How to draw transparent image in c#

Question

I made a ListBox.

There are image and Text in ListBox Like picture.

And i want to draw transparent Image on listbox When mousemove.

private void ListBox_DrawItem(object sender, DrawItemEventArgs e)
{
    int iWidth = image.Width;
    int iHeight = image.Height;

    ImageAttributes attr = new ImageAttributes();

    ColorMatrix matrix = new ColorMatrix();
    matrix.Matrix33 = 0.8f;
    attr.SetColorMatrix(matrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);

    e.Graphics.DrawImage(image, new Rectangle(0, 0 iWidth, iHeight), 0, 0, iWidth, iHeight, GraphicsUnit.Pixel, attr);
}

This is my code.

But I have a problem.

Although I input the same value of transparency, Transparency of each image is different..

How can I solve it ??

++ Add Code

I use Scrollbar.

int LocationY = ListBox.PreferredHeight / ListBox.Items.Count * (Index/*ListBox index*/ - ListBox.TopIndex);
if ((LocationY + image.Height) > Listbox.Height)
       LocationY = ListBox.Height - image.Height;

e.Graphics.DrawImage(image, new Rectangle(0, LocationY, iWidth, iHeight), 0, 0, iWidth, iHeight, GraphicsUnit.Pixel, attr); 

Answer 1

When ownerdrawing a ListBox it is important to understand how the DrawItem event is called.

It will be called on several occasions, always referring only to the Item it was called for.

But for better performance not all occasions will make the ListBox draw all its items!

The area you are meant to draw on is the e.Bounds rectangle.

You can still draw over the whole ListBox but as the event usually is called several times in a row you may run into some issues. One such issue is with transparent drawings, as these will stack and become less and less transparent!

Let's look a few cases:

• During a Refresh all visible Items will be drawn
• When selecting an Item the selected item and also all previously selected items will be drawn

So, after the initial display each item will have been drawn once but after selecting item3 and then item1 the first three items will all have been drawn different times: item1 : 2x, item2 :1x and item3 : 3x.

The workaound will usually be to create a clean slate before drawing an item, maybe like this:

e.Graphics.FillRectangle(Brushes.White, e.Bounds);

This however will not go well with anything you draw across the item bounds!

Any image you draw on an Item should respect the bounds. You can either squeeze an image into the item's bounds:

e.Graphics.DrawImage(image, e.Bounds, 0, 0, iWidth, iHeight, GraphicsUnit.Pixel, attr);

Or you can draw just a slice of a larger image, maybe like this:

e.Graphics.DrawImage(image, e.Bounds, 
                            0, e.Index * e.Bounds.Height, iWidth, e.Bounds.Height, 
                            GraphicsUnit.Pixel, attr);