Renderscript 中的直方图匹配

Question

为了对齐两个灰度图像的强度值（作为进一步处理的第一步），我编写了一个 Java 方法：

1. 将两个图像的位图转换为两个包含位图强度的int[] 数组（我这里只取红色分量，因为它是灰度的，即 r=g=b ）。

   public static int[] bmpToData(Bitmap bmp){
   int width = bmp.getWidth();
   int height = bmp.getHeight();
   int anzpixel = width*height;
   int [] pixels = new int[anzpixel];
   int [] data = new int[anzpixel];  
   bmp.getPixels(pixels, 0, width, 0, 0, width, height);
   for (int i = 0 ; i < anzpixel ; i++) {
   int p = pixels[i];
   int r = (p & 0xff0000) >> 16;
   //int g = (p & 0xff00) >> 8;
   //int b = p & 0xff;
   data[i] = r;
   }
   return data;
   }
   

2. 将位图 2 的累积强度分布与位图 1 的累积强度分布对齐

       //aligns the intensity distribution of a grayscale picture moving    (given by int[] //data2) the the intensity distribution of a reference picture fixed (given by // int[] data1)
   public static int[] histMatch(int[] data1, int[] data2){
   
      int anzpixel = data1.length;
      int[] histogram_fixed = new int[256];
      int[] histogram_moving = new int[256];
      int[] cumhist_fixed = new int[256];
      int[] cumhist_moving = new int[256];
      int i=0;
      int j=0;
   
      //read intensities of fixed und moving in histogram
      for (int n = 0; n < anzpixel; n++) {
         histogram_fixed[data1[n]]++;
         histogram_moving[data2[n]]++;
      }
   
      // calc cumulated distributions
      cumhist_fixed[0]=histogram_fixed[0];
      cumhist_moving[0]=histogram_moving[0];
      for ( i=1; i < 256; ++i ) {
         cumhist_fixed[i] = cumhist_fixed[i-1]+histogram_fixed[i];
         cumhist_moving[i] = cumhist_moving[i-1]+histogram_moving [i];
      }
   
      // look-up-table lut[]. For each quantile i of the moving picture search     the 
      // value j of the fixed picture where the quantile is the same as that of moving   
      int[] lut = new int[anzpixel];
      j=0;
      for ( i=0; i < 256; ++i ){
         while(cumhist_fixed[j]< cumhist_moving[i]){
            j++;
         }
   
         // check, whether the distance to the next-lower intensity is even lower, and if so, take this value
         if ((j!=0) && ((cumhist_fixed[j-1]- cumhist_fixed[i]) < (cumhist_fixed[j]- cumhist_fixed[i]))){
            lut[i]= (j-1);
         }
         else {
            lut[i]= (j);
         }
      }
   
      // apply the lut[] to moving picture.
      i=0;
      for (int n = 0; n < anzpixel; n++) {
         data2[n]=(int) lut[data2[n]];
      }
      return data2;
   }
   

3. 将int[] 数组转换回位图。

   public static Bitmap dataToBitmap(int[] data, int width, int heigth) {
    int index=0;
    Bitmap bmp = Bitmap.createBitmap(width, heigth, Bitmap.Config.ARGB_8888);
    for (int x = 0; x < width; x++) {
     for (int y = 0; y < heigth; y++) {
        index=y*width+x;
        int c = data[index];
        bmp.setPixel(x,y,Color.rgb(c, c, c));
        }
    }
    return bmp;
   }
   

虽然核心过程 2) 简单快速，但转换步骤 1) 和 3) 的效率相当低。在 Renderscript 中完成整个事情会非常酷。但是，老实说，由于缺少文档，我完全迷失了这样做，虽然有很多关于 Renderscript 可以执行的令人印象深刻的示例，但我看不到从这些可能性中受益的方法（没有书籍，没有文档）。任何建议都非常感谢！

Answer 1

作为起点，使用 Android Studio 来“Import Sample...”并选择 Basic Render Script。这将为您提供一个工作项目，我们现在将对其进行修改。

首先，让我们添加更多对MainActivity 的分配引用。我们将使用它们在 Java 和 Renderscript 之间传递图像数据、直方图和 LUT。

private Allocation mInAllocation;
private Allocation mInAllocation2;
private Allocation[] mOutAllocations;
private Allocation mHistogramAllocation;
private Allocation mHistogramAllocation2;
private Allocation mLUTAllocation;

