"Colourizing" a Bitmap in .NET

Question

If you have a System.Drawing.Bitmap instance that contains a greyscale image, is there a built in way to "colourize" it with the influence of another colour?

For example, if you had a black and white (greyscale) picture of a coffee mug and you wanted to create separate images of red, green and purple versions programmatically.

Answer 1

I don't have a code example to give but here's a way to do this. Convert each pixel from RGB to HSV and change the Hue and Saturation component on each pixel. The Hue controls the Color. The Value should stay the same. The result will be a Bitmap with the same lightness and darkness but with a different color.

Edit: here's an example. Notice the Hue and Saturation update.

        public static Color ColorFromAhsb(int a, float h, float s, float b)
    {

        if (0 > a || 255 < a)
        {
            throw new Exception("a");
        }
        if (0f > h || 360f < h)
        {
            throw new Exception("h");
        }
        if (0f > s || 1f < s)
        {
            throw new Exception("s");
        }
        if (0f > b || 1f < b)
        {
            throw new Exception("b");
        }

        if (0 == s)
        {
            return Color.FromArgb(a, Convert.ToInt32(b * 255),
              Convert.ToInt32(b * 255), Convert.ToInt32(b * 255));
        }

        float fMax, fMid, fMin;
        int iSextant, iMax, iMid, iMin;

        if (0.5 < b)
        {
            fMax = b - (b * s) + s;
            fMin = b + (b * s) - s;
        }
        else
        {
            fMax = b + (b * s);
            fMin = b - (b * s);
        }

        iSextant = (int)Math.Floor(h / 60f);
        if (300f <= h)
        {
            h -= 360f;
        }
        h /= 60f;
        h -= 2f * (float)Math.Floor(((iSextant + 1f) % 6f) / 2f);
        if (0 == iSextant % 2)
        {
            fMid = h * (fMax - fMin) + fMin;
        }
        else
        {
            fMid = fMin - h * (fMax - fMin);
        }

        iMax = Convert.ToInt32(fMax * 255);
        iMid = Convert.ToInt32(fMid * 255);
        iMin = Convert.ToInt32(fMin * 255);

        switch (iSextant)
        {
            case 1:
                return Color.FromArgb(a, iMid, iMax, iMin);
            case 2:
                return Color.FromArgb(a, iMin, iMax, iMid);
            case 3:
                return Color.FromArgb(a, iMin, iMid, iMax);
            case 4:
                return Color.FromArgb(a, iMid, iMin, iMax);
            case 5:
                return Color.FromArgb(a, iMax, iMin, iMid);
            default:
                return Color.FromArgb(a, iMax, iMid, iMin);
        }

    }

    private void Form1_Load(object sender, EventArgs e)
    {
        var bmp = new Bitmap("c:\\bw.bmp");

        foreach (int y in Enumerable.Range(0, bmp.Height))
        { 
            foreach (int x in Enumerable.Range(0,bmp.Width))
            {
                var p = bmp.GetPixel(x, y);
                var h = p.GetHue();

                var c = ColorFromAhsb(p.A, p.GetHue() + 200, p.GetSaturation() + 0.5f, p.GetBrightness());
                bmp.SetPixel(x, y, c);                    
            }
        }
        pictureBox1.Image = bmp;
        //bmp.Dispose();

    }