Graphics transparency on PictureBox

Question

First of all, this is not about making the PictureBox control transparent. It's about bitmap transparency on the fully opaque "canvas".

The PictureBox will always have the size of 300*300 with white background. No transparency is needed for the control.

What I need is the way to draw the transparent rectangle (or whatever else) onto the pictureBox, so anything that was already there will be seen "through" the rectangle.

Say I have a following code

Bitmap bmp = new Bitmap(300, 300);
Graphics g = Graphics.FromImage(bmp);
g.FillRectangle(new SolidBrush(Color.White), 0, 0, 300, 300);
g.FillRectangle(new SolidBrush(Color.Red), 100, 100, 100, 100);
pictureBox.Image = bmp;

This will draw a red rectangle in the middle of the white canvas. Now, I need another (transparent) "layer" on the picture containing another rectangle, but one that is transparent.

I can try

Brush brush = new SolidBrush(Color.FromArgb(128, 0, 80, 0));
g.FillRectangle(brush, 50, 50, 200, 200);

Since I am using a color by specifying its alpha = 128, the resulting rectangle should be transparent so the first red rectangle should be seen through this other green one.

However, this does not happen correctly. I can see the red rectangle behind the new green one, but the part of the green rectangle that does not overlap the red one will remain completely opaque. However, if I set the alpha value of the color to some extremely small value (say 1-5), the whole rectangle will look transparent. This is not normal in my opinion - that 5/255 is only half transparent and that 128/255 is not transparent at all... And if there was a string drawed previously with g.DrawString(), the string is either displayed behind the green rectangle or it is not, depending on the level of transparency. For example if the Alpha is greater than or equals (around) 40, the string is not visible at all, and if it is less than 40, then it will show, more visible for smaller alpha values, down to alpha = 0.

How is this brush (when created from Argb color) applied? Am I missing something? To me it seems that setting a transparent brush makes the background "more visible" instead of setting the object "less visible".

Thanks for any replies with suggestions.

[EDIT] It seems I had a nasty bug in application logic, so the drawing routine happened in a loop, so when I accumulated certain number of transparent rectangles, they became more and more thick.

The code, when taken out of the loop, works correctly.

My bad.

serhio serhio · Accepted Answer · 2010-01-13T10:16:06

alt text http://lh4.ggpht.com/_1TPOP7DzY1E/S02ivAoGgTI/AAAAAAAAC6s/ZQvZQ5GdwSU/s800/Capture4.png

is done by this code:

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        Bitmap bmp = new Bitmap(300, 300);
        Graphics g = Graphics.FromImage(bmp);
        g.FillRectangle(new SolidBrush(Color.White), 0, 0, 300, 300);
        g.FillEllipse(new SolidBrush(Color.Blue), 25, 25, 100, 200);
        g.FillRectangle(new SolidBrush(Color.Red), 100, 100, 300, 100);
        g.DrawString("this is a STRING", SystemFonts.DefaultFont, 
            Brushes.Black, new Point(150, 150));
        pictureBox1.Image = bmp;

        Brush brush = new SolidBrush(Color.FromArgb(40, 0, 80, 0));
        g.DrawRectangle(Pens.Black, 50, 50, 200, 200);
        g.FillRectangle(brush, 50, 50, 200, 200);
    }

The green part is not opaque as you can see... The string is perfectly visible.

To me it seems that setting a transparent brush makes the background "more visible" instead of setting the object "less visible".

background "more visible" and object "less visible" are the same thing...

Graphics transparency on PictureBox

1 Answers