Add initial support for subsampeling in interleave mode sample

Author: vbaderks

src/JpegLSEncoder.cs

@@ -354,9 +354,11 @@ public void EncodeComponents(ReadOnlySpan<byte> source, int sourceComponentCount
         }
         else
         {
-            int scanStride = CheckStrideAndSourceLength(source.Length, stride, sourceComponentCount);
+            int scanWidth = GetScanWidth(_encodedComponentCount);
+            int scanHeight = GetScanHeight(_encodedComponentCount);
+            int scanStride = CheckStrideAndSourceLength(source.Length, stride, scanWidth, scanHeight, sourceComponentCount);
             _writer.WriteStartOfScanSegment(sourceComponentCount, NearLossless, InterleaveMode);
-            EncodeScan(source, scanStride, FrameInfo.Width, FrameInfo.Height, sourceComponentCount, explicitCodingParameters);
+            EncodeScan(source, scanStride, scanWidth, scanHeight, sourceComponentCount, explicitCodingParameters);
         }
 
         _encodedComponentCount += sourceComponentCount;
@@ -651,9 +653,9 @@ private void CheckInterleaveModeAgainstComponentCount(int componentCount)
             ThrowHelper.ThrowArgumentException(ErrorCode.InvalidArgumentInterleaveMode);
     }
 
-    private int CheckStrideAndSourceLength(int sourceLength, int stride, int sourceComponentCount)
+    private int CheckStrideAndSourceLength(int sourceLength, int stride, int scanWidth, int scanHeight, int sourceComponentCount)
     {
-        int minimumStride = CalculateMinimumStride(sourceComponentCount);
+        int minimumStride = CalculateMinimumStride(scanWidth, sourceComponentCount);
 
         if (stride == AutoCalculateStride)
         {
@@ -667,8 +669,8 @@ private int CheckStrideAndSourceLength(int sourceLength, int stride, int sourceC
 
         int notUsedBytesAtEnd = stride - minimumStride;
         int minimumSourceLength = InterleaveMode == InterleaveMode.None
-            ? (stride * sourceComponentCount * FrameInfo.Height) - notUsedBytesAtEnd
-            : (stride * FrameInfo.Height) - notUsedBytesAtEnd;
+            ? (stride * sourceComponentCount * scanHeight) - notUsedBytesAtEnd
+            : (stride * scanHeight) - notUsedBytesAtEnd;
 
         if (sourceLength < minimumSourceLength)
             ThrowHelper.ThrowArgumentException(ErrorCode.InvalidArgumentSize);
@@ -698,13 +700,20 @@ private int CheckStrideAndSourceLengthInterleaveModeNone(int sourceLength, int s
         return stride;
     }
 
-    private int CalculateMinimumStride(int sourceComponentCount)
+    private int CalculateMinimumStride(int scanWidth, int sourceComponentCount)
     {
-        int stride = FrameInfo.Width * BitToByteCount(FrameInfo.BitsPerSample);
-        if (_interleaveMode == InterleaveMode.None)
-            return stride;
+        switch (_interleaveMode)
+        {
+            case InterleaveMode.None:
+                return FrameInfo.Width * BitToByteCount(FrameInfo.BitsPerSample);
+
+            case InterleaveMode.Line:
+                return FrameInfo.Width * BitToByteCount(FrameInfo.BitsPerSample) * sourceComponentCount;
 
-        return stride * sourceComponentCount;
+            default:
+                Debug.Assert(_interleaveMode == InterleaveMode.Sample);
+                return scanWidth * BitToByteCount(FrameInfo.BitsPerSample) * sourceComponentCount;
+        }
     }
 
     private void DetermineMaxSamplingFactors()

test/EncodeTest.cs

@@ -439,6 +439,34 @@ public void EncodeSubSamplingInterleaveNone()
         CheckOutput(component2, destination[(2 * 2 * 2)..], decoder, 1, 2 * 1);
     }
 
+    [Fact]
+    public void EncodeSubSamplingInterleaveSample()
+    {
+        JpegLSEncoder encoder = new() { FrameInfo = new FrameInfo(2, 2, 8, 3), InterleaveMode = InterleaveMode.None };
+
+        Memory<byte> encodedData = new byte[encoder.EstimatedDestinationSize];
+        encoder.Destination = encodedData;
+
+        byte[] component0 = [24, 23, 22, 21];
+        byte[] component1And2 = [25, 26];
+
+        encoder.SetSamplingFactor(0, 2, 2);
+        encoder.SetSamplingFactor(1, 1, 1);
+        encoder.SetSamplingFactor(2, 1, 1);
+        encoder.EncodeComponents(component0, 1);
+        encoder.InterleaveMode = InterleaveMode.Sample;
+        encoder.EncodeComponents(component1And2, 2);
+
+        JpegLSDecoder decoder = new() { Source = encoder.EncodedData };
+        decoder.ReadHeader();
+
+        Span<byte> destination = new byte[decoder.GetDestinationSize()];
+        decoder.Decode(destination);
+
+        CheckOutput(component0, destination, decoder, 1, 2 * 2);
+        CheckOutput(component1And2, destination[(2 * 2)..], decoder, 1, 1 * 2);
+    }
+
     private static void CheckOutput(Span<byte> source, Span<byte> destination, JpegLSDecoder decoder, int componentCount, int componentSize)
     {
         for (int component = 0; component < componentCount; ++component)