Java多线程对图像进行阈值处理

Question

I have a school project, where we should use paralel programing to make some alghorithm work faster. For example I have chosen Image Thresholding.

So I created Java program, that does it normaly (load image, loop through all the pixels, counts threshold value, again loop through all the pixels and set black or white color, if it's greater or lower than threshold value). This takes me ~5seconds on my notebook with a picture about 4000x3000 and ~49 seconds with image 11500x11500.

Then I created another program, which should be using threads, to make them loops finish faster.

Now I'm creating 4 threads, where each of them proceses 1/4 of the image. First they are adding threshold vlaues into synchronized arraylist and after all of them are finished, I calculate the threshold value. Then I create another 4 threads, and they are again procesing 1/4 of the image each and setting black or white into the picture.

This tooks me 12seconds with the 4000x3000 image and throws java.lang.OutOfMemoryError: Java heap space (in all threads) with the 11500x11500 one.

 public class PprPrahovaniParalelne{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        final Threshold image = new Threshold(nactiObrazek("ryba.jpg"));


        final int width = image.getImage().getWidth();
        final int height = image.getImage().getHeight();


        Thread t1 = new Thread(){
            int threshold;
            public void run(){
                System.out.println("Thread 1 - Started");
                for(int y = 0; y < height/4;y++){
                    for(int x = 0; x < width;x++){
                        Color color = new Color(image.getImage().getRGB(x,y));
                        threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
                        image.addThreshold(threshold); 
                    }
                }
                System.out.println("Thread 1 - finished");
            }
        };

        Thread t2 = new Thread(){
            int threshold;
            @Override
            public void run(){
                for(int y = height/4; y < height/4*2;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        image.addThreshold(threshold); 
                    }
                }
                System.out.println("Thread 2 - finished");
            }
        };

        Thread t3 = new Thread(){
            int threshold;
            @Override
            public void run(){
                for(int y = height/4*2; y < height/4*3;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        image.addThreshold(threshold); 
                    }
                }
                System.out.println("Thread 3 - finished");
            }
        };

        Thread t4 = new Thread(){
            int threshold;
            @Override
            public void run(){
                for(int y = height/4*3; y < height;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        image.addThreshold(threshold); 
                    }
                }
                System.out.println("Thread 4 - finished");
            }
        };

        t1.start();
        t2.start();
        t4.start();
        t3.start();

        try{
            t1.join();
            t2.join();
            t3.join();
            t4.join();
        }catch(InterruptedException e){
            e.printStackTrace();
        }


        image.countThreshold();
        System.out.println("Threshold je: " + image.getThreshold());

        Thread t5 = new Thread(){
            Color cerna = new Color(255,255,255);
            Color bila = new Color(0,0,0);
            int threshold;
            @Override
            public void run(){
                for(int y = 0; y < height/4;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        if(threshold > image.getThreshold()){
                            image.getImage().setRGB(x, y, cerna.getRGB());
                        }else{
                            image.getImage().setRGB(x, y, bila.getRGB());
                        }
                    }
                }                
                System.out.println("Thread 5 - finished");
            }
        };

        Thread t6 = new Thread(){
            Color cerna = new Color(255,255,255);
            Color bila = new Color(0,0,0);
            int threshold;
            @Override
            public void run(){
                for(int y = height/4; y < height/4*2;y++){
                    for(int x = 0; x < width;x++){
                        Color color = new Color(image.getImage().getRGB(x,y));
                        threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
                        if(threshold > image.getThreshold()){
                            image.getImage().setRGB(x, y, cerna.getRGB());
                        }else{
                            image.getImage().setRGB(x, y, bila.getRGB());
                        }
                    }
                }                
                System.out.println("Thread 6 - finished");
            }
        };

        Thread t7 = new Thread(){
            Color cerna = new Color(255,255,255);
            Color bila = new Color(0,0,0);
            int threshold;
            @Override
            public void run(){
                for(int y = height/4*2; y < height/4*3;y++){
                    for(int x = 0; x < width;x++){
                        Color color = new Color(image.getImage().getRGB(x,y));
                        threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
                        if(threshold > image.getThreshold()){
                            image.getImage().setRGB(x, y, cerna.getRGB());
                        }else{
                            image.getImage().setRGB(x, y, bila.getRGB());
                        }
                    }
                }                
                System.out.println("Thread 7 - finished");
            }
        };

        Thread t8 = new Thread(){
            Color cerna = new Color(255,255,255);
            Color bila = new Color(0,0,0);
            int threshold;
            @Override
            public void run(){
                for(int y = height/4*3; y < height;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        if(threshold > image.getThreshold()){
                            image.getImage().setRGB(x, y, cerna.getRGB());
                        }else{
                            image.getImage().setRGB(x, y, bila.getRGB());
                        }
                    }
                }                
                System.out.println("Thread 8 - finished");
            }
        };

        t5.start();
        t6.start();
        t7.start();
        t8.start();

        try{
            t5.join();
            t6.join();
            t7.join();
            t8.join();
        }catch(InterruptedException e){
            e.printStackTrace();
        }

        File hotovo = new File("ryba_prahovanej.jpg");
        ImageIO.write(image.getImage(), "jpg", hotovo);

    }


    public static BufferedImage nactiObrazek(String nazev){
        BufferedImage img = null;
        try {
            img = ImageIO.read(new File(nazev));
        } catch (IOException e) {
        }
        return img;        
    }    
}

