在位图中创建Windows设置（DPI）独立文本

Question

I am wondering if it is possible to create Bitmaps containing text in C# (.NET 4.0) where the text is totally independent of any Windows configuration or settings.

I am trying around with that issue for weeks but did not really find a solution. To investigate the facts I have created a small console application in VS2010 containing the following code:

class Program
{
    private const TextFormatFlags DefaultTextFormatFlags = TextFormatFlags.NoPadding | TextFormatFlags.Left | TextFormatFlags.NoPrefix;

    private static int[] aPpi = { 72, 96, 192 };
    private static bool[] aSetPpi = { false, true };
    private static GraphicsUnit[] aGU = { GraphicsUnit.Pixel, GraphicsUnit.Point };
    private static int[] aSize = { 10, 14, 100, 200 };

    static void Main(string[] args)
    {
        int idx = 0;

        foreach (int ppi in aPpi)
        {
            foreach (bool b in aSetPpi)
            {
                foreach (GraphicsUnit gu in aGU)
                {
                    foreach (int size in aSize)
                    {
                        idx++;
                        CreatePic(idx, ppi, b, gu, size);
                    }
                }
            }
        }
    }

    private static void CreatePic(int id, int ppi, bool setppi, GraphicsUnit unit, int size)
    {
        Bitmap bm;

        Graphics g;
        Font font;
        Color txtColor;
        Color background;
        string fn;

        txtColor = Color.Blue;
        background = Color.Transparent;

        bm = new Bitmap(1000, 1000,  PixelFormat.Format32bppArgb);

        if (setppi)
        {
            bm.SetResolution(ppi, ppi);
        }

        g = Graphics.FromImage(bm);
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.NearestNeighbor;
        g.PageUnit = GraphicsUnit.Pixel;

        font = new Font("Roboto Lt", size, FontStyle.Regular, unit);

        TextRenderer.DrawText(g, "0T1ÄgÁj", font, new Point(0, 0), txtColor, background, DefaultTextFormatFlags);

        font.Dispose();

        g.Dispose();

        fn = string.Format("p_{0,-5}_{1,-4}_{2,-4}_{3,-4}.png", unit, size, setppi, ppi);
        bm.Save(fn, ImageFormat.Png);
        bm.Dispose();
    }
}

It creates PNG files each containing a test text in several combinations of font size, bitmap resolution and font size unit. At first I executed it while Windows was set to normal font size (Screen resolution setting to 100%), the second run was done with Screen resolution set to 125%.

I basically had 2 expectations:

1. When the Font size is specified with "Pixel" as unit, the windows resolution does not have any influence on the result in the bitmap. It will be always the same for same font sizes also independent of the resolution of the bitmap.
2. When the Font size is specified with "Point" as unit and additionally the Bitmap resolution was explicitly set, the Windows screen resolution setting does not have any influence on the result in the bitmap.

But the results did not meet my expectations. 1. When the Font size was specified with "Pixel" as unit, the windows resolution caused different results for 14px size, but not for 10px, 100px or 200px. 2. When the Font size is specified with "Point" as unit it did not have any effect to set the Bitmap resolution explicitly.

Until now I did not found any explanation for that.

Can anyone tell me how I can create Text in Bitmaps always looking like Windows resolution is set to 100% even if it is not?

Thank you in advance!

Answer 1

Take a look at the Win32 API LogFont structure and the lfHeight member to get an idea as to how windows is fulfilling your request for height. Basically you'll want to use the equation given there and a constant DPI. You're specifying a screen device context so when you change from 100% to 125% etc.. the DPI of the screen is changing and affecting the size of the font returned by the Windows font mapper.