gistfile1.txt

var source = new Bitmap("sample.jpg");

            var pixelData = source.PlatformImpl.GetPixels();

            var stride = source.PixelWidth * 4;

            float value = 1f;

            IPixelFilter filter = new CompositeFilter(new ContrastFilter(10f), new GreenFilter());

            var start = DateTime.Now;
            unsafe
            {
                for (int y = 0; y < source.PixelHeight; ++y)
                {
                    byte* row = (byte*)pixelData + (y * stride);

                    int columnOffset = 0;

                    for (int x = 0; x < source.PixelWidth; ++x)
                    {
                        var (r, g, b, a) = filter.Transform((row[columnOffset], row[columnOffset + 1], row[columnOffset + 2], row[columnOffset + 3]));

                        row[columnOffset] = r;
                        row[columnOffset + 1] = g;
                        row[columnOffset + 2] = b;
                        row[columnOffset + 3] = a;

                        columnOffset += 4;
                    }
                }
            }
            var finish = DateTime.Now;

            Console.WriteLine($"{(finish - start).Milliseconds}");

            ImageSource = source;
            
            public interface IPixelFilter
    {
        (byte r, byte g, byte b, byte a) Transform((byte r, byte g, byte b, byte a) pixel);
    }

    public class ContrastFilter : IPixelFilter
    {
        public ContrastFilter (float contrastLevel)
        {
            this.contrastLevel = contrastLevel;
        }

        private float contrastLevel;

        public float ContrastLevel
        {
            get { return contrastLevel; }
            set { this.contrastLevel = value; }
        }

        public (byte r, byte g, byte b, byte a) Transform((byte r, byte g, byte b, byte a) pixel)
        {
            float Red = pixel.r / 255.0f;
            float Green = pixel.g / 255.0f;
            float Blue = pixel.b / 255.0f;
            Red = (((Red - 0.5f) * contrastLevel) + 0.5f) * 255.0f;
            Green = (((Green - 0.5f) * contrastLevel) + 0.5f) * 255.0f;
            Blue = (((Blue - 0.5f) * contrastLevel) + 0.5f) * 255.0f;

            int iR = (int)Red;
            iR = iR > 255 ? 255 : iR;
            iR = iR < 0 ? 0 : iR;
            int iG = (int)Green;
            iG = iG > 255 ? 255 : iG;
            iG = iG < 0 ? 0 : iG;
            int iB = (int)Blue;
            iB = iB > 255 ? 255 : iB;
            iB = iB < 0 ? 0 : iB;

            return ((byte)iR, (byte)iG, (byte)iB, pixel.a);
        }
    }

    public class GreenFilter : IPixelFilter
    {
        public (byte r, byte g, byte b, byte a) Transform((byte r, byte g, byte b, byte a) pixel)
        {
            return (0, pixel.g, 0, pixel.a);
        }
    }

    public class CompositeFilter : IPixelFilter
    {
        private List<IPixelFilter> _filters;

        public CompositeFilter(params IPixelFilter[] filters)
        {
            _filters = new List<IPixelFilter>(filters);
        }

        public (byte r, byte g, byte b, byte a) Transform((byte r, byte g, byte b, byte a) pixel)
        {
            foreach(var filter in _filters)
            {
                pixel = filter.Transform(pixel);
            }

            return pixel;
        }
    }