From d92e0bfb5e2e47131808a6e9d549ade1b26d1f34 Mon Sep 17 00:00:00 2001
From: Saxon <saxon@ausocean.org>
Date: Mon, 9 Sep 2019 11:28:43 +0930
Subject: [PATCH] codec/h264/h264dec/decode.go: wrote function picOrderCntType0
 to handle derivation of TopFieldOrderCnt and BottomFieldOrderCnt when
 pic_order_cnt_type == 0

---
 codec/h264/h264dec/decode.go | 35 +++++++++++++++++++++++++++++++++++
 codec/h264/h264dec/slice.go  | 11 ++++++++++-
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/codec/h264/h264dec/decode.go b/codec/h264/h264dec/decode.go
index 22cfc181..5a914263 100644
--- a/codec/h264/h264dec/decode.go
+++ b/codec/h264/h264dec/decode.go
@@ -23,3 +23,38 @@ LICENSE
 */
 
 package h264dec
+
+// picOrderCntType0 is used to return topFieldOrderCnt and bottomFieldOrderCnt
+// when pic_order_cnt_type i.e vid.PicOrderCntType == 0, using the process
+// defined in section 8.2.1.1 of the specifications.
+func picOrderCntType0(vid VideoStream, ctx SliceContext) (topFieldOrderCnt, bottomFieldOrderCnt int) {
+	prevPicOrderCntMsb, prevPicOrderCntLsb := 0, 0
+	topFieldOrderCnt, bottomFieldOrderCnt = -1, -1
+
+	// NB: We're currently only handling IDRs so panic.
+	if !vid.currPic.isIDR {
+		panic("not implemented")
+	}
+
+	switch {
+	case (ctx.PicOrderCntLsb < prevPicOrderCntLsb) && ((prevPicOrderCntLsb - ctx.PicOrderCntLsb) >= (vid.maxPicOrderCntLsb / 2)):
+		vid.picOrderCntMsb = prevPicOrderCntMsb + vid.maxPicOrderCntLsb
+	case (ctx.PicOrderCntLsb > prevPicOrderCntLsb) && ((ctx.PicOrderCntLsb - prevPicOrderCntLsb) > (vid.maxPicOrderCntLsb / 2)):
+		vid.picOrderCntMsb = prevPicOrderCntMsb - vid.maxPicOrderCntLsb
+	default:
+		vid.picOrderCntMsb = prevPicOrderCntMsb
+	}
+
+	if !vid.currPic.isBottomField {
+		topFieldOrderCnt = vid.picOrderCntMsb + ctx.PicOrderCntLsb
+	}
+
+	if !vid.currPic.isTopField {
+		if !ctx.FieldPic {
+			bottomFieldOrderCnt = topFieldOrderCnt + ctx.DeltaPicOrderCntBottom
+		} else {
+			bottomFieldOrderCnt = vid.picOrderCntMsb + ctx.PicOrderCntLsb
+		}
+	}
+	return
+}
diff --git a/codec/h264/h264dec/slice.go b/codec/h264/h264dec/slice.go
index 4952cfd9..83e3b7bf 100644
--- a/codec/h264/h264dec/slice.go
+++ b/codec/h264/h264dec/slice.go
@@ -35,12 +35,21 @@ const (
 	chroma444
 )
 
+type picture struct {
+	isIDR         bool
+	isBottomField bool
+	isTopField    bool
+}
+
 type VideoStream struct {
 	*SPS
 	*PPS
 	Slices []*SliceContext
 
-	ChromaArrayType int
+	ChromaArrayType   int
+	currPic           picture
+	maxPicOrderCntLsb int
+	picOrderCntMsb    int
 }
 
 type SliceContext struct {