1、并行算法介绍
Reduce:
对于一般可以加法运算，如进行8个数相加：
串行计算
由于对于符合交换律，其运算顺序可以改变，可以使用并行计算，如下:
并行计算
对比串行运算，与并行运算的速度：
| 运算量 | 8 | ... | N |
|---|---|---|---|
| 并行Step | 7 | ... | N-1 |
| 串行Step | 3 | ... | log2(N) |
对于并行计算的实现，可参照udacity的课程GitHub:reduce.cu
Scan
Scan 可称为prefix sum，即求前N项（或N-1项）的和，如下：
prefix sum
串行计算流程如上求和无太大差异，而并行计算起来相当讲技巧，以N=8举例:
黑\红实线：加法 黑虚线：赋值
详细可阅读：Parallel prefix sum (scan) with CUDA
2、图像的直方图均衡
假设存在8个灰度级的图像如下图，那么它实际可见的范围为1~4
| 1 | 2 | 3 | 4 |
|---|
| 1 | 2 | 3 | 4 |
|---|
| 1 | 2 | 3 | 4 |
|---|
| 1 | 2 | 3 | 4 |
|---|
频数图如下：
image.png
histogram_equalize.png
即原来的灰度级为1映射后为0，2映射后为2，3映射后为3，4映射后为5
均衡后新灰度频数表如下：
| 灰度级 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|---|
| 频数 | 4 | 0 | 4 | 4 | 0 | 4 | 0 | 0 |
image.png
映射结果：
| 0 | 2 | 3 | 5 |
|---|
| 0 | 2 | 3 | 5 |
|---|
| 0 | 2 | 3 | 5 |
|---|
| 0 | 2 | 3 | 5 |
|---|
总的来说没有整个过程没有改变像素点灰度值点之间大小、数量关系，或者说将可视区间拉伸，让色彩对比度更加明显。
3、并行图像直方图均衡化
1)统计每个灰度级像素的数目
__global__ void histogram_init(char *src,unsigned int *out)
//针对512*512大小灰度图 开启512*512线程，计算256个灰度级相应频数
2)应用Parallel Scan(prefix sum) 计算出映射关系
__global__ void scan_downsweep(unsigned int *in,unsigned int *out,int n)
//输入256点频数表，输出均衡化后256对应得映射值
3)将原图按照映射关系映射到结果图
__global__ void equal_remap(char *img,char *out,unsigned int *idetity)
//根据映射表，重新映射图像
程序效果图如下：
histogram_result
源代码：
#include "cuda.h"
#include "cuda_runtime.h"
#include "cuda_runtime_api.h"
#include "device_functions.h"
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/opencv.hpp"
#include "stdio.h"
#include <vector>
using namespace std;
using namespace cv;
#define IMAGE_DIR "/home/dzqiu/Documents/image/chaozhou.JPG"
__global__ void histogram_init(char *src,unsigned int *out)
{
int x = threadIdx.x + blockDim.x * blockIdx.x;
int y = threadIdx.y + blockDim.y * blockDim.y;
int offset = x + y * gridDim.x * blockDim.x;
unsigned char value = src[offset];
//原子操作，否则会出错。
atomicAdd(&out[value],1);
}
//refer: Parallel Prefix Sum (Scan) with CUDA, Mark Harris, April 2007
//link : http: //developer.download.nvidia.com/compute/cuda/2_2/sdk/website/projects/scan/doc/scan.pdf
__global__ void scan_downsweep(unsigned int *in,unsigned int *out,int n)
{
int Idx = threadIdx.x ;
extern __shared__ float sdata[];
sdata[Idx] = in[Idx];
__syncthreads();
unsigned int offset=1;
for(unsigned int i=n>>1;i>0;i>>=1) //build sum iin place up the tree
{
__syncthreads();
if(Idx<i)
{
int ai = offset*(2*Idx+1)-1;
int bi = offset*(2*Idx+2)-1;
sdata[bi] += sdata[ai];
}
offset *= 2 ;
}
if(Idx==0) sdata[n-1]=0; //travers down tree &build scan
for(unsigned int i=1;i<n;i<<=1)
{
offset >>= 1;
__syncthreads();
if(Idx<i)
{
int ai = offset*(2*Idx+1)-1;
int bi = offset*(2*Idx+2)-1;
int tmp =sdata[ai];
