Added Argb IPackedPixel

.editorconfig

@@ -0,0 +1,3 @@
+[*.cs]
+indent_style = space
+indent_size = 4

README.md

@@ -58,6 +58,7 @@ git clone https://github.com/JimBobSquarePants/ImageSharp
  - [x] YCbCr
 - IPackedPixel\<TPacked\> representations of color models. Compatible with Microsoft XNA Game Studio and MonoGame.
  - [x] Alpha8 
+ - [x] Argb 
  - [x] Bgr565 
  - [x] Bgra444 
  - [x] Bgra565 

src/ImageSharp/Colors/PackedPixel/Argb.cs

@@ -0,0 +1,309 @@
+using System;
+using System.Numerics;
+
+namespace ImageSharp
+{
+    /// <summary>
+    /// Packed pixel type containing four 8-bit unsigned normalized values ranging from 0 to 255.
+    /// The color components are stored in alpha, red, green, and blue order.
+    /// </summary>
+    /// <remarks>
+    /// This struct is fully mutable. This is done (against the guidelines) for the sake of performance,
+    /// as it avoids the need to create new values for modification operations.
+    /// </remarks>
+    public struct Argb : IPackedPixel<uint>, IEquatable<Argb>
+    {
+        const int BlueShift = 0;
+        const uint BlueMask = 0xFFFFFF00;
+        const int GreenShift = 8;
+        const uint GreenMask = 0xFFFF00FF;
+        const int RedShift = 16;
+        const uint RedMask = 0xFF00FFFF;
+        const int AlphaShift = 24;
+        const uint AlphaMask = 0x00FFFFFF;
+
+        /// <summary>
+        /// The maximum byte value.
+        /// </summary>
+        readonly static Vector4 MaxBytes = new Vector4(255);
+
+        /// <summary>
+        /// The half vector value.
+        /// </summary>
+        readonly static Vector4 Half = new Vector4(0.5F);
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Color"/> struct.
+        /// </summary>
+        /// <param name="r">The red component.</param>
+        /// <param name="g">The green component.</param>
+        /// <param name="b">The blue component.</param>
+        /// <param name="a">The alpha component.</param>
+        public Argb(byte r, byte g, byte b, byte a = 255)
+        {
+            PackedValue = Pack(r, g, b, a);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Color"/> struct.
+        /// </summary>
+        /// <param name="r">The red component.</param>
+        /// <param name="g">The green component.</param>
+        /// <param name="b">The blue component.</param>
+        /// <param name="a">The alpha component.</param>
+        public Argb(float r, float g, float b, float a = 1)
+        {
+            PackedValue = Pack(r, g, b, a);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Color"/> struct.
+        /// </summary>
+        /// <param name="vector">
+        /// The vector containing the components for the packed vector.
+        /// </param>
+        public Argb(Vector3 vector)
+        {
+            PackedValue = Pack(ref vector);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Color"/> struct.
+        /// </summary>
+        /// <param name="vector">
+        /// The vector containing the components for the packed vector.
+        /// </param>
+        public Argb(Vector4 vector)
+        {
+            PackedValue = Pack(ref vector);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Color"/> struct.
+        /// </summary>
+        /// <param name="packed">
+        /// The packed value.
+        /// </param>
+        public Argb(uint packed = 0)
+        {
+            PackedValue = packed;
+        }
+        /// <summary>
+        /// Gets or sets the red component.
+        /// </summary>
+        public byte R
+        {
+            get
+            {
+                return (byte)(PackedValue >> RedShift);
+            }
+
+            set
+            {
+                PackedValue = PackedValue & RedMask | (uint)value << RedShift;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the green component.
+        /// </summary>
+        public byte G
+        {
+            get
+            {
+                return (byte)(PackedValue >> GreenShift);
+            }
+
+            set
+            {
+                PackedValue = PackedValue & GreenMask | (uint)value << GreenShift;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the blue component.
+        /// </summary>
+        public byte B
+        {
+            get
+            {
+                return (byte)(PackedValue >> BlueShift);
+            }
+
+            set
+            {
+                PackedValue = PackedValue & BlueMask | (uint)value << BlueShift;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the alpha component.
+        /// </summary>
+        public byte A
+        {
+            get
+            {
+                return (byte)(PackedValue >> AlphaShift);
+            }
+
+            set
+            {
+                PackedValue = PackedValue & AlphaMask | (uint)value << AlphaShift;
+            }
+        }
+
+        /// <inheritdoc/>
+        public void PackFromVector4(Vector4 vector)
+        {
+            PackedValue = Pack(ref vector);
+        }
+
+        /// <inheritdoc/>
+        public Vector4 ToVector4()
+        {
+            return new Vector4(R, G, B, A) / MaxBytes;
+        }
+
+        /// <inheritdoc/>
+        public uint PackedValue { get; set; }
+
+        /// <inheritdoc/>
+        public void PackFromBytes(byte x, byte y, byte z, byte w)
+        {
+            PackedValue = Pack(x, y, z, w);
+        }
+
+        /// <summary>
+        /// Packs the four floats into a <see cref="uint"/>.
+        /// </summary>
+        /// <param name="x">The x-component</param>
+        /// <param name="y">The y-component</param>
+        /// <param name="z">The z-component</param>
+        /// <param name="w">The w-component</param>
+        /// <returns>The <see cref="uint"/></returns>
+        static uint Pack(float x, float y, float z, float w)
+        {
+            var value = new Vector4(x, y, z, w);
+            return Pack(ref value);
+        }
+        /// <summary>
+        /// Packs the four floats into a <see cref="uint"/>.
+        /// </summary>
+        /// <param name="x">The x-component</param>
+        /// <param name="y">The y-component</param>
+        /// <param name="z">The z-component</param>
+        /// <param name="w">The w-component</param>
+        /// <returns>The <see cref="uint"/></returns>
+        static uint Pack(byte x, byte y, byte z, byte w)
+        {
+            return (uint)(x << RedShift | y << GreenShift | z << BlueShift | w << AlphaShift);
+        }
+
+        /// <summary>
+        /// Packs a <see cref="Vector3"/> into a uint.
+        /// </summary>
+        /// <param name="vector">The vector containing the values to pack.</param>
+        /// <returns>The <see cref="uint"/> containing the packed values.</returns>
+        static uint Pack(ref Vector3 vector)
+        {
+            var value = new Vector4(vector, 1);
+            return Pack(ref value);
+        }
+
+        /// <summary>
+        /// Packs a <see cref="Vector4"/> into a uint.
+        /// </summary>
+        /// <param name="vector">The vector containing the values to pack.</param>
+        /// <returns>The <see cref="uint"/> containing the packed values.</returns>
+        static uint Pack(ref Vector4 vector)
+        {
+            vector = Vector4.Clamp(vector, Vector4.Zero, Vector4.One);
+            vector *= MaxBytes;
+            vector += Half;
+            return (uint)(((byte)vector.X << RedShift)
+                        | ((byte)vector.Y << GreenShift)
+                        | ((byte)vector.Z << BlueShift)
+                        | (byte)vector.W << AlphaShift);
+        }
+
+        /// <inheritdoc/>
+        public void ToBytes(byte[] bytes, int startIndex, ComponentOrder componentOrder)
+        {
+            switch(componentOrder) {
+            case ComponentOrder.ZYX:
+                bytes[startIndex] = B;
+                bytes[startIndex + 1] = G;
+                bytes[startIndex + 2] = R;
+                break;
+            case ComponentOrder.ZYXW:
+                bytes[startIndex] = B;
+                bytes[startIndex + 1] = G;
+                bytes[startIndex + 2] = R;
+                bytes[startIndex + 3] = A;
+                break;
+            case ComponentOrder.XYZ:
+                bytes[startIndex] = R;
+                bytes[startIndex + 1] = G;
+                bytes[startIndex + 2] = B;
+                break;
+            case ComponentOrder.XYZW:
+                bytes[startIndex] = R;
+                bytes[startIndex + 1] = G;
+                bytes[startIndex + 2] = B;
+                bytes[startIndex + 3] = A;
+                break;
+            default:
+                throw new NotSupportedException();
+            }
+        }
+
+        /// <summary>
+        /// Compares two <see cref="Color"/> objects for equality.
+        /// </summary>
+        /// <param name="left">
+        /// The <see cref="Color"/> on the left side of the operand.
+        /// </param>
+        /// <param name="right">
+        /// The <see cref="Color"/> on the right side of the operand.
+        /// </param>
+        /// <returns>
+        /// True if the <paramref name="left"/> parameter is equal to the <paramref name="right"/> parameter; otherwise, false.
+        /// </returns>
+        public static bool operator ==(Argb left, Argb right)
+        {
+            return left.PackedValue == right.PackedValue;
+        }
+
+        /// <summary>
+        /// Compares two <see cref="Color"/> objects for equality.
+        /// </summary>
+        /// <param name="left">The <see cref="Color"/> on the left side of the operand.</param>
+        /// <param name="right">The <see cref="Color"/> on the right side of the operand.</param>
+        /// <returns>
+        /// True if the <paramref name="left"/> parameter is not equal to the <paramref name="right"/> parameter; otherwise, false.
+        /// </returns>
+        public static bool operator !=(Argb left, Argb right)
+        {
+            return left.PackedValue != right.PackedValue;
+        }
+
+        /// <inheritdoc/>
+		public override bool Equals(object obj)
+        {
+            return obj is Argb && Equals((Argb)obj);
+        }
+
+        /// <inheritdoc/>
+        public bool Equals(Argb other)
+        {
+            return PackedValue == other.PackedValue;
+        }
+
+        /// <inheritdoc/>
+        public override int GetHashCode()
+        {
+            // ReSharper disable once NonReadonlyMemberInGetHashCode
+            return PackedValue.GetHashCode();
+        }
+    }
+}

