它需要 System.Runtime.Intrinsics.Experimental.dll 并且不安全,但它相对简单,并且对于许多实际应用程序来说可能足够快。
/// <summary>Load 4 pixels of RGB</summary>
static unsafe Vector128<int> load4( byte* src )
{
return Sse2.LoadVector128( (int*)src );
}
/// <summary>Pack red channel of 8 pixels into ushort values in [ 0xFF00 .. 0 ] interval</summary>
static Vector128<ushort> packRed( Vector128<int> a, Vector128<int> b )
{
Vector128<int> mask = Vector128.Create( 0xFF );
a = Sse2.And( a, mask );
b = Sse2.And( b, mask );
return Sse2.ShiftLeftLogical128BitLane( Sse41.PackUnsignedSaturate( a, b ), 1 );
}
/// <summary>Pack green channel of 8 pixels into ushort values in [ 0xFF00 .. 0 ] interval</summary>
static Vector128<ushort> packGreen( Vector128<int> a, Vector128<int> b )
{
Vector128<int> mask = Vector128.Create( 0xFF00 );
a = Sse2.And( a, mask );
b = Sse2.And( b, mask );
return Sse41.PackUnsignedSaturate( a, b );
}
/// <summary>Pack blue channel of 8 pixels into ushort values in [ 0xFF00 .. 0 ] interval</summary>
static Vector128<ushort> packBlue( Vector128<int> a, Vector128<int> b )
{
a = Sse2.ShiftRightLogical128BitLane( a, 1 );
b = Sse2.ShiftRightLogical128BitLane( b, 1 );
Vector128<int> mask = Vector128.Create( 0xFF00 );
a = Sse2.And( a, mask );
b = Sse2.And( b, mask );
return Sse41.PackUnsignedSaturate( a, b );
}
/// <summary>Load 8 pixels, split into RGB channels.</summary>
static unsafe void loadRgb( byte* src, out Vector128<ushort> red, out Vector128<ushort> green, out Vector128<ushort> blue )
{
var a = load4( src );
var b = load4( src + 16 );
red = packRed( a, b );
green = packGreen( a, b );
blue = packBlue( a, b );
}
const ushort mulRed = (ushort)( 0.29891 * 0x10000 );
const ushort mulGreen = (ushort)( 0.58661 * 0x10000 );
const ushort mulBlue = (ushort)( 0.11448 * 0x10000 );
/// <summary>Compute brightness of 8 pixels</summary>
static Vector128<short> brightness( Vector128<ushort> r, Vector128<ushort> g, Vector128<ushort> b )
{
r = Sse2.MultiplyHigh( r, Vector128.Create( mulRed ) );
g = Sse2.MultiplyHigh( g, Vector128.Create( mulGreen ) );
b = Sse2.MultiplyHigh( b, Vector128.Create( mulBlue ) );
var result = Sse2.AddSaturate( Sse2.AddSaturate( r, g ), b );
return Vector128.AsInt16( Sse2.ShiftRightLogical( result, 8 ) );
}
/// <summary>Convert buffer from RGBA to grayscale.</summary>
/// <remarks>
/// <para>If your image has line paddings, you'll want to call this once per line, not for the complete image.</para>
/// <para>If width of the image is not multiple of 16 pixels, you'll need to do more work to handle the last few pixels of every line.</para>
/// </remarks>
static unsafe void convertToGrayscale( byte* src, byte* dst, int count )
{
byte* srcEnd = src + count * 4;
while( src < srcEnd )
{
loadRgb( src, out var r, out var g, out var b );
var low = brightness( r, g, b );
loadRgb( src + 32, out r, out g, out b );
var hi = brightness( r, g, b );
var bytes = Sse2.PackUnsignedSaturate( low, hi );
Sse2.Store( dst, bytes );
src += 64;
dst += 16;
}
}
然而,等效的 C++ 实现会更快。 C# 在内联这些函数方面做得不错,即convertToGrayscale
不包含任何函数调用。
但该函数的代码远非最佳。 .NET 无法传播常量,对于幻数,它在循环内发出如下代码:
mov r8d,962Ch
vmovd xmm1,r8d
vpbroadcastw xmm1,xmm1
生成的代码仅使用 16 个寄存器中的 6 个。有足够的可用寄存器来容纳所有涉及的幻数。
此外.NET 还发出许多冗余指令,这些指令只是对数据进行打乱:
vmovaps xmm2, xmm0
vmovaps xmm3, xmm1