lavc/pngenc: Support writing colorspace tags.

These are defined in http://www.w3.org/TR/PNG/#11addnlcolinfo.

libavcodec/pngenc.c

@@ -231,6 +231,81 @@ static int png_write_row(PNGEncContext *s, const uint8_t *data, int size)
     return 0;
 }
 
+#define AV_WB32_PNG(buf, n) (AV_WB32(buf, round((n) * 100000)))
+static int png_get_chrm(enum AVColorPrimaries prim,  uint8_t *buf)
+{
+    double rx, ry, gx, gy, bx, by, wx = 0.3127, wy = 0.3290;
+    switch (prim) {
+        case AVCOL_PRI_BT709:
+            rx = 0.640; ry = 0.330;
+            gx = 0.300; gy = 0.600;
+            bx = 0.150; by = 0.060;
+            break;
+        case AVCOL_PRI_BT470M:
+            rx = 0.670; ry = 0.330;
+            gx = 0.210; gy = 0.710;
+            bx = 0.140; by = 0.080;
+            wx = 0.310; wy = 0.316;
+            break;
+        case AVCOL_PRI_BT470BG:
+            rx = 0.640; ry = 0.330;
+            gx = 0.290; gy = 0.600;
+            bx = 0.150; by = 0.060;
+            break;
+        case AVCOL_PRI_SMPTE170M:
+        case AVCOL_PRI_SMPTE240M:
+            rx = 0.630; ry = 0.340;
+            gx = 0.310; gy = 0.595;
+            bx = 0.155; by = 0.070;
+            break;
+        case AVCOL_PRI_BT2020:
+            rx = 0.708; ry = 0.292;
+            gx = 0.170; gy = 0.797;
+            bx = 0.131; by = 0.046;
+            break;
+        default:
+            return 0;
+    }
+
+    AV_WB32_PNG(buf     , wx); AV_WB32_PNG(buf + 4 , wy);
+    AV_WB32_PNG(buf + 8 , rx); AV_WB32_PNG(buf + 12, ry);
+    AV_WB32_PNG(buf + 16, gx); AV_WB32_PNG(buf + 20, gy);
+    AV_WB32_PNG(buf + 24, bx); AV_WB32_PNG(buf + 28, by);
+    return 1;
+}
+
+static int png_get_gama(enum AVColorTransferCharacteristic trc, uint8_t *buf)
+{
+    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:
+            return 0;
+    }
+
+    AV_WB32_PNG(buf, 1.0 / gamma);
+    return 1;
+}
+
 static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
                         const AVFrame *pict, int *got_packet)
 {
@@ -358,6 +433,18 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     }
     png_write_chunk(&s->bytestream, MKTAG('p', 'H', 'Y', 's'), s->buf, 9);
 
+    /* write colorspace information */
+    if (pict->color_primaries == AVCOL_PRI_BT709 &&
+        pict->color_trc == AVCOL_TRC_IEC61966_2_1) {
+        s->buf[0] = 1; /* rendering intent, relative colorimetric by default */
+        png_write_chunk(&s->bytestream, MKTAG('s', 'R', 'G', 'B'), s->buf, 1);
+    }
+
+    if (png_get_chrm(pict->color_primaries, s->buf))
+        png_write_chunk(&s->bytestream, MKTAG('c', 'H', 'R', 'M'), s->buf, 32);
+    if (png_get_gama(pict->color_trc, s->buf))
+        png_write_chunk(&s->bytestream, MKTAG('g', 'A', 'M', 'A'), s->buf, 4);
+
     /* put the palette if needed */
     if (color_type == PNG_COLOR_TYPE_PALETTE) {
         int has_alpha, alpha, i;