avutil: Extract gamma determination from PNG encoder for future use. Adds private avpriv_get_gamma_from_trc() function to libavutil.

Signed-off-by: Kevin Wheatley <kevin.j.wheatley@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngenc.c

@@ -28,6 +28,7 @@
 #include "libavutil/avassert.h"
 #include "libavutil/libm.h"
 #include "libavutil/opt.h"
+#include "libavutil/color_utils.h"
 
 #include <zlib.h>
 
@@ -277,31 +278,9 @@ static int png_get_chrm(enum AVColorPrimaries prim,  uint8_t *buf)
 
 static int png_get_gama(enum AVColorTransferCharacteristic trc, uint8_t *buf)
 {
-    double gamma;
+    double gamma = avpriv_get_gamma_from_trc(trc);
-    switch (trc) {
+    if (gamma <= 1e-6)
-        case AVCOL_TRC_BT709:
+        return 0;
-        case AVCOL_TRC_SMPTE170M:
-        case AVCOL_TRC_SMPTE240M:
-        case AVCOL_TRC_BT1361_ECG:
-        case AVCOL_TRC_BT2020_10:
-        case AVCOL_TRC_BT2020_12:
-            /* these share a segmented TRC, but gamma 1.961 is a close
-              approximation, and also more correct for decoding content */
-            gamma = 1.961;
-            break;
-        case AVCOL_TRC_GAMMA22:
-        case AVCOL_TRC_IEC61966_2_1:
-            gamma = 2.2;
-            break;
-        case AVCOL_TRC_GAMMA28:
-            gamma = 2.8;
-            break;
-        case AVCOL_TRC_LINEAR:
-            gamma = 1.0;
-            break;
-        default:
-            return 0;
-    }
 
     AV_WB32_PNG(buf, 1.0 / gamma);
     return 1;

libavutil/Makefile

@@ -88,6 +88,7 @@ OBJS = adler32.o                                                        \
        cast5.o                                                          \
        camellia.o                                                       \
        channel_layout.o                                                 \
+       color_utils.o                                                    \
        cpu.o                                                            \
        crc.o                                                            \
        des.o                                                            \

libavutil/color_utils.c

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2015 Kevin Wheatley <kevin.j.wheatley@gmail.com>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libavutil/color_utils.h"
+#include "libavutil/pixfmt.h"
+
+double avpriv_get_gamma_from_trc(enum AVColorTransferCharacteristic trc)
+{
+    double gamma;
+    switch (trc) {
+        case AVCOL_TRC_BT709:
+        case AVCOL_TRC_SMPTE170M:
+        case AVCOL_TRC_SMPTE240M:
+        case AVCOL_TRC_BT1361_ECG:
+        case AVCOL_TRC_BT2020_10:
+        case AVCOL_TRC_BT2020_12:
+            /* these share a segmented TRC, but gamma 1.961 is a close
+              approximation, and also more correct for decoding content */
+            gamma = 1.961;
+            break;
+        case AVCOL_TRC_GAMMA22:
+        case AVCOL_TRC_IEC61966_2_1:
+            gamma = 2.2;
+            break;
+        case AVCOL_TRC_GAMMA28:
+            gamma = 2.8;
+            break;
+        case AVCOL_TRC_LINEAR:
+            gamma = 1.0;
+            break;
+        default:
+            gamma = 0.0; // Unknown value representation
+    }
+    return gamma;
+}

libavutil/color_utils.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2015 Kevin Wheatley <kevin.j.wheatley@gmail.com>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_COLOR_UTILS_H
+#define AVUTIL_COLOR_UTILS_H
+
+
+#include "libavutil/pixfmt.h"
+
+/**
+ * Determine a suitable 'gamma' value to match the supplied
+ * AVColorTransferCharacteristic.
+ *
+ * See Apple Technical Note TN2257 (https://developer.apple.com/library/mac/technotes/tn2257/_index.html)
+ *
+ * @return Will return an approximation to the simple gamma function matching
+ *         the supplied Transfer Characteristic, Will return 0.0 for any
+ *         we cannot reasonably match against.
+ */
+double avpriv_get_gamma_from_trc(enum AVColorTransferCharacteristic trc);
+
+#endif

libavutil/version.h

@@ -56,7 +56,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  54
-#define LIBAVUTIL_VERSION_MINOR  19
+#define LIBAVUTIL_VERSION_MINOR  20
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \