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in Technique[技术] by (71.8m points)

c# - TextRenderer.DrawText in Bitmap vs OnPaintBackground

If I use TextRenderer.DrawText() using the Graphics object provided in the OnPaintBackground my text looks perfect. If I create my own Bitmap and use the Graphics object obtained from my Bitmap my text looks terrible. It looks like it is anti-aliasing the text using black, not the bitmap's background color. I can avoid this problem if I use Graphics.DrawString(), but this method has horrible kerning problems. What should I do? How can I get TextRenderer.DrawText() to anti-alias properly using the Bitmap's contents?

Looks terrible:

Bitmap bmp = new Bitmap(100, 100, PixelFormat.Format32bppArgb);
using (Graphics g = Graphics.FromImage(bmp))
{
    g.Clear(Color.Red);
    TextFormatFlags tf = TextFormatFlags.Left;
    TextRenderer.DrawText(g, @"C:DevelopmentTestinglag", font, clip, Color.White,
                          Color.Transparent, tf);
}

Looks good, but I want to render this onto a bitmap, NOT onto the control's surface:

protected override void OnPaintBackground(PaintEventArgs e)
{
    e.Graphics.Clear(Color.Red);
    TextFormatFlags tf = TextFormatFlags.Left;
    TextRenderer.DrawText(e.Graphics, @"C:DevelopmentTestinglag", font, clip,
                          Color.White, Color.Transparent, tf);
}

What is the difference?

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by (71.8m points)

The answer is not to use TextRenderer. TextRenderer is a wrapper for the GDI (not GDI+) implementation of text rendering, which has lots of features, but doesn't interoperate well with in-memory DCs as you have discovered.

Use Graphics.DrawString & Graphics.MeasureString, but remember to pass it StringFormat.GenericTypographic to get accurate size and positioning.

The reason TextRenderer was introduced initially was that GDI+ didn't support all the complex scripts that GDI's Uniscribe engine did. Over time however GDI+ support for complex scripts has been expanded, and these days there aren't any good reasons left to use TextRenderer (it's not even the faster of the two anymore, in fact quite the opposite it appears).

Really, though, unless you are running into serious, measurable performance issues just use Graphics.DrawString.


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