Enable PickBestIntra16 and PickBestUv

src/ImageSharp/Formats/WebP/Lossy/Vp8CostArray.cs

@@ -8,7 +8,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         /// <summary>
         /// Initializes a new instance of the <see cref="Vp8CostArray"/> class.
         /// </summary>
-        public Vp8CostArray() => this.Costs = new ushort[WebpConstants.NumCtx * (67 + 1)];
+        public Vp8CostArray() => this.Costs = new ushort[67 + 1];
 
         public ushort[] Costs { get; }
     }

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

@@ -415,11 +415,10 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 this.MakeIntra4Preds();
                 for (mode = 0; mode < maxMode; ++mode)
                 {
-                    int alpha;
                     histos[curHisto] = new Vp8Histogram();
                     histos[curHisto].CollectHistogram(src, this.YuvP.AsSpan(Vp8Encoding.Vp8I4ModeOffsets[mode]), 0, 1);
 
-                    alpha = histos[curHisto].GetAlpha();
+                    var alpha = histos[curHisto].GetAlpha();
                     if (alpha > bestModeAlpha)
                     {
                         bestModeAlpha = alpha;
@@ -522,7 +521,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 for (int x = 0; x < 4; ++x)
                 {
                     int ctx = this.TopNz[x] + this.LeftNz[y];
-                    res.SetCoeffs(rd.YAcLevels.AsSpan(x + (y * 4)));
+                    res.SetCoeffs(rd.YAcLevels.AsSpan((x + (y * 4)) * 16, 16));
                     r += res.GetResidualCost(ctx);
                     this.TopNz[x] = this.LeftNz[y] = (res.Last >= 0) ? 1 : 0;
                 }
@@ -572,7 +571,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                     for (int x = 0; x < 2; ++x)
                     {
                         int ctx = this.TopNz[4 + ch + x] + this.LeftNz[4 + ch + y];
-                        res.SetCoeffs(rd.UvLevels.AsSpan((ch * 2) + x + (y * 2)));
+                        res.SetCoeffs(rd.UvLevels.AsSpan(((ch * 2) + x + (y * 2)) * 16, 16));
                         r += res.GetResidualCost(ctx);
                         this.TopNz[4 + ch + x] = this.LeftNz[4 + ch + y] = (res.Last >= 0) ? 1 : 0;
                     }

src/ImageSharp/Formats/WebP/Lossy/Vp8Encoder.cs

@@ -311,7 +311,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
 
                 // Warning! order is important: first call VP8Decimate() and
                 // *then* decide how to code the skip decision if there's one.
-                if (!this.Decimate(it, info, this.rdOptLevel) || dontUseSkip)
+                if (!this.Decimate(it, ref info, this.rdOptLevel) || dontUseSkip)
                 {
                     this.CodeResiduals(it, info);
                 }
@@ -411,7 +411,8 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 this.Proba.FinalizeTokenProbas();
             }
 
-            this.Proba.CalculateLevelCosts();  // Finalize costs.
+            // Finalize costs.
+            this.Proba.CalculateLevelCosts();
         }
 
         private long OneStatPass(int width, int height, int yStride, int uvStride, Vp8RdLevel rdOpt, int nbMbs, PassStats stats)
@@ -425,12 +426,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             long distortion = 0;
             long pixelCount = nbMbs * 384;
 
+            it.Init();
             this.SetLoopParams(stats.Q);
             do
             {
                 var info = new Vp8ModeScore();
                 it.Import(y, u, v, yStride, uvStride, width, height, false);
-                if (this.Decimate(it, info, rdOpt))
+                if (this.Decimate(it, ref info, rdOpt))
                 {
                     // Just record the number of skips and act like skipProba is not used.
                     ++this.Proba.NbSkip;
@@ -877,7 +879,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             return bestAlpha; // Mixed susceptibility (not just luma).
         }
 
-        private bool Decimate(Vp8EncIterator it, Vp8ModeScore rd, Vp8RdLevel rdOpt)
+        private bool Decimate(Vp8EncIterator it, ref Vp8ModeScore rd, Vp8RdLevel rdOpt)
         {
             rd.InitScore();
 
@@ -886,13 +888,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             it.MakeLuma16Preds();
             it.MakeChroma8Preds();
 
-            // TODO: disabled picking best mode because its still bugged.
-            /* if (rdOpt > Vp8RdLevel.RdOptNone)
+            if (rdOpt > Vp8RdLevel.RdOptNone)
             {
                 QuantEnc.PickBestIntra16(it, ref rd, this.SegmentInfos, this.Proba);
                 if (this.method >= 2)
                 {
-                    QuantEnc.PickBestIntra4(it, ref rd, this.SegmentInfos, this.Proba, this.maxI4HeaderBits);
+                    // TODO: there is still a bug in PickBestIntra4, therefore disabled.
+                    // QuantEnc.PickBestIntra4(it, ref rd, this.SegmentInfos, this.Proba, this.maxI4HeaderBits);
                 }
 
                 QuantEnc.PickBestUv(it, ref rd, this.SegmentInfos, this.Proba);
@@ -905,9 +907,6 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 // quantization/reconstruction.
                 QuantEnc.RefineUsingDistortion(it, this.SegmentInfos, rd, this.method >= 2, this.method >= 1, this.MbHeaderLimit);
             }
-            */
-
-            QuantEnc.RefineUsingDistortion(it, this.SegmentInfos, rd, this.method >= 2, this.method >= 1, this.MbHeaderLimit);
 
             bool isSkipped = rd.Nz == 0;
             it.SetSkip(isSkipped);

src/ImageSharp/Formats/WebP/Lossy/Vp8Residual.cs

@@ -47,7 +47,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 }
             }
 
-            this.Coeffs = coeffs.ToArray();
+            this.Coeffs = coeffs.Slice(0, 16).ToArray();
         }
 
         // Simulate block coding, but only record statistics.