Correct AV1 intra prediction

HEIF_IMPLEMENTATION_PLAN.md

@@ -58,6 +58,7 @@ This snapshot pins or classifies the available references and failures; it does
 | --- | --- | --- | --- |
 | `Av1YuvConverter.ConvertToRgb`, `ConvertFromRgb`, scalar row conversion, and chroma reconstruction | H.273 formulas 20-31 and the identity, YCgCo, and non-constant-luminance matrix formulas; AV1 section 6.4.2 chroma sample positions | libavif `src/reformat.c` and `src/colr.c` at `092276ce89098ead06db80975173191e5fee1826`; libaom `aom/aom_image.h` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Scalar behavioral oracle for 8-bit full/limited-range conversion. Decode covers monochrome, YUV 4:2:0, 4:2:2, and 4:4:4 with AV1 chroma sample positioning; encode remains YUV 4:4:4 at this snapshot. Later high-bit-depth and SIMD paths must match it. |
 | `Av1FrameBuffer` high-bit-depth sample layout and `Av1YuvConverter` 10/12-bit output conversion | AV1 section 6.4.1 bit depth and H.273 sample-range scaling | libaom `aom_scale/yv12config.h`, `av1/common/idct.c`, and `av1/common/reconintra.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f`; libavif `src/avif.c` and `src/reformat.c` at `092276ce89098ead06db80975173191e5fee1826` | Establish two-byte native sample storage with sample-unit strides for 10/12-bit reconstruction and use the same scalar color model at every supported bit depth. |
+| `Av1PredictionDecoder` and the scalar DC, directional, Paeth, and smooth intra predictors | AV1 section 7.11.2 intra prediction | libaom `aom_dsp/intrapred.c` and `av1/common/reconintra.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Behavioral oracle for neighbor addressing, directional upsampling, Paeth selection, one-axis smooth normalization, and chroma-from-luma row strides. Existing managed scalar predictors remain the implementation base. The WIP rectangular smooth digest expectations encode width/height-swapped weights and must be replaced only from an independently generated oracle, not regenerated from this implementation. |
 
 This table is intentionally incomplete. Add a row before each additional AV1 or HEVC algorithm is ported or materially reshaped.
 

src/ImageSharp/Formats/Heif/Av1/Prediction/Av1DirectionalZone2Predictor.cs

@@ -26,7 +26,7 @@ internal class Av1DirectionalZone2Predictor
     }
 
     public static void PredictScalar(Av1TransformSize transformSize, Span<byte> destination, nuint stride, Span<byte> above, Span<byte> left, bool upsampleAbove, bool upsampleLeft, int dx, int dy)
-        => new Av1DirectionalZone2Predictor(transformSize).PredictScalar(destination, stride, above, left, upsampleAbove, upsampleAbove, dx, dy);
+        => new Av1DirectionalZone2Predictor(transformSize).PredictScalar(destination, stride, above, left, upsampleAbove, upsampleLeft, dx, dy);
 
     /// <summary>
     /// SVT: svt_av1_dr_prediction_z1_c

src/ImageSharp/Formats/Heif/Av1/Prediction/Av1PaethPredictor.cs

@@ -24,7 +24,7 @@ internal class Av1PaethPredictor : IAv1Predictor
     }
 
     public static void PredictScalar(Av1TransformSize transformSize, Span<byte> destination, nuint stride, Span<byte> above, Span<byte> left)
-        => new Av1DcPredictor(transformSize).PredictScalar(destination, stride, above, left);
+        => new Av1PaethPredictor(transformSize).PredictScalar(destination, stride, above, left);
 
     public void PredictScalar(Span<byte> destination, nuint stride, Span<byte> above, Span<byte> left)
     {
@@ -34,15 +34,21 @@ internal class Av1PaethPredictor : IAv1Predictor
         Guard.MustBeSizedAtLeast(destination, (int)this.blockHeight * (int)stride, nameof(destination));
         ref byte leftRef = ref left[0];
         ref byte aboveRef = ref above[0];
-        int yTopLeft = above[-1];
+
+        // The caller preserves the top-left sample immediately before the top row.
+        int yTopLeft = Unsafe.Subtract(ref aboveRef, 1);
         ref byte destinationRef = ref destination[0];
         for (nuint r = 0; r < this.blockHeight; r++)
         {
             for (nuint c = 0; c < this.blockWidth; c++)
             {
-                destinationRef = PredictSingle(Unsafe.Add(ref leftRef, r), Unsafe.Add(ref aboveRef, c), yTopLeft);
-                destinationRef = ref Unsafe.Add(ref destinationRef, stride);
+                Unsafe.Add(ref destinationRef, c) = PredictSingle(
+                    Unsafe.Add(ref leftRef, r),
+                    Unsafe.Add(ref aboveRef, c),
+                    yTopLeft);
             }
+
+            destinationRef = ref Unsafe.Add(ref destinationRef, stride);
         }
     }
 

src/ImageSharp/Formats/Heif/Av1/Prediction/Av1PredictionDecoder.cs

@@ -38,8 +38,7 @@ internal class Av1PredictionDecoder
         int blockModeInfoColumnOffset,
         int blockModeInfoRowOffset)
     {
-        int bytesPerPixel = (bitDepth == Av1BitDepth.EightBit && !this.is16BitPipeline) ? 2 : 1;
-        int stride = pixelStride * bytesPerPixel;
+        int stride = pixelStride;
 
         // Deviation from SVT: Buffer starts at PREVIOUS row.
         Span<byte> topNeighbor = pixelBuffer;
@@ -190,9 +189,9 @@ internal class Av1PredictionDecoder
                 destinationBuffer[i] = (byte)Av1Math.Clamp(alphaQ0 + predictedBuffer[i], 0, maxPixelValue);
             }
 
