C# - Crop Transparent/White space

Question

I'm trying to remove all white or transparent pixels from an image, leaving the actual image (cropped). I've tried a few solutions, but none seem to work. Any suggestions or am I going to spend the night writing image cropping code?

Answer 1

In WPF we have a WriteableBitmap class. Is this what are you looking for ? If it is the case please have a look at http://blogs.msdn.com/b/jgalasyn/archive/2008/04/17/using-writeablebitmap-to-display-a-procedural-texture.aspx

Answer 2

Per-pixel check should do the trick. Scan each line to find empty line from the top & bottom, scan each row to find left & right constraints (this can be done in one pass with either rows or columns). When the constraint is found - copy the part of the image to another buffer.

Answer 3

public Bitmap CropBitmap(Bitmap original)
{
    // determine new left
    int newLeft = -1;
    for (int x = 0; x < original.Width; x++)
    {
        for (int y = 0; y < original.Height; y++)
        {
            Color color = original.GetPixel(x, y);
            if ((color.R != 255) || (color.G != 255) || (color.B != 255) || 
                (color.A != 0))
            {
                // this pixel is either not white or not fully transparent
                newLeft = x;
                break;
            }
        }
        if (newLeft != -1)
        {
            break;
        }

        // repeat logic for new right, top and bottom

    }

    Bitmap ret = new Bitmap(newRight - newLeft, newTop - newBottom);
    using (Graphics g = Graphics.FromImage(ret)
    {
        // copy from the original onto the new, using the new coordinates as
        // source coordinates for the original
        g.DrawImage(...);
    }

    return ret
}

Note that this function will be slow as dirt. GetPixel() is unbelievably slow, and accessing the Width and Height properties of a Bitmap inside a loop is also slow. LockBits would be the proper way to do this - there are tons of examples here on StackOverflow.

Answer 4

So, what you want to do is find the top, left most non white/transparent pixel and the bottom, right most non white/transparent pixel. These two coordinates will give you a rectangle that you can then extract.

  // Load the bitmap
  Bitmap originalBitmap = Bitmap.FromFile("d:\\temp\\test.bmp") as Bitmap;

  // Find the min/max non-white/transparent pixels
  Point min = new Point(int.MaxValue, int.MaxValue);
  Point max = new Point(int.MinValue, int.MinValue);

  for (int x = 0; x < originalBitmap.Width; ++x)
  {
    for (int y = 0; y < originalBitmap.Height; ++y)
    {
      Color pixelColor = originalBitmap.GetPixel(x, y);
      if (!(pixelColor.R == 255 && pixelColor.G == 255 && pixelColor.B == 255)
        || pixelColor.A < 255)
      {
        if (x < min.X) min.X = x;
        if (y < min.Y) min.Y = y;

        if (x > max.X) max.X = x;
        if (y > max.Y) max.Y = y;
      }
    }
  }

  // Create a new bitmap from the crop rectangle
  Rectangle cropRectangle = new Rectangle(min.X, min.Y, max.X - min.X, max.Y - min.Y);
  Bitmap newBitmap = new Bitmap(cropRectangle.Width, cropRectangle.Height);
  using (Graphics g = Graphics.FromImage(newBitmap))
  {
    g.DrawImage(originalBitmap, 0, 0, cropRectangle, GraphicsUnit.Pixel);
  }