For loops, application freezes

Question

Well, I'm building an application, I use it for loops to basically read every pixel of an image looking for patterns in pixel color (simple things). In any case, for some reason, my application simply blocks and never returns to its normal state. I repeat the code again and again without seeing any real problems.

The only thing I noticed was the for loop in ScanPixelsLater, which might be removed earlier. I commented on the code as much as possible,

private Point topLeftc, bottomLeftc, topRightc, bottomRightc;

/// <summary>
/// Starts the initial looping process, designed only to loop through ONCE, ScanPixelsLater takes over
/// </summary>
/// <param name="img">Image to scan</param>
public void ScanPixels(Bitmap img)
{
    int whitePixel = 0;

    for (int y = 100; y < img.Height; y++)
    {

        for (int x = 100; x < img.Width; x++)
        {

            if (img.GetPixel(x, y) == Color.FromArgb(255, 255, 255, 255))
            {
                // img.SetPixel(x, y, Color.Green);
                whitePixel++;

            }
            else { whitePixel = 0; }


            if (whitePixel == 18)
            {
                whitePixel = 0;
                topLeftc = new Point(x - 18, y);
                DetectNextWhiteLine(topLeftc, img);

            }
        }
    }


}


/// <summary>
/// First creates the TopRight value via using the last pixel in x range, and using the current Y value
/// Then a Y loop is started 10 pixels down, within this loop is another X loop which scans the X range
/// If 10 consecutive white pixels are found, the TopLeft X value and the current Y value are used to map the 
/// BottomLeft and BottomRight coordinates. Finally a new Point is created which starts (x = 1) and (y = currentYValue + 2)
/// The ScanPixelsLater method is then called, passing the new Point (newLocation).
/// 
/// </summary>
/// <param name="p">The x and y value of where the pixels were found</param>
/// <param name="img">Image being used</param>
private void DetectNextWhiteLine(Point p, Bitmap img)
{

    int whitePixel = 0;
    topRightc = new Point(img.Width, topLeftc.Y);

    for (int y = p.Y + 10; y < img.Height; y++)
    {

        for (int x = p.X - 5; x < img.Width; x++)
        {
            if (img.GetPixel(x, y) == Color.FromArgb(255, 255, 255, 255))
            {
                whitePixel++;
            }
            else
            {
                whitePixel = 0;
            }

            if (whitePixel == 10)
            {
                bottomLeftc = new Point(topLeftc.X, y);
                bottomRightc = new Point(img.Width, y);
                Cords.Add(new Coordinates(topLeftc, topRightc, bottomLeftc, bottomRightc));

                Point newLocation = new Point(1, y + 2);
                calls++;
                ScanPixelsLater(newLocation, img); //rescan the image from new y axis 
            }
        }
    }

}



/// <summary>
/// Loops through the pixels based on the p parameter, if 15 white pixels are found, the location (x & y) is
/// passed to the DetectNextWhiteLine method which fixes the next line with a similar number of white pixels.
/// </summary>
/// <param name="p">The Point(x,y) at which to start scanning</param>
/// <param name="img"></param>
private void ScanPixelsLater(Point p, Bitmap img)
{

    int whitePixel = 0;

    for (int y = p.Y; y < img.Height; y++)
    {

        for (int x = p.X; x < img.Width; x++)
        {
            if (img.GetPixel(x, y) == Color.FromArgb(255, 255, 255, 255))
            {
                whitePixel++;
            }
            else
            {
                whitePixel = 0;
            }

            if (whitePixel == 15)
            {
                bottomLeftc = new Point(topLeftc.X, y);
                topLeftc = new Point(x - 15, y);
                calls++;
                DetectNextWhiteLine(topLeftc, img);
            }

        }
    }

    // only want this to execute after all the pixels within the entire img have been read
    // possibly executing early.

    DrawWhiteLines(img);
    AppArgs aa = new AppArgs(true);
    Change(this, aa); // custom event handler, fired behind form to update GUI

}

Answer 1

So, to understand why your application hangs, you need to learn a little about how WinForm applications work.

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, . , ( -, , ). GDI + GetPixel , , . , LockBits. , .

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Answer 2

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, , , GetPixel . . . "getpixel slow" google stackoverflow .

: ... (ScanPixels), , DetectNextWhiteLine, , , , ScanPixelsLater, . 6- O (n ^ 6), (GetPixel). . , , 1000 ^ 6 * ~ 100 :)