segmentation / ivote3

  #ifndef STIM_CUDA_GRADIENT3_H
  #define STIM_CUDA_GRADIENT3_H
  
  #include <iostream>
  #include <cuda.h>
  #include <stim/cuda/cudatools.h>
  #include <stim/cuda/cudatools/error.h>
  
  template<typename T>

  __global__ void gradient3(T* out, T* in, int x, int y, int z){

  
  	//calculate x,y,z coordinates for this thread
  	int xi = blockIdx.x * blockDim.x + threadIdx.x;

  	//find the grid size along y
  	int grid_y = y / blockDim.y;
  	int blockidx_y = blockIdx.y % grid_y;
  	int yi = blockidx_y * blockDim.y + threadIdx.y;
  	int zi = blockIdx.y / grid_y;

  	int i = zi * x * y + yi * x + xi;
  	
  	//return if the pixel is outside of the image

  	if(xi >= x || yi >= y || zi>=z) return;

  
  	//calculate indices for the forward difference
  	int i_xp = zi * x * y + yi * x + (xi + 1);
  	int i_yp = zi * x * y + (yi + 1) * x + xi;
  	int i_zp = (zi + 1) * x * y + yi * x + xi;
  

  	//calculate indices for the backward difference
  	int i_xn = zi * x * y + yi * x + (xi - 1);
  	int i_yn = zi * x * y + (yi - 1) * x + xi;
  	int i_zn = (zi - 1) * x * y + yi * x + xi;
  

  	//use forward differences if a coordinate is zero
  	if(xi == 0)
  		out[i * 3 + 0] = in[i_xp] - in[i];
  	if(yi == 0)
  		out[i * 3 + 1] = in[i_yp] - in[i];
  	if (zi==0)
  		out[i * 3 + 2] = in[i_zp] - in[i];
  

  	//use backward differences if the coordinate is at the maximum edge
  	if(xi == x-1)
  		out[i * 3 + 0] = in[i] - in[i_xn];
  	if(yi == y-1)
  		out[i * 3 + 1] = in[i] - in[i_yn];
  	if(zi == z-1)
  		out[i * 3 + 2] = in[i] - in[i_zn];
  
  	//otherwise use central differences
  	if(xi > 0 && xi < x-1)
  		out[i * 3 + 0] = (in[i_xp] - in[i_xn]) / 2;

  	if(yi > 0 && yi < y-1)
  		out[i * 3 + 1] = (in[i_yp] - in[i_yn]) / 2;
  	if(zi > 0 && zi < z-1)

  		out[i * 3 + 2] = (in[i_zp] - in[i_zn]) / 2;

  }
  
  template<typename T>
  

  void gpu_gradient3(T* gpuGrad, T* gpuI, size_t x, size_t y, size_t z){

  	unsigned int max_threads = stim::maxThreadsPerBlock();

  	dim3 threads(sqrt (max_threads),sqrt (max_threads));
  	dim3 blocks(x / threads.x + 1, (y / threads.y + 1) * z);

  	
  	//call the GPU kernel to determine the gradient

  	gradient3<T> <<< blocks, threads >>>(gpuGrad, gpuI, x, y, z);

  
  }
  
  template<typename T>

  void cpu_gradient3(T* out, T* in, size_t x, size_t y, size_t z){

  
  	//get the number of pixels in the image

  	size_t pixels = x * y * z;
  	size_t bytes = pixels * sizeof(T);

  
  	//allocate space on the GPU for the input image
  	T* gpuIn;
  	HANDLE_ERROR(cudaMalloc(&gpuIn, bytes));
  
  	//copy the image data to the GPU
  	HANDLE_ERROR(cudaMemcpy(gpuIn, in, bytes, cudaMemcpyHostToDevice));
  
  	//allocate space on the GPU for the output gradient image
  	T* gpuOut;
  	cudaMalloc(&gpuOut, bytes * 3);		//the output image will have two channels (x, y)
  
  	//call the GPU version of this function

  	gpu_gradient3(gpuOut, gpuIn, x, y, z);	

  
  	//copy the results to the CPU
  	cudaMemcpy(out, gpuOut, bytes * 3, cudaMemcpyDeviceToHost);
  
  	//free allocated memory
  	cudaFree(gpuIn);
  	cudaFree(gpuOut);
  }
  
  
  
  #endif