,7 +982,11 @@ static int encode_residual_ch(FlacEncodeContext *s, int ch)
             for (i = last-step; i <= last+step; i += step) {
                 if (i < min_order-1 || i >= max_order || bits[i] < UINT32_MAX)
                     continue;
-                if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(i) <= 32) {
+                if (s->avctx->bits_per_raw_sample > 24) {
+                    if (lpc_encode_with_overflow_detect(res, smp, n, i+1,
+                                                        coefs[i], shift[i]))
+                        continue;
+                } else if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(i) <= 32) {
                     s->flac_dsp.lpc32_encode(res, smp, n, i+1, coefs[i], shift[i]);
                 } else {
                     s->flac_dsp.lpc16_encode(res, smp, n, i+1, coefs[i], shift[i]);
@@ -951,7 +1027,11 @@ static int encode_residual_ch(FlacEncodeContext *s, int ch)
                 if (diffsum >8)
                     continue;
 
-                if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(opt_order - 1) <= 32) {
+                if (s->avctx->bits_per_raw_sample > 24) {
+                    if (lpc_encode_with_overflow_detect(res, smp, n, opt_order,
+                                                        lpc_try, shift[opt_order-1]))
+                        continue;
+                } else if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(opt_order-1) <= 32) {
                     s->flac_dsp.lpc16_encode(res, smp, n, opt_order, lpc_try, shift[opt_order-1]);
                 } else {
                     s->flac_dsp.lpc32_encode(res, smp, n, opt_order, lpc_try, shift[opt_order-1]);
@@ -972,7 +1052,16 @@ static int encode_residual_ch(FlacEncodeContext *s, int ch)
     for (i = 0; i < sub->order; i++)
         sub->coefs[i] = coefs[sub->order-1][i];
 
-    if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(opt_order) <= 32) {
+    if (s->avctx->bits_per_raw_sample > 24) {
+        if (lpc_encode_with_overflow_detect(res, smp, n, sub->order,
+                                            sub->coefs, sub->shift)) {
+            /* No coefs found that do not cause integer overflow,
+             * so return a verbatim subframe instead */
+            sub->type = sub->type_code = FLAC_SUBFRAME_VERBATIM;
+            memcpy(res, smp, n * sizeof(int32_t));
+            return subframe_count_exact(s, sub, 0);
+        }
+    } else if (s->bps_code * 4 + s->options.lpc_coeff_precision + av_log2(sub->order) <= 32) {
         s->flac_dsp.lpc16_encode(res, smp, n, sub->order, sub->coefs, sub->shift);
     } else {
         s->flac_dsp.lpc32_encode(res, smp, n, sub->order, sub->coefs, sub->shift);
@@ -1227,12 +1316,22 @@ static void write_subframes(FlacEncodeContext *s)
         if (sub->type == FLAC_SUBFRAME_CONSTANT) {
             put_sbits(&s->pb, sub->obits, res[0]);
         } else if (sub->type == FLAC_SUBFRAME_VERBATIM) {
-            while (res < frame_end)
-                put_sbits(&s->pb, sub->obits, *res++);
+            if (sub->obits < 32) {
+                while (res < frame_end)
+                    put_sbits(&s->pb, sub->obits, *res++);
+            } else {
+                while (res < frame_end)
+                    put_bits32(&s->pb, *res++);
+            }
         } else {
             /* warm-up samples */
-            for (i = 0; i < sub->order; i++)
-                put_sbits(&s->pb, sub->obits, *res++);
+            if (sub->obits < 32) {
+                for (i = 0; i < sub->order; i++)
+                    put_sbits(&s->pb, sub->obits, *res++);
+            } else {
+                for (i = 0; i < sub->order; i++)
+                    put_bits32(&s->pb, *res++);
+            }
 
             /* LPC coefficients */
             if (sub->type == FLAC_SUBFRAME_LPC) {
@@ -1305,7 +1404,7 @@ static int update_md5_sum(FlacEncodeContext *s, const void *samples)
                             (const uint16_t *) samples, buf_size / 2);
         buf = s->md5_buffer;
 #endif
-    } else {
+    } else if (s->avctx->bits_per_raw_sample <= 24) {
         int i;
         const int32_t *samples0 = samples;
         uint8_t *tmp            = s->md5_buffer;
@@ -1315,6 +1414,15 @@ static int update_md5_sum(FlacEncodeContext *s, const void *samples)
             AV_WL24(tmp + 3*i, v);
         }
         buf = s->md5_buffer;
+    } else {
+        /* s->avctx->bits_per_raw_sample <= 32 */
+        int i;
+        const int32_t *samples0 = samples;
+        uint8_t *tmp            = s->md5_buffer;
+
+        for (i = 0; i < s->frame.blocksize * s->channels; i++)
+            AV_WL32(tmp + 4*i, samples0[i]);
+        buf = s->md5_buffer;
     }
     av_md5_update(s->md5ctx, buf, buf_size);
 
-- 
2.30.2

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