Problem with color manipulation on byte level

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Hello,
I have one more problem with my current project:
I want to color a bitmap by applying a scale to it. The colors of the scale are stored in a SynchronizedCollection, to make things Thread safe. When I want to color my Bitmap everything works fine except, that there are colors in the bitmap which are not in the colors of the scale (which should be impossible). So I think there is something wrong with my Bytewise manipulation of the Bitmap. I can't find the fault, so maybe you can help me:

C#

private unsafe Bitmap COLOR_BMP(double min_press, double max_press, double f_p_b, Bitmap bmp)
        {
            try
            {
                Bitmap result = new Bitmap(bmp.Width, bmp.Height, PixelFormat.Format32bppArgb);
                BitmapData bitmapData = result.LockBits(new Rectangle(0, 0, result.Width, result.Height), ImageLockMode.ReadWrite, result.PixelFormat);
                IntPtr ptr = bitmapData.Scan0;
                int bytesPerPixel = System.Drawing.Bitmap.GetPixelFormatSize(result.PixelFormat) / 8;
                int numBytes = bitmapData.Stride * bmp.Height;
                byte[] rgbValues = new byte[numBytes];
                System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, numBytes);
                int list_ct = 0;

                for (int counter = 0; counter < rgbValues.Length; counter++)
                {
                    if (BR_LIST[list_ct] >= BORDER)
                    {
                        double brightness = BR_LIST[list_ct];
                        double force = brightness * f_p_b;
                        double percent = force / (max_press - min_press);
                        int pos = (int)(percent * (double)SCALE_LIST.Count);
                        Color color = SCALE_LIST[pos];

                        if (counter % 4 == 0)   //A
                            rgbValues[counter] = color.A;
                        if (counter % 4 == 1)   //R
                            rgbValues[counter] = color.R;
                        if (counter % 4 == 2)   //G
                            rgbValues[counter] = color.G;
                        if (counter % 4 == 3)   //B
                        {
                            rgbValues[counter] = color.B;
                            list_ct++;
                        }
                    }
                    else
                    {
                        rgbValues[counter] = 0;
                        counter = counter + 3;
                        list_ct++;
                    }
                        
                }

                System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, numBytes);
                result.UnlockBits(bitmapData);
                return result;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
                return bmp;
            }
        }

BR_LIST[] holds one value per pixel, so one value for every 4 bytes of the bitmap

The result looks like this:

CLICK ME

As you can see, there are colors in the image, that are not in the scaling above :-(

Posted 12-Oct-15 23:46pm

DANIEL_PD

Updated 12-Oct-15 23:47pm

v2

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andres_fprado · Answer 1 · 2015-10-29T19:27:00

The problem is that the computer is little-endian, which means that although the format is ARGB, the order the bytes are stored in memory is B - G - R - A (i.e., B is stored in a lower memory address than G, then R, and then A).

Your code is assuming the opposite ordering of bytes. To get the intended result, you should just invert the assignment order:

C#

if (counter % 4 == 0)   //B    
    rgbValues[counter] = color.B;
if (counter % 4 == 1)   //G
    rgbValues[counter] = color.G;
if (counter % 4 == 2)   //R
    rgbValues[counter] = color.R;
if (counter % 4 == 3)   //A
{
    rgbValues[counter] = color.A;
    list_ct++;
}

This should solve the problem.