Question

我有一个后台线程，它在给定文件夹中创建图像的灰度缩略图。我看到的问题是后台线程中的Graphics.DrawImage（）调用似乎在某种程度上阻止了主UI线程上的Graphics操作。

我可能会误解我在这里看到的东西，并且在今晚晚些时候没有机会进行任何深入的剖析，尽管我不希望能找到太多。

我试图想出尽可能少的repro案例。如果您使用下面的表单替换默认项目中的表单（并在文件夹中有一些图像进行测试），您会注意到动画标签在窗口来回反弹时会断断续续。然而，如果您取消注释顶部的#define以使子控件动画而不是重新绘制窗口内容，它将非常顺利地运行。

任何人都可以在这里看到我做错了什么或帮我弄清楚如何在更新循环中避免这种口吃？

//#define USE_LABEL_CONTROL

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Threading;
using System.Windows.Forms;
using Timer = System.Windows.Forms.Timer;

namespace ThreadTest
{
    public partial class Form1 : Form
    {
        private const string ImageFolder = "c:\\pics";
        private const string ImageType = "*.jpg";

        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            this.Size = new Size(300, 300);

            string[] ImageFiles = Directory.GetFiles(ImageFolder, 
                                                        ImageType, 
                                                        SearchOption.AllDirectories);

            // kick off a thread to create grayscale thumbnails of all images
            this.thumbnailThread = new Thread(this.thumbnailThreadFunc);
            this.thumbnailThread.Priority = ThreadPriority.Lowest;
            this.thumbnailThread.Start(ImageFiles);

            // set a timer to start us off...
            this.startTimer = new Timer();
            this.startTimer.Interval = 500;
            this.startTimer.Tick += this.startTimer_Tick;
            this.startTimer.Start();

#if USE_LABEL_CONTROL
            this.label.Location = this.labelRect.Location;
            this.label.Size = this.labelRect.Size;
            this.label.Text = "Loaded: 0";
            this.label.BorderStyle = BorderStyle.FixedSingle;
            this.Controls.Add(this.label);
#endif

            base.OnLoad(e);
        }

        void startTimer_Tick(object sender, EventArgs e)
        {
            // kill the timer
            this.startTimer.Stop();

            // update ourself in a loop
            while (this.IsHandleCreated)
            {
                int NextTick = Environment.TickCount + 50;

                // update the label position
                this.labelRect.Offset(this.currentLabelDirection, 0);
                if (this.labelRect.Right == this.ClientRectangle.Right ||
                    this.labelRect.Left == 0)
                {
                    this.currentLabelDirection = -this.currentLabelDirection;
                }

                // update the display
#if USE_LABEL_CONTROL
                this.label.Text = "Loaded: " + this.thumbs.Count;
                this.label.Location = this.labelRect.Location;
#else
                using (Graphics Dest = this.CreateGraphics())
                {
                    this.redrawControl(Dest, this.ClientRectangle);
                }
#endif

                Application.DoEvents();
                Thread.Sleep(Math.Max(0, NextTick - Environment.TickCount));
            }
        }

        private void thumbnailThreadFunc(object ThreadData)
        {
            string[] ImageFiles = (string[]) ThreadData;
            foreach (string ImageFile in ImageFiles)
            {
                if (!this.IsHandleCreated)
                {
                    return;
                }

                using (Image SrcImg = Image.FromFile(ImageFile))
                {
                    Rectangle SrcRect = new Rectangle(Point.Empty, SrcImg.Size);

                    Rectangle DstRect = new Rectangle(Point.Empty, new Size(300, 200));
                    Bitmap DstImg = new Bitmap(DstRect.Width, DstRect.Height);
                    using (Graphics Dst = Graphics.FromImage(DstImg))
                    {
                        using (ImageAttributes Attrib = new ImageAttributes())
                        {
                            Attrib.SetColorMatrix(this.grayScaleMatrix);
                            Dst.DrawImage(SrcImg, 
                                            DstRect, 
                                            0, 0, SrcRect.Width, SrcRect.Height, 
                                            GraphicsUnit.Pixel, 
                                            Attrib);
                        }
                    }

                    lock (this.thumbs)
                    {
                        this.thumbs.Add(DstImg);
                    }
                }
            }
        }

#if !USE_LABEL_CONTROL
        private void redrawControl (Graphics Dest, Rectangle UpdateRect)
        {
            Bitmap OffscreenImg = new Bitmap(this.ClientRectangle.Width, 
                                                this.ClientRectangle.Height);
            using (Graphics Offscreen = Graphics.FromImage(OffscreenImg))
            {
                Offscreen.FillRectangle(Brushes.White, this.ClientRectangle);
                Offscreen.DrawRectangle(Pens.Black, this.labelRect);
                Offscreen.DrawString("Loaded: " + this.thumbs.Count,
                                        SystemFonts.MenuFont,
                                        Brushes.Black,
                                        this.labelRect);
            }
            Dest.DrawImageUnscaled(OffscreenImg, 0, 0);
            OffscreenImg.Dispose();
        }

        protected override void OnPaintBackground(PaintEventArgs e)
        {
            return;
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            this.redrawControl(e.Graphics, e.ClipRectangle);
        }
#endif


        private ColorMatrix grayScaleMatrix = new ColorMatrix(new float[][] 
                                                        {
                                                            new float[] {.3f, .3f, .3f, 0, 0},
                                                            new float[] {.59f, .59f, .59f, 0, 0},
                                                            new float[] {.11f, .11f, .11f, 0, 0},
                                                            new float[] {0, 0, 0, 1, 0},
                                                            new float[] {0, 0, 0, 0, 1}
                                                        });
        private Thread thumbnailThread;
        private Timer startTimer;
        private List<Bitmap> thumbs = new List<Bitmap>();
        private Label label = new Label();
        private int currentLabelDirection = 1;
        private Rectangle labelRect = new Rectangle(0, 125, 75, 20);
    }
}

Answer 1

事实证明，答案是使用多个进程来处理后台GDI +任务。如果你在VS2010中的并发分析器下运行上面的代码，你会看到前台线程阻塞在后台线程中由DrawImage（）调用保护的关键部分。

该线程还讨论了这个问题，并指出由于它使用了一个临界区，因此锁是按进程进行的，后台任务可以使用多个进程而不是线程来并行化：

Parallelizing GDI+ Image Resizing .net

Answer 2

每当遇到性能问题时，您都需要考虑尽可能多的因素。在您的情况下，标签和重绘实现之间存在相当大的差异。

标签版本仅重绘屏幕的一小部分（或两个小部分）。
重绘版本正在做更多工作并重新绘制更大的区域。

这本身可能是问题所在，具体取决于您正在使用的计算机的功能。

为什么后台线程中的图形操作阻止主UI线程中的图形操作？

2 个答案: