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/// Copyright (c) 2008 Jeffrey Powers for Fluxcapacity Open Source.
/// Under the MIT License, details: License.txt..
// NOTE: Compile with DYNAMIC_IDCT for a decode performance boost.
// May not yield a perceptible boost for small images,
// since there is some overhead in emitting CIL dynamically.
using System;
using System.Reflection.Emit;
using System.Reflection;
namespace FluxJpeg.Core
{
/// <summary>
/// Implements the Discrete Cosine Transform with dynamic CIL
/// </summary>
public partial class DCT
{
private float[] _temp = new float[64];
// Cosine matrix and transposed cosine matrix
private static readonly float[,] c = buildC();
private static readonly float[,] cT = buildCT();
internal DCT()
{
#if DYNAMIC_IDCT
dynamicIDCT = dynamicIDCT ?? EmitIDCT();
#endif
}
/// <summary>
/// Precomputes cosine terms in A.3.3 of
/// http://www.w3.org/Graphics/JPEG/itu-t81.pdf
///
/// Closely follows the term precomputation in the
/// Java Advanced Imaging library.
/// </summary>
private static float[,] buildC()
{
float[,] c = new float[8, 8];
for (int i = 0; i < 8; i++) // i == u or v
{
for (int j = 0; j < 8; j++) // j == x or y
{
c[i, j] = i == 0 ?
0.353553391f : /* 1 / SQRT(8) */
(float)(0.5 * Math.Cos(((2.0 * j + 1) * i * Math.PI) / 16.0));
}
}
return c;
}
private static float[,] buildCT()
{
// Transpose i,k <-- j,i
float[,] cT = new float[8, 8];
for (int i = 0; i < 8; i++)
for (int j = 0; j < 8; j++)
cT[j, i] = c[i, j];
return cT;
}
public static void SetValueClipped(byte[,] arr, int i, int j, float val)
{
// Clip into the 0...255 range & round
arr[i, j] = val < 0 ? (byte)0
: val > 255 ? (byte)255
: (byte)(val + 0.5);
}
/// See figure A.3.3 IDCT (informative) on A-5.
/// http://www.w3.org/Graphics/JPEG/itu-t81.pdf
internal byte[,] FastIDCT(float[] input)
{
byte[,] output = new byte[8, 8];
#if DYNAMIC_IDCT
// Fastest, dynamic MSIL stream
dynamicIDCT(input, _temp, output);
#else
#region Slower, easy-to-read, pure C# IDCT
float temp, val = 0;
int idx = 0;
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
val = 0;
for(int k = 0; k < 8; k++)
{
val += input[i * 8 + k] * c[k, j];
}
_temp[idx++] = val;
}
}
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
temp = 128f;
for (int k = 0; k < 8; k++)
{
temp += cT[i, k] * _temp[k * 8 + j];
}
if (temp < 0) output[i, j] = 0;
else if (temp > 255) output[i, j] = 255;
else output[i, j] = (byte)(temp + 0.5); // Implements rounding
}
}
#endregion
#endif
return output;
}
#if DYNAMIC_IDCT
/// <summary>
/// Generates a pure-IL nonbranching stream of instructions
/// that perform the inverse DCT. Relies on helper function
/// SetValueClipped.
/// </summary>
/// <returns>A delegate to the DynamicMethod</returns>
private static IDCTFunc EmitIDCT()
{
Type[] args = { typeof(float[]), typeof(float[]), typeof(byte[,]) };
DynamicMethod idctMethod = new DynamicMethod("dynamicIDCT",
null, // no return type
args); // input arrays
ILGenerator il = idctMethod.GetILGenerator();
int idx = 0;
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
il.Emit(OpCodes.Ldarg_1); // 1 {temp}
il.Emit(OpCodes.Ldc_I4_S, (short)idx++); // 3 {temp, idx}
for (int k = 0; k < 8; k++)
{
il.Emit(OpCodes.Ldarg_0); // {in}
il.Emit(OpCodes.Ldc_I4_S, (short)(i * 8 + k)); // {in,idx}
il.Emit(OpCodes.Ldelem_R4); // {in[idx]}
il.Emit(OpCodes.Ldc_R4, c[k, j]); // {in[idx],c[k,j]}
il.Emit(OpCodes.Mul); // {in[idx]*c[k,j]}
if (k != 0) il.Emit(OpCodes.Add);
}
il.Emit(OpCodes.Stelem_R4); // {}
}
}
var meth = typeof(DCT).GetMethod("SetValueClipped",
BindingFlags.Static | BindingFlags.Public, null,
CallingConventions.Standard,
new Type[] {
typeof(byte[,]), // arr
typeof(int), // i
typeof(int), // j
typeof(float) } // val
, null);
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
il.Emit(OpCodes.Ldarg_2); // {output}
il.Emit(OpCodes.Ldc_I4_S, (short)i); // {output,i}
il.Emit(OpCodes.Ldc_I4_S, (short)j); // X={output,i,j}
il.Emit(OpCodes.Ldc_R4, 128.0f); // {X,128.0f}
for (int k = 0; k < 8; k++)
{
il.Emit(OpCodes.Ldarg_1); // {X,temp}
il.Emit(OpCodes.Ldc_I4_S,
(short)(k * 8 + j)); // {X,temp,idx}
il.Emit(OpCodes.Ldelem_R4); // {X,temp[idx]}
il.Emit(OpCodes.Ldc_R4, cT[i, k]); // {X,temp[idx],cT[i,k]}
il.Emit(OpCodes.Mul); // {X,in[idx]*c[k,j]}
il.Emit(OpCodes.Add);
}
il.EmitCall(OpCodes.Call, meth, null);
}
}
il.Emit(OpCodes.Ret);
return (IDCTFunc)idctMethod.CreateDelegate(typeof(IDCTFunc));
}
private delegate void IDCTFunc(float[] input, float[] temp, byte[,] output);
private static IDCTFunc dynamicIDCT = null;
#endif
}
}
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