Store diffusion errors

src/ImageSharp/Formats/WebP/Lossy/QuantEnc.cs

@@ -240,6 +240,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                     rdBest.CopyScore(rdUv);
                     rd.ModeUv = mode;
                     rdUv.UvLevels.CopyTo(rd.UvLevels.AsSpan());
+                    for (int i = 0; i < 2; i++)
+                    {
+                        rd.Derr[i, 0] = rdUv.Derr[i, 0];
+                        rd.Derr[i, 1] = rdUv.Derr[i, 1];
+                        rd.Derr[i, 2] = rdUv.Derr[i, 2];
+                    }
+
                     Span<byte> tmp = dst;
                     dst = tmpDst;
                     tmpDst = tmp;
@@ -253,6 +260,9 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 // copy 16x8 block if needed.
                 LossyUtils.Vp8Copy16X8(dst, dst0);
             }
+
+            // Store diffusion errors for next block.
+            it.StoreDiffusionErrors(rd);
         }
 
         public static int ReconstructIntra16(Vp8EncIterator it, Vp8SegmentInfo dqm, Vp8ModeScore rd, Span<byte> yuvOut, int mode)
@@ -571,10 +581,9 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 c[3 * 16] += (short)(((C1 * err1) + (C2 * err2)) >> (DSHIFT - DSCALE));
                 int err3 = QuantizeSingle(c.Slice(3 * 16), mtx);
 
-                // TODO: set errors in rd
-                // rd->derr[ch][0] = (int8_t)err1;
-                // rd->derr[ch][1] = (int8_t)err2;
-                // rd->derr[ch][2] = (int8_t)err3;
+                rd.Derr[ch, 0] = err1;
+                rd.Derr[ch, 1] = err2;
+                rd.Derr[ch, 2] = err3;
             }
         }
 

src/ImageSharp/Formats/WebP/Lossy/Vp8EncIterator.cs

@@ -167,7 +167,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         public uint[] Nz { get; }
 
         /// <summary>
-        /// Gets the diffusion error.
+        /// Gets the top diffusion error.
         /// </summary>
         public sbyte[] TopDerr { get; }
 
@@ -587,6 +587,27 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
 
         public void SetSegment(int segment) => this.CurrentMacroBlockInfo.Segment = segment;
 
+        public void StoreDiffusionErrors(Vp8ModeScore rd)
+        {
+            for (int ch = 0; ch <= 1; ++ch)
+            {
+                Span<sbyte> top = this.TopDerr.AsSpan((this.X * 4) + ch, 2);
+                Span<sbyte> left = this.LeftDerr.AsSpan(ch, 2);
+
+                // restore err1
+                left[0] = (sbyte)rd.Derr[ch, 0];
+
+                // 3/4th of err3
+                left[1] = (sbyte)((3 * rd.Derr[ch, 2]) >> 2);
+
+                // err2
+                top[0] = (sbyte)rd.Derr[ch, 1];
+
+                // 1/4th of err3.
+                top[1] = (sbyte)(rd.Derr[ch, 2] - left[1]);
+            }
+        }
+
         /// <summary>
         /// Returns true if iteration is finished.
         /// </summary>

src/ImageSharp/Formats/WebP/Lossy/Vp8ModeScore.cs

@@ -25,6 +25,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             this.UvLevels = new short[(4 + 4) * 16];
 
             this.ModesI4 = new byte[16];
+            this.Derr = new int[2, 3];
         }
 
         /// <summary>
@@ -87,6 +88,11 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         /// </summary>
         public uint Nz { get; set; }
 
+        /// <summary>
+        /// Gets the diffusion errors.
+        /// </summary>
+        public int[,] Derr { get; }
+
         public void InitScore()
         {
             this.D = 0;