多线程 Mandelbrot 集代码输出不正确的图像
Multithreaded Mandelbrot set code outputs incorrect image
#include <chrono>
#include <cstdint>
#include <cstdlib>
#include <complex>
#include <fstream>
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
using std::chrono::duration_cast;
using std::chrono::milliseconds;
using std::chrono::seconds;
using std::complex;
using std::cout;
using std::endl;
using std::this_thread::sleep_for;
using std::ofstream;
using std::thread;
using std::mutex;
using std::condition_variable;
using std::unique_lock;
mutex mutex1;
mutex mutex2;
std::condition_variable done;
typedef std::chrono::steady_clock the_clock;
const int WIDTH1 = 960;
const int HEIGHT1 = 600;
const int WIDTH2 = 1920;
const int HEIGHT2 = 1200;
int finished_threads = 0;
const int MAX_ITERATIONS = 500;
uint32_t image[HEIGHT2][WIDTH2];
struct ThreadArgs { int id; int delay; };
void myThreadFunc(ThreadArgs args)
{
for (int i = 0; i < 1; i++) {
sleep_for(seconds(args.delay));
cout << args.id;
}
}
void write_tga(const char *filename)
{
unique_lock<mutex> lock(mutex2);
while (finished_threads < 2) {
done.wait(lock);
}
ofstream outfile(filename, ofstream::binary);
uint8_t header[18] = {
0, // no image ID
0, // no colour map
2, // uncompressed 24-bit image
0, 0, 0, 0, 0, // empty colour map specification
0, 0, // X origin
0, 0, // Y origin
WIDTH2 & 0xFF, (WIDTH2 >> 8) & 0xFF, // width
HEIGHT2 & 0xFF, (HEIGHT2 >> 8) & 0xFF, // height
24, // bits per pixel
0, // image descriptor
};
outfile.write((const char *)header, 18);
for (int y = 0; y < HEIGHT2; ++y)
{
for (int x = 0; x < WIDTH2; ++x)
{
uint8_t pixel[3] = {
image[y][x] & 0xFF, // blue channel
(image[y][x] >> 8) & 0xFF, // green channel
(image[y][x] >> 16) & 0xFF, // red channel
};
outfile.write((const char *)pixel, 3);
}
}
outfile.close();
if (!outfile)
{
cout << "Error writing to " << filename << endl;
exit(1);
}
}
// Render the Mandelbrot set into the image array.
// The parameters specify the region on the complex plane to plot.
void compute_mandelbrot(double left, double right, double top, double bottom)
{
unique_lock<mutex> lock(mutex2);
for (int y = 0; y < HEIGHT1; ++y)
{
for (int x = 0; x < WIDTH1; ++x)
{
complex<double> c(left + (x * (right - left) / WIDTH2),
top + (y * (bottom - top) / HEIGHT2));
// Start off z at (0, 0).
complex<double> z(0.0, 0.0);
// Iterate z = z^2 + c until z moves more than 2 units
// away from (0, 0), or we've iterated too many times.
int iterations = 0;
mutex1.lock();
while (abs(z) < 2.0 && iterations < MAX_ITERATIONS)
{
z = (z * z) + c;
++iterations;
}
mutex1.unlock();
if (iterations == MAX_ITERATIONS)
{
image[y][x] = 0x000000; // black
}
else
{
image[y][x] = 0xFFFFFF; // white
finished_threads = finished_threads + 1;
done.notify_all();
}
}
}
}
void compute_mandelbrot2(double left2, double right2, double top2, double bottom2)
{
unique_lock<mutex> lock(mutex2);
//map <int, int> val = map<int, int>(0, MAX_ITERATIONS);
//map <int, int> colourval = map<int, int>(0, MAX_ITERATIONS);
for (int y = HEIGHT1; y < HEIGHT2; ++y)
{
for (int x = HEIGHT1; x < WIDTH2; ++x)
{
complex<double> c(left2 + (x * (right2 - left2) / WIDTH2),
top2 + (y * (bottom2 - top2) / HEIGHT2));
// Start off z at (0, 0).
complex<double> z(0.0, 0.0);
// Iterate z = z^2 + c until z moves more than 2 units
int iterations = 0;
mutex1.lock();
while (abs(z) < 2.0 && iterations < MAX_ITERATIONS)
{
z = (z * z) + c;
++iterations;
}
mutex1.unlock();
if (iterations == MAX_ITERATIONS)
{
// z didn't escape from the circle.
