Bitmap unexpected exiting loop without error

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Hello Community,
I am encountering a problem while doing some calculation with a bitmap. I want to get the summed up brightness of a bitmap (659 x 494) and a matrix with the brightness values. To do that I combined some helpful articles from here.

Here is the code:

C#

public static double CALC_BRIGHTNESS(Bitmap bm)
        {
            double lum = 0;
            var tmpBmp = new Bitmap(bm);
            var width = bm.Width;
            var height = bm.Height;
            var bppModifier = bm.PixelFormat == PixelFormat.Format24bppRgb ? 3 : 4;

            var srcData = tmpBmp.LockBits(new Rectangle(0, 0, bm.Width, bm.Height), ImageLockMode.ReadOnly, bm.PixelFormat);
            var stride = srcData.Stride;
            var scan0 = srcData.Scan0;

            unsafe
            {
                int bytes = Math.Abs(stride) * tmpBmp.Height;
                byte[] p = new byte[bytes];
                System.Runtime.InteropServices.Marshal.Copy(scan0, p, 0, bytes);

                for (int y = 0; y < height; y++)
                {
                    for (int x = 0; x < width; x++)
                    {
                        int idx = (y * stride) + x * bppModifier;
                        double lumin = (0.299 * p[idx + 2] + 0.587 * p[idx + 1] + 0.114 * p[idx]) / 255.00;
                        pixels[x, y] = lumin;
                        if (lumin >= border)
                            lum += lumin;
                    }
                    
                }
            }

            tmpBmp.UnlockBits(srcData);
            tmpBmp.Dispose();
            return lum;
        }

But it is always exiting the for loops where y = 491, x = 494 and idx = 327640. Following calculations are not continued, the Form is shown and can be closed...
With a smaller picture(approx 300 x 200) I don't have any issues.
Maybe one of you can help me solve this issue.

Posted 11-Oct-15 21:57pm

DANIEL_PD

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George Jonsson · Accepted Answer · 2015-10-11T22:25:00

Solution 1

Start to put a try catch around your code and show any error in a MessageBox.

C#

public static double CALC_BRIGHTNESS(Bitmap bm)
{
    Bitmap tmpBmp = null;  // Declare this variable outside the try-catch block
                           // and call dispose in finally
    try
    {
        // Your code
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString));
    }
    finally
    {
        if (tmpBmp != null)
            tmpBmp.Dispose();
    }
}

Also how and where is the variable pixels declared?

Posted 11-Oct-15 22:25pm

George Jonsson

Updated 11-Oct-15 22:30pm

v3

Comments

DANIEL_PD 12-Oct-15 4:35am

Hello George, thanks for your answer.
I had a try...catch around the code that calls this function, but obviously it wasn't enough...

pixels is initialized like following:


public static double[,] pixels;

and in the main code:


[...]
p_bmp = (Bitmap)Image.FromFile("C:\\Glasplatte\\TEST.tiff");
pixels = new double[p_bmp.Width, p_bmp.Height];
double val = CALC_BRIGHTNESS(p_bmp);
[...]

The following error is displayed:
System.IndexOutOfRangeException: Index was outside the bounds of the array...

I found out, that int bytes = Math.Abs(stride) * tmpBmp.Height; equals 327640 and then p[idx +2] is out of range... but why is the array to small?

George Jonsson 12-Oct-15 6:03am

Well, what is the possible max-value of int idx = (y * stride) + x * bppModifier;
It has to be less than bytes.Length + 2.

DANIEL_PD 13-Oct-15 2:19am

Thanks for the Answer, I set the maximum to a fix value and it worked... Thank you

George Jonsson 13-Oct-15 5:47am

Glad it helped you.