Member Avatar for learningthings

Hi!

I have a grayscale image in my pictureBox. I want to box up the white pixels in the image using the rectangle. Can anyone teach me how to write the code. Thanks in advance ..

Best regards,

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If by "box up" you mean draw rectangles around white areas, you first need to decide what counts as white. In real images you rarely get pure 255; pick a threshold (for example, pixels with luminance >= 240). Also, small correction to : in WinForms the origin is top-left (0,0), X increases to the right, Y increases downward. is right that white areas are often irregular, so you will usually want one rectangle per connected white region.

Quick start: draw one rectangle around all white pixels (good first check that your thresholding works).

// using System.Drawing; using System.Windows.Forms;
void BoxAllWhite(PictureBox pb, byte threshold = 240)
{
    var bmp = (Bitmap)pb.Image;
    int minX = bmp.Width, minY = bmp.Height, maxX = -1, maxY = -1;

    for (int y = 0; y < bmp.Height; y++)
    for (int x = 0; x < bmp.Width; x++)
    {
        Color c = bmp.GetPixel(x, y); // OK for small images; use LockBits for speed
        int lum = (299 * c.R + 587 * c.G + 114 * c.B) / 1000; // ITU-R BT.601
        if (lum >= threshold)
        {
            if (x < minX) minX = x; if (y < minY) minY = y;
            if (x > maxX) maxX = x; if (y > maxY) maxY = y;
        }
    }

    if (maxX >= 0)
    {
        using (var g = Graphics.FromImage(bmp))
        using (var pen = new Pen(Color.Red, 1))
            g.DrawRectangle(pen, Rectangle.FromLTRB(minX, minY, maxX + 1, maxY + 1));
        pb.Image = bmp;
    }
}

To draw a rectangle per white area: threshold once into a boolean mask, then run a flood-fill (BFS/DFS) to label connected components (4- or 8-neighborhood). For each component, track minX/minY/maxX/maxY while visiting pixels and draw that rectangle. Use Bitmap.LockBits + Marshal.Copy for speed; GetPixel will be slow on large images. Finally, if pictureBox.SizeMode != Normal, either draw on the bitmap itself (as above) or map coordinates between image and control to avoid misaligned boxes. Adjust the threshold if compression noise or antialiasing leaves near-white edges, as anticipated.

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All 6 Replies

Member Avatar for ddanbe

What do you mean by I want to box up the white pixels in the image using the rectangle. ?
Coordinates start in the upper left corner with (0,0).
Y coordinates increase by moving down.
X coordinates increase by moving left.

Member Avatar for sknake

Do you mean you want to draw a rectangle around area that have all-white pixels? This will work OK if the areas are in fact only rectangular but I highly suspect you will find jagged edges with white pixels. How should that be handled?

Member Avatar for learningthings

What do you mean by I want to box up the white pixels in the image using the rectangle. ?
Coordinates start in the upper left corner with (0,0).
Y coordinates increase by moving down.
X coordinates increase by moving left.

Thanks for the reply and apologize for my poor English language :)

Box up mean indicate with rectangles where the white pixels of the image are.

I have a gray-scale image in the pictureBox and I want to draw a rectangle on the white pixels of the image. Now my problem is i don't know how to get the X-Y coordinate and size of the white pixels of the image for drawing the rectangle on them. As i m a beginer in programming, could you guide me how can i write it in c#. Thanks in advance.

best regards,

Member Avatar for sknake

Check out this thread:
http://www.daniweb.com/forums/thread215149.html

It does basically the same thing but uses a screen shot instead of a bitmap file. You can just change that section of the code out then it shows you how to find a pixel. After you have a working version of that code then post it up here and we'll go from there!

Member Avatar for learningthings

Do you mean you want to draw a rectangle around area that have all-white pixels? This will work OK if the areas are in fact only rectangular but I highly suspect you will find jagged edges with white pixels. How should that be handled?

Yes, I want to draw rectangles around area that have all-white pixels of the image. The white pixels will be within the rectangle. How can i write it in c#. Thanks a lot for the reply.

Best Regard,

Member Avatar for sknake

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