// This point is in the Mandelbrot set.
image[y][x] = 0x000000; // black
}
else
{
// z escaped within less than MAX_ITERATIONS
// iterations. This point isn't in the set.
image[y][x] = 0xFFFFFF; // white
finished_threads = finished_threads + 1;
done.notify_one();
}
}
}
}
int main(int argc, char *argv[])
{
std::thread myThread;
std::thread myThread2;
std::thread myThread3;
ThreadArgs args;
myThread = std::thread(compute_mandelbrot, -2.0, 1.0, 1.125, -1.125);
myThread3 = std::thread(compute_mandelbrot2, -2.0, 1.0, 1.125, -1.125);
myThread2 = std::thread(write_tga, "output.tga");
cout << "Please wait..." << endl;
// Start timing
the_clock::time_point start = the_clock::now();
myThread.join();
myThread3.join();
// Stop timing
the_clock::time_point end = the_clock::now();
// Compute the difference between the two times in milliseconds
auto time_taken = duration_cast<milliseconds>(end - start).count();
cout << "Computing the Mandelbrot set took " << time_taken << " ms." << endl;
myThread2.join();
return 0;
}
上面是多线程版本的代码，它输出了一个不正确的集合版本，其中大部分是黑色的，但有些是正确的，所以我不知道问题是什么：Mandlebrot threaded but the non-threaded version outputs a correct Mandelbrot setMandelbrot not threaded问题可能与我使用多线程的方式有关：我只是不知道我做错了什么。感谢任何帮助。
OK，'simple'事情先！您在内部 (x) 循环的控制语句的 compute_mandelbrot2 函数中有一个 'typo'；这一行：
for (int x = HEIGHT1; x < WIDTH2; ++x)
应该（当然）是：
for (int x = WIDTH1; x < WIDTH2; ++x) // WIDTH1 not HEIGHT1
现在来了解更多 'subtle' 内容。您正试图通过将 both 'x' 和 'y' 范围分成两半来将计算分成两半。这是行不通的，因为这需要 四个 个线程，每个线程处理相关的 quarter 情节。为了保持只使用两半，两个线程函数 中的 'y' 范围必须 覆盖整个图（但 'x' 范围可以适当地分成两部分).
因此，您的外部 ('y') 循环控制语句应涵盖 整个 范围 both 线程函数，它们都应该是这样的：
for (int y = 0; y < HEIGHT2; ++y) {
//...
我已经用上述三个更改测试了您的代码，它生成了正确的 Mandelbrot 集图像。随时要求进一步澄清 and/or 解释。
#include <chrono>
#include <cstdint>
#include <cstdlib>
#include <complex>
#include <fstream>
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
using std::chrono::duration_cast;
using std::chrono::milliseconds;
using std::chrono::seconds;
using std::complex;
using std::cout;
using std::endl;
using std::this_thread::sleep_for;
using std::ofstream;
using std::thread;
using std::mutex;
using std::condition_variable;
using std::unique_lock;
mutex mutex1;
mutex mutex2;
std::condition_variable done;
typedef std::chrono::steady_clock the_clock;
const int WIDTH1 = 960;
const int HEIGHT1 = 600;
const int WIDTH2 = 1920;
const int HEIGHT2 = 1200;
int finished_threads = 0;
const int MAX_ITERATIONS = 500;
uint32_t image[HEIGHT2][WIDTH2];
struct ThreadArgs { int id; int delay; };
void myThreadFunc(ThreadArgs args)
{
for (int i = 0; i < 1; i++) {
sleep_for(seconds(args.delay));
cout << args.id;
}
}
void write_tga(const char *filename)
{
unique_lock<mutex> lock(mutex2);
while (finished_threads < 2) {
done.wait(lock);
}
ofstream outfile(filename, ofstream::binary);
uint8_t header[18] = {
0, // no image ID
0, // no colour map
2, // uncompressed 24-bit image
0, 0, 0, 0, 0, // empty colour map specification
0, 0, // X origin
0, 0, // Y origin
WIDTH2 & 0xFF, (WIDTH2 >> 8) & 0xFF, // width
HEIGHT2 & 0xFF, (HEIGHT2 >> 8) & 0xFF, // height
24, // bits per pixel
0, // image descriptor
};
outfile.write((const char *)header, 18);
for (int y = 0; y < HEIGHT2; ++y)
{
for (int x = 0; x < WIDTH2; ++x)
{
uint8_t pixel[3] = {
image[y][x] & 0xFF, // blue channel
(image[y][x] >> 8) & 0xFF, // green channel
(image[y][x] >> 16) & 0xFF, // red channel
};
outfile.write((const char *)pixel, 3);
}
}
outfile.close();
if (!outfile)
{
cout << "Error writing to " << filename << endl;
exit(1);
}
}
// Render the Mandelbrot set into the image array.
