Grayscale to RGB: Pixel color is set with wrong value

Question

I have a program that reads the grayscale (0 - 255) value of a pixel and changes it to an RGB color (formula see code).

Here is my code:

Bitmap img = new Bitmap(@"somepath");
        Bitmap new_img = new Bitmap(img.Width, img.Height);
        for (int i = 0; i < img.Width; i++)
        {
            for (int j = 0; j < img.Height; j++)
            {
                Color pixel = img.GetPixel(i, j);
                Color my = new Color();

                int R_new = 0;
                if (pixel.R > 126)
                {
                    R_new = (pixel.R -127) / 128  * 255;
                }
                
                int B_new = 0;
                if (pixel.B < 128) {
                    B_new = (1 - pixel.R / 127) * 255;
                }

                int G_new = 0;
                if (pixel.G < 128) {
                    G_new = pixel.R/127 * 255;
                }


                my = Color.FromArgb(R_new, G_new, B_new);

                new_img.SetPixel(i, j, my);     
            }
        }
        new_img.Save(@"C:somepath");

In the following picture you can see the green-value of the old pixel is 6 and of the new pixel it is set with 255, which is wrong. According to the formula it should be set at 12.

Here is my question: Why is the value of the pixel set wrong?

Answer 1

You have two problems.

#1: You're passing the blue value in place of the green value. Check the expected order of arguments :

Color.FromArgb(int red, , int blue)

#2: B_new is 255 due to how integer division is handled:

For the operands of integer types, the result of the / operator is of an integer type and equals the quotient of the two operands rounded towards zero:

(1 - 6 / 127) * 255 // becomes 255

So, 6 / 127 equals 0.047244094488189 but it's rounded down to zero... making your formula:

(1 - 0) * 255 // becomes 255

Make at least one of your division terms a floating point type and you'll get a decimal value:

(1 - pixel.R / ) * 255 // becomes 242.9527559055118

... which you'll then have to convert to an int to conform to the method signature of Color.FromArgb(int, int, int) .