c++ - OpenMP 分形生成器
问题描述
我一直在尝试创建 julia 集的 openMP 变体,但是在运行多个线程时无法创建连贯的图像,我一直在尝试解决看起来像竞态但找不到错误的问题. 有问题的输出看起来像所需的输出以及整个图片的“扫描线”。
如果代码不够清晰,我也附上了代码。
#include <iostream>
#include <math.h>
#include <fstream>
#include <sstream>
#include <omp.h>
#include <QtWidgets>
#include <QElapsedTimer>
using namespace std;
double newReal(int x, int imageWidth){
return 1.5*(x - imageWidth / 2)/(0.5 * imageWidth);
}
double newImaginary(int y, int imageHeight){
return (y - imageHeight / 2) / (0.5 * imageHeight);
}
int julia(double& newReal, double& newImaginary, double& oldReal, double& oldImaginary, double cRe, double cIm,int maxIterations){
int i;
for(i = 0; i < maxIterations; i++){
oldReal = newReal;
oldImaginary = newImaginary;
newReal = oldReal * oldReal - oldImaginary * oldImaginary + cRe;
newImaginary = 2 * oldReal * oldImaginary + cIm;
if((newReal * newReal + newImaginary * newImaginary) > 4) break;
}
return i;
}
int main(int argc, char *argv[])
{
int fnum=atoi(argv[1]);
int numThr=atoi(argv[2]);
// int imageHeight=atoi(argv[3]);
// int imageWidth=atoi(arg[4]);
// int maxIterations=atoi(argv[5]);
// double cRe=atof(argv[3]);
// double cIm=atof(argv[4]);
//double cRe, cIm;
int imageWidth=10000, imageHeight=10000, maxIterations=3000;
double newRe, newIm, oldRe, oldIm,cRe,cIm;
cRe = -0.7;
cIm = 0.27015;
string fname;
QElapsedTimer time;
QImage img(imageHeight, imageWidth, QImage::Format_RGB888);//Qimagetesting
img.fill(QColor(Qt::black).rgb());//Qimagetesting
time.start();
int i,x,y;
int r, gr, b;
#pragma omp parallel for shared(imageHeight,imageWidth,newRe,newIm) private(x,y,i) num_threads(3)
for(y = 0; y < imageHeight; y++)
{
for(x = 0; x < imageWidth; x++)
{
newRe = newReal(x,imageWidth);
newIm = newImaginary(y,imageHeight);
i= julia(newRe, newIm, oldRe, oldIm, cRe, cIm, maxIterations);
r = (3*i % 256);
gr = (2*(int)sqrt(i) % 256);
b = (i % 256);
img.setPixel(x, y, qRgb(r, gr, b));
}
}
//stringstream s;
//s << fnum;
//fname= "julia" + s.str();
//fname+=".png";
//img.save(fname.c_str(),"PNG", 100);
img.save("julia.png","PNG", 100);
cout<< "Finished"<<endl;
cout<<time.elapsed()/1000.00<<" seconds"<<endl;
}
解决方案
正如评论中所指出的,您有两个主要问题:
newRe
并且newIm
是共享的,但不应该是r
,gr
和b
的访问权限未指定(我认为默认共享)- 有并发调用
QImage::setPixel
要纠正这个问题,请不要犹豫,将omp for
循环嵌套在一个omp parallel
块中。
for
在循环之前声明私有变量:
为了防止并发调用QImage::setPixel
,因为这个函数不是线程安全的,你可以把它放在一个临界区,用#pragma omp critical
.
int main(int argc, char *argv[])
{
int imageWidth=1000, imageHeight=1000, maxIterations=3000;
double cRe = -0.7;
double cIm = 0.27015;
QElapsedTimer time;
QImage img(imageHeight, imageWidth, QImage::Format_RGB888);//Qimagetesting
img.fill(Qt::black);
time.start();
#pragma omp parallel
{
/* all folowing values will be private */
int i,x,y;
int r, gr, b;
double newRe, newIm, oldRe, oldIm;
#pragma omp for
for(y = 0; y < imageHeight; y++)
{
for(x = 0; x < imageWidth; x++)
{
newRe = newReal(x,imageWidth);
newIm = newImaginary(y,imageHeight);
i= julia(newRe, newIm, oldRe, oldIm, cRe, cIm, maxIterations);
r = (3*i % 256);
gr = (2*(int)sqrtf(i) % 256);
b = (i % 256);
#pragma omp critical
img.setPixel(x, y, qRgb(r, gr, b));
}
}
}
img.save("julia.png","PNG", 100);
cout<<time.elapsed()/1000.00<<" seconds"<<endl;
return 0;
}
更进一步,您可以节省一些 cpu 时间,替换::setPixel
为::scanLine
:
#pragma omp for
for(y = 0; y < imageHeight; y++)
{
uchar *line = img.scanLine(y);
for(x = 0; x < imageWidth; x++)
{
newRe = newReal(x,imageWidth);
newIm = newImaginary(y,imageHeight);
i= julia(newRe, newIm, oldRe, oldIm, cRe, cIm, maxIterations);
r = (3*i % 256);
gr = (2*(int)sqrtf(i) % 256);
b = (i % 256);
*line++ = r;
*line++ = gr;
*line++ = b;
}
}
编辑:
由于 julia 集似乎具有围绕(0,0)
点的中心对称性,因此您只能执行一半的微积分:
int half_heigt = imageHeight / 2;
#pragma omp for
// compute only for first half of image
for(y = 0; y < half_heigt; y++)
{
for(x = 0; x < imageWidth; x++)
{
newRe = newReal(x,imageWidth);
newIm = newImaginary(y,imageHeight);
i= julia(newRe, newIm, oldRe, oldIm, cRe, cIm, maxIterations);
r = (3*i % 256);
gr = (2*(int)sqrtf(i) % 256);
b = (i % 256);
#pragma omp critical
{
// set the point
img.setPixel(x, y, qRgb(r, gr, b));
// set the symetric point
img.setPixel(imageWidth-1-x, imageHeight-1-y, qRgb(r, gr, b));
}
}
}
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