// The parameters specify the region on the complex plane to plot.
void compute_mandelbrot(double left, double right, double top, double bottom)
{
unique_lock<mutex> lock(mutex2);
for (int y = 0; y < HEIGHT1; ++y)
{
for (int x = 0; x < WIDTH1; ++x)
{
complex<double> c(left + (x * (right - left) / WIDTH2),
top + (y * (bottom - top) / HEIGHT2));
// Start off z at (0, 0).
complex<double> z(0.0, 0.0);
// Iterate z = z^2 + c until z moves more than 2 units
// away from (0, 0), or we've iterated too many times.
int iterations = 0;
mutex1.lock();
while (abs(z) < 2.0 && iterations < MAX_ITERATIONS)
{
z = (z * z) + c;
++iterations;
}
mutex1.unlock();
if (iterations == MAX_ITERATIONS)
{
image[y][x] = 0x000000; // black
}
else
{
image[y][x] = 0xFFFFFF; // white
finished_threads = finished_threads + 1;
done.notify_all();
}
}
}
}
void compute_mandelbrot2(double left2, double right2, double top2, double bottom2)
{
unique_lock<mutex> lock(mutex2);
//map <int, int> val = map<int, int>(0, MAX_ITERATIONS);
//map <int, int> colourval = map<int, int>(0, MAX_ITERATIONS);
for (int y = HEIGHT1; y < HEIGHT2; ++y)
{
for (int x = HEIGHT1; x < WIDTH2; ++x)
{
complex<double> c(left2 + (x * (right2 - left2) / WIDTH2),
top2 + (y * (bottom2 - top2) / HEIGHT2));
// Start off z at (0, 0).
complex<double> z(0.0, 0.0);
// Iterate z = z^2 + c until z moves more than 2 units
int iterations = 0;
mutex1.lock();
while (abs(z) < 2.0 && iterations < MAX_ITERATIONS)
{
z = (z * z) + c;
++iterations;
}
mutex1.unlock();
if (iterations == MAX_ITERATIONS)
{
// z didn't escape from the circle.
// This point is in the Mandelbrot set.
image[y][x] = 0x000000; // black
}
else
{
// z escaped within less than MAX_ITERATIONS
// iterations. This point isn't in the set.
image[y][x] = 0xFFFFFF; // white
finished_threads = finished_threads + 1;
done.notify_one();
}
}
}
}
int main(int argc, char *argv[])
{
std::thread myThread;
std::thread myThread2;
std::thread myThread3;
ThreadArgs args;
myThread = std::thread(compute_mandelbrot, -2.0, 1.0, 1.125, -1.125);
myThread3 = std::thread(compute_mandelbrot2, -2.0, 1.0, 1.125, -1.125);
myThread2 = std::thread(write_tga, "output.tga");
cout << "Please wait..." << endl;
// Start timing
the_clock::time_point start = the_clock::now();
myThread.join();
myThread3.join();
// Stop timing
the_clock::time_point end = the_clock::now();
// Compute the difference between the two times in milliseconds
auto time_taken = duration_cast<milliseconds>(end - start).count();
cout << "Computing the Mandelbrot set took " << time_taken << " ms." << endl;
myThread2.join();
return 0;
}
上面是多线程版本的代码，它输出了一个不正确的集合版本，其中大部分是黑色的，但有些是正确的，所以我不知道问题是什么：Mandlebrot threaded but the non-threaded version outputs a correct Mandelbrot setMandelbrot not threaded问题可能与我使用多线程的方式有关：我只是不知道我做错了什么。感谢任何帮助。
OK，'simple'事情先！您在内部 (x) 循环的控制语句的 compute_mandelbrot2 函数中有一个 'typo'；这一行：
for (int x = HEIGHT1; x < WIDTH2; ++x)
应该（当然）是：
for (int x = WIDTH1; x < WIDTH2; ++x) // WIDTH1 not HEIGHT1
现在来了解更多 'subtle' 内容。您正试图通过将 both 'x' 和 'y' 范围分成两半来将计算分成两半。这是行不通的，因为这需要 四个 个线程，每个线程处理相关的 quarter 情节。为了保持只使用两半，两个线程函数 中的 'y' 范围必须 覆盖整个图（但 'x' 范围可以适当地分成两部分).
因此，您的外部 ('y') 循环控制语句应涵盖 整个 范围 both 线程函数，它们都应该是这样的：
for (int y = 0; y < HEIGHT2; ++y) {
//...
我已经用上述三个更改测试了您的代码，它生成了正确的 Mandelbrot 集图像。随时要求进一步澄清 and/or 解释。