Apply color conversion when decoding PNG

Author: Socolin

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -7,9 +7,12 @@
 using System.Globalization;
 using System.IO.Compression;
 using System.IO.Hashing;
+using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
+using SixLabors.ImageSharp.ColorProfiles;
+using SixLabors.ImageSharp.ColorProfiles.Icc;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Compression.Zlib;
 using SixLabors.ImageSharp.Formats.Png.Chunks;
@@ -323,6 +326,11 @@ protected override Image<TPixel> Decode<TPixel>(BufferedReadStream stream, Cance
                 PngThrowHelper.ThrowNoData();
             }
 
+            if (this.Options.TryGetIccProfileForColorConversion(metadata.IccProfile, out IccProfile? iccProfile))
+            {
+                ApplyIccProfile(image, iccProfile, CompactSrgbV4Profile.Profile);
+            }
+
             return image;
         }
         catch
@@ -2153,4 +2161,58 @@ private static bool IsXmpTextData(ReadOnlySpan<byte> keywordBytes)
 
     private void SwapScanlineBuffers()
         => (this.scanline, this.previousScanline) = (this.previousScanline, this.scanline);
+
+    // FIXME: Maybe this could be a .Mutate(x => x.ApplyIccProfile(destinationProfile)) ? Nothing related to png here
+    private static void ApplyIccProfile<TPixel>(Image<TPixel> image, IccProfile sourceProfile, IccProfile destinationProfile)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        ColorConversionOptions options = new()
+        {
+            SourceIccProfile = sourceProfile,
+            TargetIccProfile = destinationProfile,
+        };
+
+        ColorProfileConverter converter = new(options);
+
+        image.ProcessPixelRows(pixelAccessor =>
+            {
+                using IMemoryOwner<float> rgbBuffer = image.Configuration.MemoryAllocator.Allocate<float>(pixelAccessor.Width * 3);
+                using IMemoryOwner<float> alphaBuffer = image.Configuration.MemoryAllocator.Allocate<float>(pixelAccessor.Width);
+                Span<float> rgbPacked = rgbBuffer.Memory.Span;
+                ref float rgbPackedRef = ref MemoryMarshal.GetReference(rgbPacked);
+                Span<float> alphaPacked = alphaBuffer.Memory.Span;
+                ref float alphaPackedRef = ref MemoryMarshal.GetReference(alphaPacked);
+
+                for (int y = 0; y < pixelAccessor.Height; y++)
+                {
+                    Span<TPixel> pixelsRow = pixelAccessor.GetRowSpan(y);
+                    int rgbIdx = 0;
+                    for (int x = 0; x < pixelsRow.Length; x++, rgbIdx += 3)
+                    {
+                        Vector4 rgba = pixelsRow[x].ToScaledVector4();
+                        Unsafe.Add(ref rgbPackedRef, rgbIdx) = rgba.X;
+                        Unsafe.Add(ref rgbPackedRef, rgbIdx + 1) = rgba.Y;
+                        Unsafe.Add(ref rgbPackedRef, rgbIdx + 2) = rgba.Z;
+                        Unsafe.Add(ref alphaPackedRef, x) = rgba.W;
+                    }
+
+                    Span<Rgb> source = MemoryMarshal.Cast<float, Rgb>(rgbPacked);
+                    Span<Rgb> destination = MemoryMarshal.Cast<float, Rgb>(rgbPacked);
+                    converter.Convert<Rgb, Rgb>(source, destination);
+
+                    rgbIdx = 0;
+                    for (int x = 0; x < pixelsRow.Length; x++, rgbIdx += 3)
+                    {
+                        float r = Unsafe.Add(ref rgbPackedRef, rgbIdx);
+                        float g = Unsafe.Add(ref rgbPackedRef, rgbIdx + 1);
+                        float b = Unsafe.Add(ref rgbPackedRef, rgbIdx + 2);
+                        float a = Unsafe.Add(ref alphaPackedRef, x);
+
+                        pixelsRow[x] = TPixel.FromScaledVector4(new Vector4(r, g, b, a));
+                    }
+                }
+            }
+        );
+    }
+
 }