然后在onCreate() 中加载另一个图像，您还需要将其添加到/res/drawables/。

    mBitmapIn2 = loadBitmap(R.drawable.cat_480x400);

在createScript() 中创建额外的分配：

    mInAllocation2 = Allocation.createFromBitmap(mRS, mBitmapIn2);
    mHistogramAllocation = Allocation.createSized(mRS, Element.U32(mRS), 256);
    mHistogramAllocation2 = Allocation.createSized(mRS, Element.U32(mRS), 256);
    mLUTAllocation = Allocation.createSized(mRS, Element.U32(mRS), 256);

现在是主要部分（RenderScriptTask）：

            /*
             * Invoke histogram kernel for both images
             */
            mScript.bind_histogram(mHistogramAllocation);
            mScript.forEach_compute_histogram(mInAllocation);

            mScript.bind_histogram(mHistogramAllocation2);
            mScript.forEach_compute_histogram(mInAllocation2);


            /*
             * Variables copied verbatim from your code.
             */
            int []histogram_fixed = new int[256];
            int []histogram_moving = new int[256];
            int[] cumhist_fixed = new int[256];
            int[] cumhist_moving = new int[256];
            int i=0;
            int j=0;

            // copy computed histograms to Java side
            mHistogramAllocation.copyTo(histogram_fixed);
            mHistogramAllocation2.copyTo(histogram_moving);

            // your code again...
            // calc cumulated distributions
            cumhist_fixed[0]=histogram_fixed[0];
            cumhist_moving[0]=histogram_moving[0];

            for ( i=1; i < 256; ++i ) {
                cumhist_fixed[i] = cumhist_fixed[i-1]+histogram_fixed[i];
                cumhist_moving[i] = cumhist_moving[i-1]+histogram_moving [i];
            }

            // look-up-table lut[]. For each quantile i of the moving picture search     the
            // value j of the fixed picture where the quantile is the same as that of moving
            int[] lut = new int[256];
            j=0;
            for ( i=0; i < 256; ++i ){
                while(cumhist_fixed[j]< cumhist_moving[i]){
                    j++;
                }

                // check, whether the distance to the next-lower intensity is even lower, and if so, take this value
                if ((j!=0) && ((cumhist_fixed[j-1]- cumhist_fixed[i]) < (cumhist_fixed[j]- cumhist_fixed[i]))){
                    lut[i]= (j-1);
                }
                else {
                    lut[i]= (j);
                }
            }

            // copy the LUT to Renderscript side
            mLUTAllocation.copyFrom(lut);
            mScript.bind_LUT(mLUTAllocation);

            // Apply LUT to the destination image
            mScript.forEach_apply_histogram(mInAllocation2, mInAllocation2);


            /*
             * Copy to bitmap and invalidate image view
             */
            //mOutAllocations[index].copyTo(mBitmapsOut[index]);

            // copy back to Bitmap in preparation for viewing the results
            mInAllocation2.copyTo((mBitmapsOut[index]));

情侣笔记：

• 在您的部分代码中，我还修复了 LUT 分配大小 - 只需要 256 个位置，
• 如您所见，我将累积直方图和 LUT 的计算留在了 Java 端。由于数据依赖性和计算规模小，这些很难有效地并行化，但考虑到后者，我认为这不是问题。

最后是 Renderscript 代码。唯一不明显的部分是使用 rsAtomicInc() 来增加直方图 bin 中的值 - 这是必要的，因为可能有许多线程试图同时增加同一个 bin。

#pragma version(1)
#pragma rs java_package_name(com.example.android.basicrenderscript)
#pragma rs_fp_relaxed

int32_t *histogram;
int32_t *LUT;

void __attribute__((kernel)) compute_histogram(uchar4 in)
{
    volatile int32_t *addr = &histogram[in.r];
    rsAtomicInc(addr);
}

uchar4 __attribute__((kernel)) apply_histogram(uchar4 in)
{
    uchar val = LUT[in.r];
    uchar4 result;
    result.r = result.g = result.b = val;
    result.a = in.a;

    return(result);
}