C# Bitmap difficulties

Question

What I am trying to do is check an image row or column, and if it contains all white pixels then trim that row or column. I am not sure why, but when I run this code snippet, img.Width in TrimLeft is 1, before subtracting 1 from it. The actual image width is 175. I end up with an ArgumentException. I have a similar method for trimming the right side, and that works fine.

class ImageHandler
{

    public Bitmap img;
    public List<int[]> pixels = new List<int[]>();

    public ImageHandler(String path)
    {
        img = new Bitmap(path);
        GetPixels();
    }

    public void TrimLeft()
    {
        while (CheckColIfWhite(0, 0))
        {
            Rectangle cropBox = new Rectangle(1, 0, (img.Width-1), img.Height);
            Bitmap cropdImg = CropImage(img, cropBox);
            img = cropdImg;
        }
    }

    public bool CheckColIfWhite(int colx, int starty)
    {
        bool allPixelsWhite = false;
        int whitePixels = 0;

        for (int y = starty; y < img.Height; y++)
        {
            if (pixels[y][colx] >= 200) { whitePixels++; }
            else { return false; }
            if (whitePixels == img.Height) { allPixelsWhite = true; }
        }
        return allPixelsWhite;
    }

    public void GetPixels()
    {
        for (int y = 0; y < img.Height; y++)
        {
            int[] line = new int[img.Width];
            for (int x = 0; x < img.Width; x++)
            {
                line[x] = (int) img.GetPixel(x, y).R;
            }
            pixels.Add(line);
        }
    }

    public Bitmap CropImage(Bitmap tImg, Rectangle area) 
    {
        Bitmap bmp = new Bitmap(tImg);
        Bitmap bmpCrop = bmp.Clone(area, bmp.PixelFormat);
        return bmpCrop;
    }
}

Answer 1

Your method seems like it will be remarkably inefficient - if the image is 175 pixels wide, and entirely white, you're going to create 175 copies of it, before (probably) failing when trying to create a 0 pixel wide image.

Why not examine each column in turn until you find a non-white column, and then perform a single crop at that time. Untested code, and with other changes hopefully obvious:

public Bitmap CropImage (Bitmap image)
{
    int top = 0;
    int bottom = image.Height-1;
    int left = 0;
    int right = image.Width-1;
    while(left < right && CheckColIfWhite(image,left))
         left++;
    if(left==right) return null; //Entirely white
    while(CheckColIfWhite(image,right)) //Because left stopped, we know right will also
         right--;
    while(CheckRowIfWhite(image,top))
         top++;
    while(CheckRowIfWhite(image,bottom))
         bottom--;
    return CropImage(image,new Rectangle(left,top,right-left+1,bottom-top+1));
}

(E.g. I'm now passing the image around, I've modified CheckColIfWhite and CheckRowIfWhite to take the image also, and to assume that one parameter is always fixed at 0)

---

Also, not sure why you're extracting the pixel array beforehand, so I'm putting my re-written CheckColIfWhite too:

public bool CheckColIfWhite(Bitmap image,int colx)
{
    for (int y = 0; y < image.Height; y++)
    {
        if (image.GetPixel(colx,y).R < 200)
            return false;
    }
    return true;
}