Winforms transparent form issue

Question

I am trying to draw a form that has a transparent background and a blurred dropshadow from a main body.

The way I did this is to use the form as the bounding box, then draw the actual form's body inside that with margin on the sides for the shadow to show. The shadow expands from behind the drawn body.

So, I have 1 form in which I draw inside. I want the form's background to be transparent so that only the drawn body and the shadow appears.

I have some problems with the shadow. The shadow is a bitmap in which I apply AForge's gaussian blur to create the shadow effect. The bitmap is placed behind the drawn body so that it appears as the shadow.

Here is the problem: The form's backcolor remains drawn where the shadow is. The shadow bitmap is transparent except for the blurred blacks.

I have tried setting backcolor to transparent, or a misc color and then transparencykey to the same color. results stays the same.

Picture of what I want to achieve:

(light pink is not a part of the program, imagine it as the desktop)

Picture of what I am getting:

(I am only drawing the shadow for this example, not the white body(which works fine), the form's background refuses to become transparent where the shadow exists..)

This is the code for creating the shadow's bitmap.

    private void _CreateShadow()
    {
        shadowImg = new Bitmap(m_ShadowRect.Width, m_ShadowRect.Height, PixelFormat.Format32bppArgb);
        Graphics g = Graphics.FromImage(shadowImg);
        int diameter = Math.Min(shadowImg.Width, shadowImg.Height);
        g.DrawEllipse(m_ShadowPen, m_ShadowWidth, m_ShadowWidth, m_BodyRect.Width - m_ShadowWidth, m_BodyRect.Height - m_ShadowWidth);
        filter.ApplyInPlace(shadowImg);
    }

    private void _DrawShadow(Graphics g)
    {
        g.DrawImage(shadowImg, m_ShadowRect.X, m_ShadowRect.Y);
    }

    protected override void OnPaint(PaintEventArgs pe)
    {
        base.OnPaint(pe);

        _DrawShadow(pe.Graphics);
        _DrawBody(pe.Graphics);
    }

Please, any pointers?

You can use Layered Windows. Using a layered window can significantly improve performance and visual effects for a window that has a complex shape, animates its shape, or wishes to use alpha blending effects. Here is an example which creates a shaped form using a png image: [C# Windows Form Transparent Background Image](http://stackoverflow.com/a/33531201/3110834) — Reza Aghaei, Jul 17 '16 at 06:59
@RezaAghaei Thank you, this approach is the same as the one commented below. Great answer — Alx, Jul 18 '16 at 18:39

score 1 · Accepted Answer · answered Jul 17 '16 at 03:07

1

Have a look at this file in a github repository, mainly the lines 123 - 131, 253 - 415. The code uses GDI+ methods to draw transparent bitmap that replaces the form background.

You cannot draw anything transparent on a Form because all the methods (like OnPaint) use Format24bppRgb not Format32bppArgb, which is what you need.

answered Jul 17 '16 at 03:07

Is there any way I can use the form's original onpaint (for other drawing than the shadow bitmap) along with the proposed method? I have tried to use both at the same time, but then the program just renders everything completely transparent – Alx Jul 18 '16 at 18:44
I have experimented with `OnPaint` in the past and the results depend on what OS version, Windows 8 can't draw alpha colors, but Window 10, can. Alpha Colors only work if you don't put `base.OnPaint(e);`. Also you have to clear everything when redrawing, otherwise the trasparent colors layer over each other. – Jul 19 '16 at 01:10

Winforms transparent form issue

1 Answers1