.

src/Verify.Tests/Compare/Png/PngDecoderTests.cs

@@ -144,6 +144,52 @@ public void Large_256x256()
         Assert.Equal(rgba, image.Rgba);
     }
 
+    [Fact]
+    public void Small_GrayAlpha()
+    {
+        const int width = 4;
+        const int height = 4;
+        var ga = new byte[width * height * 2];
+        for (var i = 0; i < width * height; i++)
+        {
+            ga[i * 2] = (byte)(i * 16);
+            ga[i * 2 + 1] = (byte)(255 - i * 8);
+        }
+
+        var png = PngTestHelper.EncodeGrayAlpha(width, height, ga);
+        var image = PngDecoder.Decode(new MemoryStream(png));
+        Assert.Equal(width, image.Width);
+        Assert.Equal(height, image.Height);
+        for (var i = 0; i < width * height; i++)
+        {
+            var g = ga[i * 2];
+            Assert.Equal(g, image.Rgba[i * 4]);
+            Assert.Equal(g, image.Rgba[i * 4 + 1]);
+            Assert.Equal(g, image.Rgba[i * 4 + 2]);
+            Assert.Equal(ga[i * 2 + 1], image.Rgba[i * 4 + 3]);
+        }
+    }
+
+    [Fact]
+    public void Multiple_Idat_Chunks()
+    {
+        const int width = 32;
+        const int height = 32;
+        var rgba = new byte[width * height * 4];
+        new Random(7).NextBytes(rgba);
+        var png = PngTestHelper.EncodeRgbaMultipleIdat(width, height, rgba, chunkSize: 64);
+        var image = PngDecoder.Decode(new MemoryStream(png));
+        Assert.Equal(rgba, image.Rgba);
+    }
+
+    [Fact]
+    public void Rejects_Missing_Idat()
+    {
+        var png = PngTestHelper.EncodeWithoutIdat(1, 1);
+        var exception = Assert.Throws<Exception>(() => PngDecoder.Decode(new MemoryStream(png)));
+        Assert.Contains("IDAT", exception.Message);
+    }
+
     [Fact]
     public void Rejects_Bad_Signature()
     {

src/Verify.Tests/Compare/Png/PngTestHelper.cs

@@ -29,6 +29,13 @@ public static byte[] EncodeGray(int width, int height, byte[] gray)
         return BuildPng(ihdr, null, null, raw);
     }
 
+    public static byte[] EncodeGrayAlpha(int width, int height, byte[] grayAlpha)
+    {
+        var raw = AddFilterBytes(grayAlpha, width * 2, height);
+        var ihdr = BuildIhdr(width, height, colorType: 4);
+        return BuildPng(ihdr, null, null, raw);
+    }
+
     public static byte[] EncodePaletted(int width, int height, byte[] indices, byte[] palette, byte[]? trns = null)
     {
         var raw = AddFilterBytes(indices, width, height);
@@ -61,6 +68,34 @@ static byte[] BuildIhdr(int width, int height, byte colorType)
         return data;
     }
 
+    public static byte[] EncodeWithoutIdat(int width, int height)
+    {
+        var ihdr = BuildIhdr(width, height, colorType: 6);
+        using var stream = new MemoryStream();
+        stream.Write(signature, 0, signature.Length);
+        WriteChunk(stream, "IHDR", ihdr);
+        WriteChunk(stream, "IEND", []);
+        return stream.ToArray();
+    }
+
+    public static byte[] EncodeRgbaMultipleIdat(int width, int height, byte[] rgba, int chunkSize)
+    {
+        var raw = AddFilterBytes(rgba, width * 4, height);
+        var compressed = ZlibCompress(raw);
+        var ihdr = BuildIhdr(width, height, colorType: 6);
+        using var stream = new MemoryStream();
+        stream.Write(signature, 0, signature.Length);
+        WriteChunk(stream, "IHDR", ihdr);
+        for (var offset = 0; offset < compressed.Length; offset += chunkSize)
+        {
+            var length = Math.Min(chunkSize, compressed.Length - offset);
+            WriteChunk(stream, "IDAT", compressed.AsSpan(offset, length).ToArray());
+        }
+
+        WriteChunk(stream, "IEND", []);
+        return stream.ToArray();
+    }
+
     static byte[] BuildPng(byte[] ihdr, byte[]? plte, byte[]? trns, byte[] raw)
     {
         var compressed = ZlibCompress(raw);

src/Verify/Compare/Png/PngDecoder.cs

@@ -110,6 +110,11 @@ public static PngImage Decode(Stream stream)
                         throw new("PNG missing IHDR.");
                     }
 
+                    if (idat.Length == 0)
+                    {
+                        throw new("PNG missing IDAT.");
+                    }
+
                     ReadExact(stream, crc);
                     idat.Position = 0;
                     return Reconstruct(idat, width, height, colorType, palette, transparency);
@@ -128,6 +133,11 @@ public static PngImage Decode(Stream stream)
             throw new("PNG missing IHDR.");
         }
 
+        if (idat.Length == 0)
+        {
+            throw new("PNG missing IDAT.");
+        }
+
         idat.Position = 0;
         return Reconstruct(idat, width, height, colorType, palette, transparency);
     }