更改 c# 中图像的像素颜色

Question

I am trying to replace black pixels with red in an image. Here is my image My_Image

Here is my code

public static Bitmap ChangeColor(Bitmap scrBitmap)
{            
    Color newColor = Color.Red;
    Color actualColor;            
    Bitmap newBitmap = new Bitmap(scrBitmap.Width, scrBitmap.Height);

    for (int i = 0; i < scrBitmap.Width; i++)
    {
        for (int j = 0; j < scrBitmap.Height; j++)
        {
            actualColor = scrBitmap.GetPixel(i, j);

            if (actualColor.R == 0 && actualColor.G == 0 && actualColor.B == 0)
                newBitmap.SetPixel(i, j, Color.FromArgb(actualColor.A, Color.Red));
        }
    }

    return newBitmap;
}

On Load

Bitmap image = new Bitmap(@"D:\test.jpg");
pictureBox1.Image = ChangeColor(image);

I just want to change the color of the black background to red but it's not working. What am I missing?

Edit: I tried the solution of This Post but it changes the color of the whole image.

Edit2: I tried @JonasH solution

if (0.3 * actualColor.R + 0.59 * actualColor.G + 0.11 * actualColor.B < 10)
                        newBitmap.SetPixel(i, j, Color.FromArgb(actualColor.A, Color.Red));
                    else
                        newBitmap.SetPixel(i, j, actualColor);

It worked but not perfectly. Some pixels which are not pure black but close enough to black are still not changed. Here is the

Answer 1

The most obvious problem is that you create a new bitmap, but do not copy the color for non black pixels. So you probably should do:

if (actualColor.R == 0 && actualColor.G == 0 && actualColor.B == 0)
       newBitmap.SetPixel(i, j, Color.FromArgb(actualColor.A, Color.Red));
else
    newBitmap.SetPixel(i, j, actualColor);

An alternative would be to update the source bitmap in-place instead of creating a new one.

You might also want to change the color checks to something like

if (0.3 * actualColor.R + 0.59 * actualColor.G + 0.11*actualColor.B < threshold)

To include all pixels with a luminosity lower than some threshold