Overloading control drawing in c# has text rendering problems

Question

I've sub-classed a control (ToolStripStatusLabel) to try and override the way it paints. At the moment I would expect this code to effectively do nothing, but it leads to a strange output:

protected override void OnPaint(PaintEventArgs e)
{
  // Create a temp image to draw to and then put that onto the control transparently
  using (Bitmap bmp = new Bitmap(this.Width, this.Height))
  {
    using (Graphics newGraphics = Graphics.FromImage(bmp))
    {
      // Paint the control to the temp graphics
      PaintEventArgs newEvent = new PaintEventArgs(newGraphics, e.ClipRectangle);
      base.OnPaint(newEvent);

      // Copy the temp image to the control
      e.Graphics.Clear(this.BackColor);
      e.Graphics.DrawImage(bmp, new Rectangle(0, 0, this.Width, this.Height), 0, 0, bmp.Width, bmp.Height, GraphicsUnit.Pixel);//, imgAttr);
    }
  }
}

When I run this code the text on the control comes out very strangely, the expected image is at the top, the actual output is at the bottom:

It looks like when the control is drawing the text the alpha blending with the antialiased text is going wrong.

Things that I've tried:

Setting the CompositingMode of e.graphics and newGraphics
Setting the TextRenderingHint.
setting the pixel format of newGraphics to 32Bpp ARGB and Premultiplied ARGB
Clearing the newGraphics with the controls background colour before asking the base class to render.

Try `PixelFormat.Format32bppPArgb` for the `Bitmap`, or clear the bitmap instead of the control before drawing. If I'm not mistaken this is the kind of issue that requires premultiplied alpha. — Joey, Jan 23 '18 at 15:56
I forgot to mention that I've tried explicitly setting the pixel format - I tried 32bppARGB, but changing it to 32bppPARGB didn't have any effect. I have also tried clearing the newGraphics context before getting the control to render — steeveeet, Jan 23 '18 at 17:18
@joey Re the background clearing: that certainly helps when using DrawString() (i've just done a test with that), but strangely it makes no difference to the results of the base class drawing. I wonder what Microsoft are doing in there! — steeveeet, Jan 23 '18 at 17:49
If you are trying to customize the appearance of `ToolStripStatusLabel`, creating a renderer is the way to go. Take a look at [this post](https://stackoverflow.com/a/38155401/3110834) for example. — Reza Aghaei, Jan 23 '18 at 18:09
Also you should know you can use a `Label` inside the `StatusStrip`. Take a look at [this post](https://stackoverflow.com/a/38359163/3110834) for example. — Reza Aghaei, Jan 23 '18 at 18:11

steeveeet · Accepted Answer · 2018-01-31T09:09:15.557

TL;DR: You'll need to render the text yourself with a custom renderer that re-implements OnRenderItemText, probably using graphics.DrawString() to do the drawing ultimately.

Another option would be to use a label in the status bar (as mentioned by @Reza Aghaei). Then you can set UseCompatibleTextRendering to true to make it use GDI+ rather than GDI

This seems to be an inherent problem in the way that the text is rendered at the lowest level. If you add a normal ToolStripStatusLabel and set its TextDirection to be Vertical90 then you get the same result, where the anti-aliasing of the text appears to have no alpha with the background.

Looking at the source you'll see that a very similar bit of code is called where the text is rendered to a bitmap and then in this case rotated:

            using (Bitmap textBmp = new Bitmap(textSize.Width, textSize.Height,PixelFormat.Format32bppPArgb)) {

                using (Graphics textGraphics = Graphics.FromImage(textBmp)) {
                    // now draw the text..
                    textGraphics.TextRenderingHint = TextRenderingHint.AntiAlias;
                    TextRenderer.DrawText(textGraphics, text, textFont, new Rectangle(Point.Empty, textSize), textColor, textFormat);
                    textBmp.RotateFlip((e.TextDirection == ToolStripTextDirection.Vertical90) ? RotateFlipType.Rotate90FlipNone :  RotateFlipType.Rotate270FlipNone);
                    g.DrawImage(textBmp, textRect);
                }
            }

So this seems like a fundamental problem when text is rendered to a bitmaps graphics context (as opposed to the control's graphics context). Ultimately the code that is called is:

        using( WindowsGraphicsWrapper wgr = new WindowsGraphicsWrapper( dc, flags ))
        {
            using (WindowsFont wf = WindowsGraphicsCacheManager.GetWindowsFont( font, fontQuality )) {
                wgr.WindowsGraphics.DrawText( text, wf, bounds, foreColor, GetIntTextFormatFlags( flags ) );
            }
        }

Which I think is wading off into GDI (as opposed to GDI+) which has trouble with alpha on text.

Your best bet is to write a custom renderer that re-implements OnRenderItemText, probably with some 'inspiration' from the source from the default implementation.

Overloading control drawing in c# has text rendering problems

1 Answers1