avutil/csp: add av_csp_approximate_eotf_gamma()

2.2 gamma was used for bt.1886 aproximation as the most common target in
real world.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2025-10-xx - xxxxxxxxxx - lavu 60.14.100 - csp.h
+  Add av_csp_approximate_eotf_gamma().
+
 2025-08-xx - xxxxxxxxxx - lavf 62.6.100 - oggparsevorbis.h oggparseopus.h oggparseflac.h
   Drop header packets from secondary chained ogg/{flac, opus, vorbis} streams
   from demuxer output.

libavutil/csp.c

@@ -157,6 +157,27 @@ double av_csp_approximate_trc_gamma(enum AVColorTransferCharacteristic trc)
     return 0.0;
 }
 
+static const double approximate_eotf_gamma[AVCOL_TRC_NB] = {
+    [AVCOL_TRC_BT709] = 2.2,
+    [AVCOL_TRC_SMPTE170M] = 2.2,
+    [AVCOL_TRC_SMPTE240M] = 2.2,
+    [AVCOL_TRC_BT1361_ECG] = 2.2,
+    [AVCOL_TRC_BT2020_10] = 2.2,
+    [AVCOL_TRC_BT2020_12] = 2.2,
+    [AVCOL_TRC_GAMMA22] = 2.2,
+    [AVCOL_TRC_IEC61966_2_1] = 2.2,
+    [AVCOL_TRC_GAMMA28] = 2.8,
+    [AVCOL_TRC_LINEAR] = 1.0,
+    [AVCOL_TRC_SMPTE428] = 2.6,
+};
+
+double av_csp_approximate_eotf_gamma(enum AVColorTransferCharacteristic trc)
+{
+    if ((unsigned)trc >= AVCOL_TRC_NB)
+        return 0.0;
+    return approximate_eotf_gamma[trc];
+}
+
 #define BT709_alpha 1.099296826809442
 #define BT709_beta 0.018053968510807
 

libavutil/csp.h

@@ -133,6 +133,22 @@ enum AVColorPrimaries av_csp_primaries_id_from_desc(const AVColorPrimariesDesc *
  */
 double av_csp_approximate_trc_gamma(enum AVColorTransferCharacteristic trc);
 
+/**
+ * Determine a suitable EOTF 'gamma' value to match the supplied
+ * AVColorTransferCharacteristic.
+ *
+ * This function returns the gamma value (exponent) for a simple pure power
+ * function approximation of the supplied AVColorTransferCharacteristic, or 0.
+ * if no reasonable approximation exists.
+ *
+ * EOTF(v) = (L_w - L_b) * v^gamma + L_b
+ *
+ * @return Will return an approximation to the simple gamma function matching
+ *         the supplied Transfer Characteristic EOTF, Will return 0.0 for any
+ *         we cannot reasonably match against.
+ */
+double av_csp_approximate_eotf_gamma(enum AVColorTransferCharacteristic trc);
+
 /**
  * Determine the function needed to apply the given
  * AVColorTransferCharacteristic to linear input.

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  60
-#define LIBAVUTIL_VERSION_MINOR  13
+#define LIBAVUTIL_VERSION_MINOR  14
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \