[FFmpeg-devel] [PATCH 2/2] lavfi/vf_colorspace: Add SMPTE ST 2084 support

["Tomas Härdin" <git@haerdin.se>]

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We need something better for proper tonemap support, but this is at
least useful for getting the HDR discussion going.

/Tomas

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From fd7a789fbffdd0f7e41b77a9d70ae0696142c6db Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomas=20H=C3=A4rdin?= <git@haerdin.se>
Date: Fri, 3 Feb 2023 14:00:38 +0100
Subject: [PATCH 2/2] lavfi/vf_colorspace: Add SMPTE ST 2084 support

This makes 10000 nits == 65535 in 16-bit,
meaning SDR white is 65535*203/10000 = 1330.
---
 libavfilter/vf_colorspace.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/libavfilter/vf_colorspace.c b/libavfilter/vf_colorspace.c
index 1e1ab5fb34..081c1c9d0e 100644
--- a/libavfilter/vf_colorspace.c
+++ b/libavfilter/vf_colorspace.c
@@ -173,6 +173,7 @@ static const struct TransferCharacteristics transfer_characteristics[AVCOL_TRC_N
     [AVCOL_TRC_IEC61966_2_4] = { 1.099, 0.018, 0.45, 4.5 },
     [AVCOL_TRC_BT2020_10] = { 1.099,  0.018,  0.45, 4.5 },
     [AVCOL_TRC_BT2020_12] = { 1.0993, 0.0181, 0.45, 4.5 },
+    [AVCOL_TRC_SMPTE2084] = { 1.0,    0,      0,    0 }, // fake entry, actual TRC uses entirely separate formula
 };
 
 static const struct TransferCharacteristics *
@@ -197,6 +198,8 @@ static int fill_gamma_table(ColorSpaceContext *s)
     double in_ialpha = 1.0 / in_alpha, in_igamma = 1.0 / in_gamma, in_idelta = 1.0 / in_delta;
     double out_alpha = s->out_txchr->alpha, out_beta = s->out_txchr->beta;
     double out_gamma = s->out_txchr->gamma, out_delta = s->out_txchr->delta;
+    double m1 = 1305.0/8192, m2 = 2523.0/32, c2 = 2413.0/128, c3 = 2392.0/128, c1 = c3 - c2 + 1;
+    double im1 = 1.0 / m1, im2 = 1.0 / m2;
 
     s->lin_lut = av_malloc(sizeof(*s->lin_lut) * 32768 * 2);
     if (!s->lin_lut)
@@ -206,7 +209,15 @@ static int fill_gamma_table(ColorSpaceContext *s)
         double v = (n - 2048.0) / 28672.0, d, l;
 
         // delinearize
-        if (v <= -out_beta) {
+        if (s->out_trc == AVCOL_TRC_SMPTE2084) {
+            // see BT.2100-2
+            if (v >= 0) {
+                double vm1 = pow(v, m1);
+                d = pow((c1 + c2 * vm1)/(1 + c3 * vm1), m2);
+            } else {
+                d = 0;
+            }
+        } else if (v <= -out_beta) {
             d = -out_alpha * pow(-v, out_gamma) + (out_alpha - 1.0);
         } else if (v < out_beta) {
             d = out_delta * v;
@@ -216,7 +227,16 @@ static int fill_gamma_table(ColorSpaceContext *s)
         s->delin_lut[n] = av_clip_int16(lrint(d * 28672.0));
 
         // linearize
-        if (v <= -in_beta * in_delta) {
+        if (s->in_trc == AVCOL_TRC_SMPTE2084) {
+            // see BT.2100-2
+            if (v >= 0) {
+                double vim2 = pow(v, im2);
+                // note that [0,1] here corresponds to [0,100] in SDR
+                l = pow((vim2 - c1 > 0 ? vim2 - c1 : 0) / (c2 - c3 * vim2), im1);
+            } else {
+                l = 0;
+            }
+        } else if (v <= -in_beta * in_delta) {
             l = -pow((1.0 - in_alpha - v) * in_ialpha, in_igamma);
         } else if (v < in_beta * in_delta) {
             l = v * in_idelta;
@@ -956,6 +976,7 @@ static const AVOption colorspace_options[] = {
     ENUM("iec61966-2-4", AVCOL_TRC_IEC61966_2_4, "trc"),
     ENUM("bt2020-10",    AVCOL_TRC_BT2020_10,    "trc"),
     ENUM("bt2020-12",    AVCOL_TRC_BT2020_12,    "trc"),
+    ENUM("smpte2084",    AVCOL_TRC_SMPTE2084,    "trc"),
 
     { "format",   "Output pixel format",
       OFFSET(user_format), AV_OPT_TYPE_INT,  { .i64 = AV_PIX_FMT_NONE },
-- 
2.30.2


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