[FFmpeg-devel] [PATCH 2/2] lavu/tx: add a half-complex R2C transform

[Lynne <dev@lynne.ee>]

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Required for DCT-I and DST-I.

Patch attached.


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From 2e95c3a7128b7f826db8394eb238c5405c47864b Mon Sep 17 00:00:00 2001
From: Lynne <dev@lynne.ee>
Date: Tue, 25 Jul 2023 10:57:21 +0200
Subject: [PATCH 2/2] lavu/tx: add a half-complex R2C transform

Required for DCT-I and DST-I.
---
 doc/APIchanges          |  3 ++
 libavutil/tx.c          | 13 +++++++--
 libavutil/tx.h          |  9 ++++++
 libavutil/tx_template.c | 65 +++++++++++++++++++++++++++++++++++++++--
 libavutil/version.h     |  2 +-
 5 files changed, 86 insertions(+), 6 deletions(-)

diff --git a/doc/APIchanges b/doc/APIchanges
index 5afe8bcb75..0797dcc372 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+2023-07-xx - xxxxxxxxxx - lavu 58.15.100 - tx.h
+  Add AV_TX_HALF_COMPLEX
+
 2023-07-xx - xxxxxxxxxx - lavc 60 - avcodec.h
   Deprecate AV_CODEC_FLAG_DROPCHANGED without replacement.
 
diff --git a/libavutil/tx.c b/libavutil/tx.c
index e25abf998f..d6ab8ee485 100644
--- a/libavutil/tx.c
+++ b/libavutil/tx.c
@@ -437,7 +437,8 @@ int ff_tx_decompose_length(int dst[TX_MAX_DECOMPOSITIONS], enum AVTXType type,
 
             /* Check direction for non-orthogonal codelets */
             if (((cd->flags & FF_TX_FORWARD_ONLY) && inv) ||
-                ((cd->flags & (FF_TX_INVERSE_ONLY | AV_TX_FULL_IMDCT)) && !inv))
+                ((cd->flags & (FF_TX_INVERSE_ONLY | AV_TX_FULL_IMDCT)) && !inv) ||
+                ((cd->flags & (FF_TX_FORWARD_ONLY | AV_TX_HALF_COMPLEX)) && inv))
                 continue;
 
             /* Check if the CPU supports the required ISA */
@@ -560,6 +561,8 @@ static void print_flags(AVBPrint *bp, uint64_t f)
         av_bprintf(bp, "%spreshuf", prev > 1 ? sep : "");
     if ((f & AV_TX_FULL_IMDCT) && ++prev)
         av_bprintf(bp, "%simdct_full", prev > 1 ? sep : "");
+    if ((f & AV_TX_HALF_COMPLEX) && ++prev)
+        av_bprintf(bp, "%shalf_complex", prev > 1 ? sep : "");
     if ((f & FF_TX_ASM_CALL) && ++prev)
         av_bprintf(bp, "%sasm_call", prev > 1 ? sep : "");
     av_bprintf(bp, "]");
@@ -717,7 +720,10 @@ av_cold int ff_tx_init_subtx(AVTXContext *s, enum AVTXType type,
     uint64_t req_flags = flags;
 
     /* Flags the codelet may require to be present */
-    uint64_t inv_req_mask = AV_TX_FULL_IMDCT | FF_TX_PRESHUFFLE | FF_TX_ASM_CALL;
+    uint64_t inv_req_mask = AV_TX_FULL_IMDCT |
+                            AV_TX_HALF_COMPLEX |
+                            FF_TX_PRESHUFFLE |
+                            FF_TX_ASM_CALL;
 
     /* Unaligned codelets are compatible with the aligned flag */
     if (req_flags & FF_TX_ALIGNED)
@@ -742,7 +748,8 @@ av_cold int ff_tx_init_subtx(AVTXContext *s, enum AVTXType type,
 
             /* Check direction for non-orthogonal codelets */
             if (((cd->flags & FF_TX_FORWARD_ONLY) && inv) ||
-                ((cd->flags & (FF_TX_INVERSE_ONLY | AV_TX_FULL_IMDCT)) && !inv))
+                ((cd->flags & (FF_TX_INVERSE_ONLY | AV_TX_FULL_IMDCT)) && !inv) ||
+                ((cd->flags & (FF_TX_FORWARD_ONLY | AV_TX_HALF_COMPLEX)) && inv))
                 continue;
 
             /* Check if the requested flags match from both sides */
diff --git a/libavutil/tx.h b/libavutil/tx.h
index 064edbc097..63fba8e8b4 100644
--- a/libavutil/tx.h
+++ b/libavutil/tx.h
@@ -149,6 +149,15 @@ enum AVTXFlags {
      * Ignored for all transforms but inverse MDCTs.
      */
     AV_TX_FULL_IMDCT = 1ULL << 2,
+
+    /**
+     * Perform a real to half-complex RDFT or FFT.
+     * Only the real coefficients will
+     * be output. Only available for forward RDFTs.
+     * Output array must have enough space to hold N complex values
+     * (regular size for a real to complex transform).
+     */
+    AV_TX_HALF_COMPLEX = 1ULL << 3,
 };
 
