Fix decoding very small lossy webp images

src/ImageSharp/Formats/WebP/Vp8Decoder.cs

@@ -50,12 +50,15 @@ namespace SixLabors.ImageSharp.Formats.WebP
             uint height = pictureHeader.Height;
 
             // TODO: use memory allocator
-            this.CacheY = new byte[width * height]; // TODO: this is way too much mem, figure out what the min req is.
+            int extraRows = WebPConstants.FilterExtraRows[2]; // TODO: assuming worst case: complex filter
-            this.CacheU = new byte[width * height];
+            int extraY = extraRows * this.CacheYStride;
-            this.CacheV = new byte[width * height];
+            int extraUv = (extraRows / 2) * this.CacheUvStride;
-            this.TmpYBuffer = new byte[width * height]; // TODO: figure out min buffer length
+            this.CacheY = new byte[width * height + extraY + 256]; // TODO: this is way too much mem, figure out what the min req is.
-            this.TmpUBuffer = new byte[width * height]; // TODO: figure out min buffer length
+            this.CacheU = new byte[width * height + extraUv + 256];
-            this.TmpVBuffer = new byte[width * height]; // TODO: figure out min buffer length
+            this.CacheV = new byte[width * height + extraUv + 256];
+            this.TmpYBuffer = new byte[width * height + extraY]; // TODO: figure out min buffer length
+            this.TmpUBuffer = new byte[width * height + extraUv]; // TODO: figure out min buffer length
+            this.TmpVBuffer = new byte[width * height + extraUv]; // TODO: figure out min buffer length
             this.Bgr = new byte[width * height * 4];
 
             for (int i = 0; i < this.YuvBuffer.Length; i++)
@@ -66,6 +69,10 @@ namespace SixLabors.ImageSharp.Formats.WebP
             for (int i = 0; i < this.CacheY.Length; i++)
             {
                 this.CacheY[i] = 205;
+            }
+
+            for (int i = 0; i < this.CacheU.Length; i++)
+            {
                 this.CacheU[i] = 205;
                 this.CacheV[i] = 205;
             }
@@ -82,7 +89,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         public Vp8SegmentHeader SegmentHeader { get; }
 
         // number of partitions minus one.
-        public uint NumPartsMinusOne { get; }
+        public int NumPartsMinusOne { get; set; }
 
         // per-partition boolean decoders.
         public Vp8BitReader[] Vp8BitReaders { get; }

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -452,13 +452,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 Span<byte> vOut = dec.CacheV.AsSpan(dec.CacheUvOffset + (mbx * 8));
                 for (int j = 0; j < 16; ++j)
                 {
-                    yDst.Slice(j * WebPConstants.Bps, 16).CopyTo(yOut.Slice(j * dec.CacheYStride));
+                    yDst.Slice(j * WebPConstants.Bps, Math.Min(16, yOut.Length)).CopyTo(yOut.Slice(j * dec.CacheYStride));
                 }
 
                 for (int j = 0; j < 8; ++j)
                 {
-                    uDst.Slice(j * WebPConstants.Bps, 8).CopyTo(uOut.Slice(j * dec.CacheUvStride));
+                    uDst.Slice(j * WebPConstants.Bps, Math.Min(8, uOut.Length)).CopyTo(uOut.Slice(j * dec.CacheUvStride));
-                    vDst.Slice(j * WebPConstants.Bps, 8).CopyTo(vOut.Slice(j * dec.CacheUvStride));
+                    vDst.Slice(j * WebPConstants.Bps, Math.Min(8, vOut.Length)).CopyTo(vOut.Slice(j * dec.CacheUvStride));
                 }
             }
         }
@@ -579,6 +579,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 yEnd -= extraYRows;
             }
 
+            if (yEnd > io.CropBottom)
+            {
+                yEnd = io.CropBottom; // make sure we don't overflow on last row.
+            }
+
             if (yStart < yEnd)
             {
                 io.Y = io.Y.Slice(io.CropLeft);
@@ -791,7 +796,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             else
             {
                 left.NoneZeroAcDcCoeffs = macroBlock.NoneZeroAcDcCoeffs = 0;
-                if (blockData.IsI4x4)
+                if (!blockData.IsI4x4)
                 {
                     left.NoneZeroDcCoeffs = macroBlock.NoneZeroDcCoeffs = 0;
                 }
@@ -917,8 +922,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             block.NonZeroUv = nonZeroUv;
 
             // We look at the mode-code of each block and check if some blocks have less
-            // than three non-zero coeffs (code < 2). This is to avoid dithering flat and
+            // than three non-zero coeffs (code < 2). This is to avoid dithering flat and empty blocks.
-            // empty blocks.
             block.Dither = (byte)((nonZeroUv & 0xaaaa) > 0 ? 0 : q.Dither);
 
             return (nonZeroY | nonZeroUv) is 0;
@@ -1161,8 +1165,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
             int startIdx = (int)this.bitReader.PartitionLength;
             Span<byte> sz = this.bitReader.Data.AsSpan(startIdx);
             int sizeLeft = (int)size;
-            int numPartsMinusOne = (1 << (int)this.bitReader.ReadValue(2)) - 1;
+            dec.NumPartsMinusOne = (1 << (int)this.bitReader.ReadValue(2)) - 1;
-            int lastPart = numPartsMinusOne;
+            int lastPart = dec.NumPartsMinusOne;
 
             int partStart = startIdx + (lastPart * 3);
             sizeLeft -= lastPart * 3;