How to convert a PNG image to grayscale and without losing the transparency C#

Question

I want to convert a transparent png image to greyscale without losing its transparency.

The problem is the algorithm that I am using in is converting the transparent part into Black, which some picture with black character wouldn't be shown. To give you an idea.

Heres the original picture :

Look what happens when I pass it through the algorithm.

The algorithm :

       public static void ToWhiteBlack(Bitmap original)
    {
        try
        {

            for (var i = 0; i < original.Width; i++)
            {
                for (var j = 0; j < original.Height; j++)
                {
                    var originalColor = original.GetPixel(i, j);
                    var grayScale = (int) ((originalColor.R*0.3) + (originalColor.G*0.59) + (originalColor.B*0.11));
                    var corEmEscalaDeCinza = Color.FromArgb(grayScale, grayScale, grayScale);
                    original.SetPixel(i, j, corEmEscalaDeCinza);
                }
            }
        }
        catch
        {

        }

    }

You're ignorming the alpha value that determines the transparency. Just pass it to the new color. — juharr, Jul 21 '17 at 17:00
Thanks guy ! It worked. One of you answer it for me to select as a solution — Joab Santos, Jul 21 '17 at 17:04
You might want to check this out for a more preformant way to alter pixels in an image https://stackoverflow.com/questions/24701703/c-sharp-faster-alternatives-to-setpixel-and-getpixel-for-bitmaps-for-windows-f — juharr, Jul 21 '17 at 17:07

score 3 · Accepted Answer · answered Jul 21 '17 at 17:04

You need to pass the alpha value of the original color to get the transparency. Note that if you have partially transparent pixels that will be passed as well.

var corEmEscalaDeCinza = Color.FromArgb(originalColor.A, grayScale, grayScale, grayScale);

How to convert a PNG image to grayscale and without losing the transparency C#

1 Answers1

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