 /**
diff --git a/libavutil/tx_template.c b/libavutil/tx_template.c
index a4040454db..bdec53f335 100644
--- a/libavutil/tx_template.c
+++ b/libavutil/tx_template.c
@@ -1617,6 +1617,8 @@ static av_cold int TX_NAME(ff_tx_rdft_init)(AVTXContext *s,
     s->scale_d = *((SCALE_TYPE *)scale);
     s->scale_f = s->scale_d;
 
+    flags &= ~AV_TX_HALF_COMPLEX;
+
     if ((ret = ff_tx_init_subtx(s, TX_TYPE(FFT), flags, NULL, len >> 1, inv, scale)))
         return ret;
 
@@ -1700,8 +1702,51 @@ static void TX_NAME(ff_tx_rdft_ ##name)(AVTXContext *s, void *_dst,            \
     }                                                                          \
 }
 
-DECL_RDFT(r2c, 0)
-DECL_RDFT(c2r, 1)
+DECL_RDFT(r2c,  0)
+DECL_RDFT(c2r,  1)
+
+static void TX_NAME(ff_tx_rdft_r2hc)(AVTXContext *s, void *_dst,
+                                     void *_src, ptrdiff_t stride)
+{
+    const int len = s->len;
+    const int len2 = len >> 1;
+    const int len4 = len >> 2;
+    const TXSample *fact = (void *)s->exp;
+    const TXSample *tcos = fact + 8;
+    const TXSample *tsin = tcos + len4;
+    TXComplex *data = _dst;
+    TXSample *out = _dst; /* Half-complex is forward-only */
+    TXSample tmp_dc;
+
+    s->fn[0](&s->sub[0], _dst, _src, sizeof(TXComplex));
+
+    tmp_dc = data[0].re;
+
+    /* NOTE: we can avoid all multiplies here as they're all 1.0, except
+     * for one, -1.0, which is not needed */
+    out[0]    = tmp_dc + data[0].im;
+    tmp_dc -= data[0].im;
+
+    /* NOTE: unrolling this breaks non-mod8 lengths */
+    for (int i = 1; i <= len4; i++) {
+        TXSample tmp[4];
+        TXComplex sf = data[i];
+        TXComplex sl = data[len2 - i];
+
+        tmp[0] = MULT(fact[4], (sf.re + sl.re));
+        tmp[1] = MULT(fact[6], (sf.im + sl.im));
+        tmp[2] = MULT(fact[7], (sf.re - sl.re));
+        tmp[3] = tmp[1]*tcos[i] - tmp[2]*tsin[i];
+
+        out[i]       = tmp[0] + tmp[3];
+        out[len - i] = tmp[0] - tmp[3];
+    }
+
+    for (int i = 1; i < len4; i++)
+        out[len2 - i - 0] = out[len - i];
+
+    out[len2] = tmp_dc;
+}
 
 static const FFTXCodelet TX_NAME(ff_tx_rdft_r2c_def) = {
     .name       = TX_NAME_STR("rdft_r2c"),
@@ -1733,6 +1778,21 @@ static const FFTXCodelet TX_NAME(ff_tx_rdft_c2r_def) = {
     .prio       = FF_TX_PRIO_BASE,
 };
 
+static const FFTXCodelet TX_NAME(ff_tx_rdft_r2hc_def) = {
+    .name       = TX_NAME_STR("rdft_r2hc"),
+    .function   = TX_NAME(ff_tx_rdft_r2hc),
+    .type       = TX_TYPE(RDFT),
+    .flags      = AV_TX_UNALIGNED | AV_TX_HALF_COMPLEX | AV_TX_INPLACE |
+                  FF_TX_OUT_OF_PLACE | FF_TX_FORWARD_ONLY,
+    .factors[0] = 2,
+    .nb_factors = 1,
+    .min_len    = 2,
+    .max_len    = TX_LEN_UNLIMITED,
+    .init       = TX_NAME(ff_tx_rdft_init),
+    .cpu_flags  = FF_TX_CPU_FLAGS_ALL,
+    .prio       = FF_TX_PRIO_BASE,
+};
+
 static av_cold int TX_NAME(ff_tx_dct_init)(AVTXContext *s,
                                            const FFTXCodelet *cd,
                                            uint64_t flags,
@@ -1997,6 +2057,7 @@ const FFTXCodelet * const TX_NAME(ff_tx_codelet_list)[] = {
     &TX_NAME(ff_tx_mdct_naive_inv_def),
     &TX_NAME(ff_tx_mdct_inv_full_def),
     &TX_NAME(ff_tx_rdft_r2c_def),
+    &TX_NAME(ff_tx_rdft_r2hc_def),
     &TX_NAME(ff_tx_rdft_c2r_def),
     &TX_NAME(ff_tx_dctII_def),
     &TX_NAME(ff_tx_dctIII_def),
diff --git a/libavutil/version.h b/libavutil/version.h
index 24af520e08..9e798b0e3f 100644
--- a/libavutil/version.h
+++ b/libavutil/version.h
@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  58
-#define LIBAVUTIL_VERSION_MINOR  14
+#define LIBAVUTIL_VERSION_MINOR  15
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
-- 
2.40.1


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