Question

I am deriving a custom control from TVirtualDrawTree and I am overriding the DoPaintBackground event to draw a background gradient effect for the tree view.

I am also overriding the DoBeforeItemPaint function so I can custom draw the tree view items. However, I can’t quite manage to get the items to paint with a transparent background.

Looking into the source for TVirtualDrawTree, it looks as though the item is copied to a TBitmap image and then copied onto the canvas, however, I have tried editing the source and setting the transparency options on the bitmap itself and it still doesn’t seem to be working.

I have also tried clearing the canvas before drawing e.g. Canvas.Brush.Style := bsClear and filling the item rect but no joy.

Answer 1

Don’t do transparency, cheat!

Drawing the gradient sounds like a lot of work: draw it to a temporary bitmap so you don’t need to re-generate it every time DoPaintBackgrdound() is called. Once you have the background in a bitmap you can BitBlt the relevant portion into the Canvas when you handle DoBeforeItemPaint, and you can BitBlt the bitmap to the whole virtual tree when you need the whole background.

This way you don’t need to deal with expensive transparency yet for the end user it looks like your items had been painted using transparency. It’s a win:win situation.