🐛 Fix issues in motion compensation decoder

pleonex · Nov 4, 2023 · 5c8755f · 5c8755f
1 parent c7296ac
commit 5c8755f
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Showing 2 changed files with 18 additions and 17 deletions.
diff --git a/src/PlayMobic/Video/Mobiclip/MotionCompensationDecoder.cs b/src/PlayMobic/Video/Mobiclip/MotionCompensationDecoder.cs
@@ -93,11 +93,9 @@ private void DecodeBlockPartioning(YuvBlock block, int mode, int macroBlockIndex
         int newLumaHeight = partitions[0].Luma.Height;
         Huffman huffmanTable = huffmanTables[(newLumaWidth, newLumaHeight)];
 
-        // Read the partitions mode encoded with huffman
-        int partitionsMode = huffmanTable.ReadCodeword(reader);
-
         // Re-run motion compensation algorithm on each partition.
         foreach (YuvBlock partition in partitions) {
+            int partitionsMode = huffmanTable.ReadCodeword(reader);
             DecodeBlock(partition, partitionsMode, macroBlockIndex);
         }
     }
@@ -118,38 +116,41 @@ private void DecodeBlockMotion(YuvBlock block, int mode, int macroBlockIdx)
         CopyMovingYuvBlock(block, src, vector);
     }
 
-    private void CopyMovingYuvBlock(YuvBlock dstBlock, YuvBlock srcFrame, Vector2D vector)
+    private void CopyMovingYuvBlock(YuvBlock dstBlock, YuvBlock srcFrame, Vector2D delta)
     {
         ComponentBlock dstLuma = dstBlock.Luma;
         ComponentBlock dstChromaU = dstBlock.ChromaU;
         ComponentBlock dstChromaV = dstBlock.ChromaV;
 
         for (int y = 0; y < dstBlock.Luma.Height; y++) {
             for (int x = 0; x < dstBlock.Luma.Width; x++) {
-                int srcLumaX = dstLuma.X + x + vector.X;
-                int srcLumaY = dstLuma.Y + y + vector.Y;
-                dstLuma[x, y] = GetHalfPelComponent(srcFrame.Luma, srcLumaX, srcLumaY);
+                int srcLumaX = dstLuma.X + x;
+                int srcLumaY = dstLuma.Y + y;
+                dstLuma[x, y] = GetHalfPelComponent(srcFrame.Luma, srcLumaX, srcLumaY, delta);
 
                 // Because of YUV 4:2:0 downsampling we divide by 2 and do it half of time
                 if ((x % 2) == 0 && (y % 2) == 0) {
-                    int dstChromaX = dstChromaU.X + (x / 2) + (vector.X / 2);
-                    int dstChromaY = dstChromaU.Y + (y / 2) + (vector.Y / 2);
-                    dstChromaU[x / 2, y / 2] = GetHalfPelComponent(srcFrame.ChromaU, dstChromaX, dstChromaY);
-                    dstChromaV[x / 2, y / 2] = GetHalfPelComponent(srcFrame.ChromaV, dstChromaX, dstChromaY);
+                    var chromaDelta = new Vector2D(delta.X / 2, delta.Y / 2);
+                    int dstChromaX = dstChromaU.X + (x / 2);
+                    int dstChromaY = dstChromaU.Y + (y / 2);
+                    dstChromaU[x / 2, y / 2] = GetHalfPelComponent(srcFrame.ChromaU, dstChromaX, dstChromaY, chromaDelta);
+                    dstChromaV[x / 2, y / 2] = GetHalfPelComponent(srcFrame.ChromaV, dstChromaX, dstChromaY, chromaDelta);
                 }
             }
         }
     }
 
-    private byte GetHalfPelComponent(ComponentBlock block, int x, int y)
+    private byte GetHalfPelComponent(ComponentBlock block, int x, int y, Vector2D delta)
     {
         // Actual positions are multiplied by two, so we can have half-pixel positions
         // without storing decimals.
-        bool isExactX = (x % 2) == 0;
-        bool isExactY = (y % 2) == 0;
+        bool isExactX = (delta.X % 2) == 0;
+        bool isExactY = (delta.Y % 2) == 0;
 
-        x /= 2;
-        y /= 2;
+        // We divide with bitwise shifting (same as /2) because when it's -1
+        // shifting 1, gives still -1 rather than 0 (it propages sign so it's still negative).
+        x += delta.X >> 1;
+        y += delta.Y >> 1;
 
         if (isExactX && isExactY) {
             return block[x, y];

diff --git a/src/PlayMobic/Video/YuvBlock.cs b/src/PlayMobic/Video/YuvBlock.cs
@@ -19,7 +19,7 @@ public readonly YuvBlock[] PartitionBy(int widthDiv, int heightDiv)
     {
         ComponentBlock[] lumas = Luma.Partition(Luma.Width / widthDiv, Luma.Height / heightDiv);
         ComponentBlock[] chromaUs = ChromaU.Partition(ChromaU.Width / widthDiv, ChromaU.Height / heightDiv);
-        ComponentBlock[] chromaVs = ChromaU.Partition(ChromaV.Width / widthDiv, ChromaV.Height / heightDiv);
+        ComponentBlock[] chromaVs = ChromaV.Partition(ChromaV.Width / widthDiv, ChromaV.Height / heightDiv);
 
         var partitions = new YuvBlock[lumas.Length];
         for (int i = 0; i < lumas.Length; i++) {