And the Threshold class:

public class Threshold {
    private BufferedImage image;
    final private List<Integer> list;
    private int threshold;

    public int getThreshold() {
        return threshold;
    }

    public List<Integer> getList(){
        return list;
    }

    public Threshold(BufferedImage obrazek) {
        this.list = Collections.synchronizedList(new ArrayList<Integer>());
        this.image = obrazek;
    }

    public void setObrazek(BufferedImage obrazek){
        this.image = obrazek;
    }

    public BufferedImage getImage(){
        return this.image;
    }

    public void addThreshold(int threshold){
        list.add(threshold);
    }

    public void countThreshold(){
        long sum = 0;
        for (Iterator<Integer> it = list.iterator(); it.hasNext();) {
            int item = it.next();
            sum += item;
        }
        this.threshold = (int) (sum/list.size());    
    }
}

So why is it that it is slower when multithreading? I'm not synchronizing here nothing except the list, since the threads shouldn't be using same indexes in the array of pixels.

Profiler picture here:

Serial:

Paralel:

Answer 1

There are couple of things you need to consider in this parallelizing case.

1. Your code only does asynchronous processing of image (Calculating threshold and creating threshold image) but your Image IO (Writing) is blocked until all threads finish processing.
2. Other more important factor is that how did you come up with 4 Threads solution. What is the reason behind you choosing that 4 Threads is the ideal amount of threads. In a CPU and Memory Intensive multi-threaded programs like yours ideal no of threads = Number of CPUs + 1 . Having more threads doesn't make your program perform faster in fact it will degrade performance.
3. Image processing is of course memory intensive, you need to increase Heap Space when running your program with large images.

Please consider the above mentioned.

EDIT

You can start off by making your code more readable and less code duplicating. You can make use of a CyclicBarrier to achieve sequential execution of parallel tasks.

import java.awt.image.*;
import java.io.*;
import java.awt.*;
import javax.imageio.*;
import java.util.concurrent.*;

public class PprPrahovaniParalelne {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        final Threshold image = new Threshold(nactiObrazek("DSC03691.jpg"));


        final int width = image.getImage().getWidth();
        final int height = image.getImage().getHeight();

    final int nCpu = Runtime.getRuntime().availableProcessors() + 1;

    ExecutorService threadPool = Executors.newFixedThreadPool(nCpu);

    System.out.println("Number of CPUs : "+nCpu);

    CyclicBarrier cyclicBarrier = new CyclicBarrier(4, new Runnable() {
            private int count = 1;

            public void run() {
                if(count == 1) {
            image.countThreshold();
                System.out.println("Threshold je: " + image.getThreshold());

        } else {
            try {
                File hotovo = new File("ryba_prahovanej.jpg");
                    ImageIO.write(image.getImage(), "jpg", hotovo);
            } catch(IOException e) {
                System.err.println("Error while writing : " + e);
            }
            threadPool.shutdownNow();
        }
        count++;
            }
        });

    threadPool.submit(new ImageProcessingTask(0, height/4, width, image, cyclicBarrier));
    threadPool.submit(new ImageProcessingTask(height/4, height/4*2, width, image, cyclicBarrier));
    threadPool.submit(new ImageProcessingTask(height/4*2, height/4*3, width, image, cyclicBarrier));
    threadPool.submit(new ImageProcessingTask(height/4*3, height, width, image, cyclicBarrier));

    }


    public static BufferedImage nactiObrazek(String nazev){
        BufferedImage img = null;
        try {
            img = ImageIO.read(new File(nazev));
        } catch (IOException e) {
        }
        return img;        
    }    
}

class ImageProcessingTask implements Runnable {

    private int start;
    private int height;
    private int width;
    private Threshold image;
    private CyclicBarrier barrier;

    public ImageProcessingTask(int start, int height, int width, Threshold image, CyclicBarrier barrier) {
        this.start = start;
        this.height = height;
        this.width = width;
        this.image = image;
        this.barrier = barrier;
    }

        public void run(){
                int threshold;
                System.out.println(Thread.currentThread().getName()+" - Started");
                for(int y = start; y < height;y++){
                    for(int x = 0; x < width;x++){
                        Color color = new Color(image.getImage().getRGB(x,y));
                        threshold = (color.getRed()+color.getGreen()+color.getBlue())/3;
                        image.addThreshold(threshold); 
                    }
                }
        try {
            int count = barrier.await();
            if(count == 0) {
                barrier.reset();
                System.out.println("Resetting Cyclic Barrier");
            }
        } catch(InterruptedException e) {
            Thread.currentThread().interrupt();
        } catch(Exception e) {
            e.printStackTrace();
        }
        Color cerna = new Color(255,255,255);
                Color bila = new Color(0,0,0);
        for(int y = start; y < height;y++){
                    for(int x = 0; x < width;x++){
                        Color barva = new Color(image.getImage().getRGB(x,y));
                        threshold = (barva.getRed()+barva.getGreen()+barva.getBlue())/3;
                        if(threshold > image.getThreshold()){
                            image.getImage().setRGB(x, y, cerna.getRGB());
                        }else{
                            image.getImage().setRGB(x, y, bila.getRGB());
                        }
                    }
                }    
        try {
            barrier.await();
        } catch(InterruptedException e) {
            Thread.currentThread().interrupt();
        } catch(Exception e) {
            e.printStackTrace();
        }
                System.out.println(Thread.currentThread().getName()+" - finished");

    }
}