tests/ImageSharp.Tests/Colors/PackedPixelTests.cs

@@ -56,6 +56,51 @@ public void Alpha8()
             Assert.Equal(bgra, new byte[] { 0, 0, 0, 128 });
         }
 
+        [Fact]
+        public void Argb()
+        {
+            // Test the limits.
+            Assert.Equal((uint)0x0, new Argb(Vector4.Zero).PackedValue);
+            Assert.Equal(0xFFFFFFFF, new Argb(Vector4.One).PackedValue);
+
+            // Test ToVector4.
+            Assert.True(Equal(Vector4.One,   new Argb(Vector4.One  ).ToVector4()));
+            Assert.True(Equal(Vector4.Zero,  new Argb(Vector4.Zero ).ToVector4()));
+            Assert.True(Equal(Vector4.UnitX, new Argb(Vector4.UnitX).ToVector4()));
+            Assert.True(Equal(Vector4.UnitY, new Argb(Vector4.UnitY).ToVector4()));
+            Assert.True(Equal(Vector4.UnitZ, new Argb(Vector4.UnitZ).ToVector4()));
+            Assert.True(Equal(Vector4.UnitW, new Argb(Vector4.UnitW).ToVector4()));
+
+            // Test clamping.
+            Assert.True(Equal(Vector4.Zero, new Argb(Vector4.One * -1234.0f).ToVector4()));
+            Assert.True(Equal(Vector4.One,  new Argb(Vector4.One * +1234.0f).ToVector4()));
+
+            var x    = +0.1f;
+            var y    = -0.3f;
+            var z    = +0.5f;
+            var w    = -0.7f;
+            var argb = new Argb(x, y, z, w);
+            Assert.Equal(0x001a0080u, argb.PackedValue);
+
+            // Test ordering
+            byte[] rgb  = new byte[3];
+            byte[] rgba = new byte[4];
+            byte[] bgr  = new byte[3];
+            byte[] bgra = new byte[4];
+
+            argb.ToBytes(rgb, 0, ComponentOrder.XYZ);
+            Assert.Equal(rgb, new byte[] { 0x1a, 0, 0x80 });
+
+            argb.ToBytes(rgba, 0, ComponentOrder.XYZW);
+            Assert.Equal(rgba, new byte[] { 0x1a, 0, 0x80, 0 });
+
+            argb.ToBytes(bgr, 0, ComponentOrder.ZYX);
+            Assert.Equal(bgr, new byte[] { 0x80, 0, 0x1a });
+
+            argb.ToBytes(bgra, 0, ComponentOrder.ZYXW);
+            Assert.Equal(bgra, new byte[] { 0x80, 0, 0x1a, 0 });
+        }
+
         [Fact]
         public void Bgr565()
         {