sdata[ai] = sdata[bi];
sdata[bi] += tmp;
}
}
__syncthreads();
//out[Idx]=sdata[Idx];
out[Idx]=((float)sdata[Idx]/512/512*256)-1;
}
__global__ void equal_remap(char *img,char *out,unsigned int *idetity)
{
int x = threadIdx.x+blockIdx.x*blockDim.x;
int y = threadIdx.y+blockIdx.y*blockDim.y;
int offset = x+y*blockDim.x*gridDim.x;
unsigned char value = img[offset];
out[offset] = (unsigned char) idetity[value];
}
#define DIM 512
#define GRAY_IDENTITY 256
int main(int argc,char** argv)
{
Mat img_src = imread(IMAGE_DIR);
Mat img_gray;
cvtColor(img_src,img_gray,CV_RGB2GRAY);
resize(img_gray,img_gray,Size(DIM,DIM));
unsigned int *dev_histogram;
cudaMalloc((void**)&dev_histogram,GRAY_IDENTITY*sizeof(int));
cudaMemset(dev_histogram,0,GRAY_IDENTITY*sizeof(int));
/*build the histogram of the image*/
char *dev_gray;
cudaMalloc((void**)&dev_gray,DIM*DIM);
cudaMemcpy(dev_gray,img_gray.data,DIM*DIM,cudaMemcpyHostToDevice);
dim3 blocks(DIM/16,DIM/16);dim3 threads(16,16);
histogram_init<<<blocks,threads>>>(dev_gray,dev_histogram);
int host_histogram[GRAY_IDENTITY]={0};
cudaMemcpy(host_histogram,dev_histogram,GRAY_IDENTITY*sizeof(int),cudaMemcpyDeviceToHost);
/*equalize the histogram*/
unsigned int *dev_histogram_equal;
cudaMalloc((void**)&dev_histogram_equal,GRAY_IDENTITY*sizeof(int));
blocks=dim3(1,1);threads=dim3(GRAY_IDENTITY,1);
scan_downsweep<<<blocks,threads,sizeof(int)*GRAY_IDENTITY>>>(dev_histogram,dev_histogram_equal,GRAY_IDENTITY);
int host_histogram_equal[GRAY_IDENTITY]={0};
cudaMemcpy(host_histogram_equal,dev_histogram_equal,GRAY_IDENTITY*sizeof(int),cudaMemcpyDeviceToHost);
for(int i=0;i<GRAY_IDENTITY/4;i++)
{
//if(host_histogram[i]!=0)
printf("bin[%3d]:%6d -> %6d| bin[%3d]:%6d -> %6d| bin[%3d]:%6d -> %6d| bin[%3d]:%6d -> %6d\n",
4*i,host_histogram[4*i],host_histogram_equal[4*i],
4*i+1,host_histogram[4*i+1],host_histogram_equal[4*i+1],
4*i+2,host_histogram[4*i+2],host_histogram_equal[4*i+2],
4*i+3,host_histogram[4*i+3],host_histogram_equal[4*i+3]);
}
/*remap the image use the equalized histogram*/
char *dev_equalImg;
cudaMalloc((void**)&dev_equalImg,DIM*DIM);
blocks=dim3(DIM/16,DIM/16);threads=dim3(16,16);
equal_remap<<<blocks,threads>>>(dev_gray,dev_equalImg,dev_histogram_equal);
Mat equalImg(DIM,DIM,CV_8UC1,Scalar(255));
cudaMemcpy(equalImg.data,dev_equalImg,DIM*DIM,cudaMemcpyDeviceToHost);
cudaFree(dev_gray);
cudaFree(dev_histogram);
cudaFree(dev_histogram_equal);
imshow("gray_img",img_gray);
imshow("histogram equalization use GPU",equalImg);
waitKey(0);
return 0;
}
源码下载：GitHub
参考：Parallel Prefix Sum (Scan) with CUDA, Mark Harris, April 2007
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