-            destinationBuffer = destinationBuffer[width..];
-            predictedBuffer = predictedBuffer[width..];
-            predictedBufferQ3 = predictedBufferQ3[width..];
+            destinationBuffer = destinationBuffer[destinationStride..];
+            predictedBuffer = predictedBuffer[predictedStride..];
+            predictedBufferQ3 = predictedBufferQ3[32..];
         }
     }
 

src/ImageSharp/Formats/Heif/Av1/Prediction/Av1SmoothHorizontalPredictor.cs

@@ -41,8 +41,7 @@ internal class Av1SmoothHorizontalPredictor : IAv1Predictor
         int rightPrediction = Unsafe.Add(ref aboveRef, this.blockWidth - 1); // estimated by top-right pixel
         ref int weights = ref Av1SmoothPredictor.Weights[(int)this.blockWidth];
 
-        // scale = 2 * 2^sm_weight_log2_scale
-        int log2Scale = 1 + Av1SmoothPredictor.WeightLog2Scale;
+        int log2Scale = Av1SmoothPredictor.WeightLog2Scale;
         int scale = 1 << Av1SmoothPredictor.WeightLog2Scale;
 
         // sm_weights_sanity_checks(sm_weights_w, sm_weights_h, scale, log2_scale + 2);

src/ImageSharp/Formats/Heif/Av1/Prediction/Av1SmoothVerticalPredictor.cs

@@ -41,8 +41,7 @@ internal class Av1SmoothVerticalPredictor : IAv1Predictor
         int belowPrediction = Unsafe.Add(ref leftRef, this.blockHeight - 1); // estimated by bottom-left pixel
         ref int weights = ref Av1SmoothPredictor.Weights[(int)this.blockHeight];
 
-        // scale = 2 * 2^sm_weight_log2_scale
-        int log2Scale = 1 + Av1SmoothPredictor.WeightLog2Scale;
+        int log2Scale = Av1SmoothPredictor.WeightLog2Scale;
         int scale = 1 << Av1SmoothPredictor.WeightLog2Scale;
 
         // sm_weights_sanity_checks(sm_weights_w, sm_weights_h, scale, log2_scale + 2);

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1PredictorTests.cs

@@ -262,6 +262,112 @@ public class Av1PredictorTests
         Assert.Equal(expectedDigest, predictorMemory.GetDestinationDigest());
     }
 
+    [Fact]
+    public void PaethFactoryUsesNearestNeighborPrediction()
+    {
+        byte[] destination = new byte[16];
+        byte[] aboveData = [50, 60, 10, 90, 40];
+        Span<byte> above = aboveData.AsSpan(1);
+        byte[] left = [20, 80, 30, 100];
+        byte[] expected =
+        [
+            20, 10, 50, 20,
+            80, 50, 90, 80,
+            30, 10, 90, 30,
+            100, 50, 100, 100,
+        ];
+
+        Av1PredictorFactory.GeneralPredictor(
+            Av1PredictionMode.Paeth,
+            Av1TransformSize.Size4x4,
+            destination,
+            4,
+            above,
+            left);
+
+        Assert.Equal(expected, destination);
+    }
+
+    [Fact]
+    public void SmoothHorizontalUsesSingleAxisNormalization()
+    {
+        byte[] destination = new byte[16];
+        byte[] above = [20, 40, 60, 80];
+        byte[] left = [20, 40, 60, 80];
+        byte[] expected =
+        [
+            20, 45, 60, 65,
+            40, 57, 67, 70,
+            60, 68, 73, 75,
+            80, 80, 80, 80,
+        ];
+
+        Av1SmoothHorizontalPredictor.PredictScalar(
+            Av1TransformSize.Size4x4,
+            destination,
+            4,
+            above,
+            left);
+
+        Assert.Equal(expected, destination);
+    }
+
+    [Fact]
+    public void SmoothVerticalUsesSingleAxisNormalization()
+    {
+        byte[] destination = new byte[16];
+        byte[] above = [20, 40, 60, 80];
+        byte[] left = [20, 40, 60, 80];
+        byte[] expected =
+        [
+            20, 40, 60, 80,
+            45, 57, 68, 80,
+            60, 67, 73, 80,
+            65, 70, 75, 80,
+        ];
+
+        Av1SmoothVerticalPredictor.PredictScalar(
+            Av1TransformSize.Size4x4,
+            destination,
+            4,
+            above,
+            left);
+
+        Assert.Equal(expected, destination);
+    }
+
+    [Fact]
+    public void DirectionalZone2StaticPredictorForwardsLeftUpsampling()
+    {
+        byte[] actual = new byte[16];
+        byte[] expected = new byte[16];
+        byte[] aboveData = new byte[128];
+        byte[] leftData = new byte[128];
+        for (int i = 0; i < aboveData.Length; i++)
+        {
+            aboveData[i] = (byte)((i * 5) + 1);
+            leftData[i] = (byte)((i * 7) + 3);
+        }
+
+        Span<byte> above = aboveData.AsSpan(64);
+        Span<byte> left = leftData.AsSpan(64);
+        Av1DirectionalZone2Predictor predictor = new(Av1TransformSize.Size4x4);
+        predictor.PredictScalar(expected, 4, above, left, false, true, 64, 64);
+
+        Av1DirectionalZone2Predictor.PredictScalar(
+            Av1TransformSize.Size4x4,
+            actual,
+            4,
+            above,
+            left,
+            false,
+            true,
+            64,
+            64);
+
+        Assert.Equal(expected, actual);
+    }
+
     private static void AssertValue(byte expected, byte actual)
     {
         Assert.NotEqual(0, actual);