Cause of Lag in Painting a Line Set by Dragging the Mouse

问题

I have written an applet in Java that allows the user to set background color, and "pen" color then click and draw in the window frame to drawn a line. I set this line by filling ovals at each x and y point of the dragging mouse. I also allows user to use the - and + keys to change the size of the line by increasing or decreasing the radius of the ovals. My issue is that there is some cause of lag in the drawing of the line. I believe it is in the mouseDrag method and the speed at which that method is executed limits how many x and y points can be attained to paint ovals on. Is there any way to alleviate this lag to allow for better lines?

import java.applet.Applet;
import java.awt.Event;
import java.awt.Color;
import java.awt.Graphics;

public class Screen extends Applet
{
    int x = -20, y = -20;
    int height = 20, width = 20;
    boolean black, blue, cyan, green, magenta, orange, pink, red, white, yellow;

    public void init()
    {
    setSize(400,400);
    setBackground(Color.white);
    }
    public void paint(Graphics g)
    {
    if (black == true)
        g.setColor(Color.BLACK);
    else if (blue == true)
        g.setColor(Color.BLUE);
    else if (cyan == true)
        g.setColor(Color.CYAN);
    else if (green == true)
        g.setColor(Color.GREEN);
    else if (magenta == true)
        g.setColor(Color.MAGENTA);
    else if (orange == true)
        g.setColor(Color.ORANGE);
    else if (pink == true)
        g.setColor(Color.PINK);
    else if (red == true)
        g.setColor(Color.RED);
    else if (white == true)
        g.setColor(Color.WHITE);
    else if (yellow == true)
        g.setColor(Color.YELLOW);

    g.fillOval(x, y, width, height);
    }
    public boolean mouseDrag(Event e, int xPos, int yPos)
    {
    x = xPos;
    y = yPos;
    paint(getGraphics());
    return true;
    }
    public boolean keyDown(Event e, int key)
    {
    black = false;
    blue = false;
    cyan = false;
    green = false;
    magenta = false;
    orange = false;
    pink = false;
    red = false;
    white = false;
    yellow = false;

    if (key == 'n')
    {
        x =-20;
        y =-20;
        update(getGraphics());
    }
    else if (key == 'x')
        black = true;
    else if (key == 'b')
        blue = true; 
    else if (key == 'c')
        cyan = true;
    else if (key == 'g')
        green = true;
    else if (key == 'm')
        magenta = true;
    else if (key == 'o')
        orange = true;
    else if (key == 'p')
        pink = true;
    else if (key == 'r')
        red = true;
    else if (key == 'w')
        white = true;
    else if (key == 'y')
        yellow = true;
    else if (key == '+')
    {
        height += 5;
        width += 5;
    }
    else if (key == '-')
    {
        height += -5;
        width += -5;
    }
    else if (key == 'X')
        setBackground(Color.BLACK);
    else if (key == 'B' )
        setBackground(Color.BLUE); 
    else if (key == 'C')
        setBackground(Color.CYAN);
    else if (key == 'G')
        setBackground(Color.GREEN);
    else if (key == 'M')
        setBackground(Color.MAGENTA);
    else if (key == 'O')
        setBackground(Color.ORANGE);
    else if (key == 'P')
        setBackground(Color.PINK);
    else if (key == 'R')
        setBackground(Color.RED);
    else if (key == 'W')
        setBackground(Color.WHITE);
    else if (key == 'Y')
        setBackground(Color.YELLOW);
    return true;
    }
}

回答1:

Your painting looks questionable:

• You're calling paint(...) directly, something that should never be done.
• You're not calling the super.paint(...) method inside of your paint override.
• Try minimizing your GUI and then restoring it, and tell me how much of your drawing disappears into the nothingness.

Instead you should:

• call repaint() when you want to suggest that the GUI be painted
• call the super method in your painting override
• create a collection of ovals, perhaps Ellipse2D, and paint them in your painting method via a for loop.
• Chuck all this AWT code and instead draw in Swing in a JPanel's paintComponent(...) method.
• Rather than draw individual ellipses, draw lines connecting points. You can change thickness by changing the Stroke.
• Save each curve drawn onto a BufferedImage which is then drawn in the paintComponent method.

---

Edit:
For a complete example, please see my answer here: Changing JPanel Graphics g color drawing line

来源：https://stackoverflow.com/questions/19781603/cause-of-lag-in-painting-a-line-set-by-dragging-the-mouse