Question

I have a source bitmap that is 1×1 and I am trying to take that image and draw it to a new bitmap. The source bitmap is all red, but for some reason the new bitmap ends up with a gradient (see image). Using the code below, shouldn’t the new bitmap be completely red? Where is it getting the white/alpha from?

alt text http://www.binaryfortress.com/Temp/Error.jpg

private void DrawImage()
{
    Bitmap bmpSOURCE = new Bitmap(1, 1, PixelFormat.Format32bppArgb);
    using (Graphics g = Graphics.FromImage(bmpSOURCE))
    {
        g.Clear(Color.Red);
    }

    Bitmap bmpTest = new Bitmap(300, 100, PixelFormat.Format32bppArgb);
    using (Graphics g = Graphics.FromImage(bmpTest))
    {
        g.CompositingMode = CompositingMode.SourceCopy;
        g.CompositingQuality = CompositingQuality.AssumeLinear;
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
        g.PageUnit = GraphicsUnit.Pixel;
        g.PixelOffsetMode = PixelOffsetMode.None;
        g.SmoothingMode = SmoothingMode.None;

        Rectangle rectDest = new Rectangle(0, 0, bmpTest.Width, bmpTest.Height);
        Rectangle rectSource = new Rectangle(0, 0, 1, 1);
        g.DrawImage(bmpSOURCE, rectDest, rectSource, GraphicsUnit.Pixel);
    }

    pictureBox1.Image = bmpTest;
}

Answer 1

This is not a good way to fill an area with a color. A better approach would be to determine the color of the pixel in the source image and use that color to fill the target.

Bitmap source = // get the source

Color color = source.GetPixel(1, 1);

Bitmap target = // get the target    

target.Clear(color);

Nonetheless, the problem is likely the InterpolationMode, as this is what’s used when scaling images. Try using Low istead of HighQualityBicubic.

g.InterpolationMode = InterpolationMode.Low;