Alpha blending with the same transparent image

Question

I'd like to draw in my iPad app as in the following image.

But what I got is the following result.

Following is my code to draw the image.

 [brushImage drawAtPoint:CGPointZero blendMode:kCGBlendModeNormal alpha:.1f];   

How do I blend a transparent image and get the result like first picture? Any suggestion?

Many thanks.

Answer 1

i guess, when you start brushing you should create a UIView, that will contain all the points; than apply the alpha to that view

(this is a useful way, because let you easily implement the undo functionality in your project)

Answer 2

You can use a non transparent image for the different parts and apply the alpha to a containing view. That way, instead of getting this:

You'll get this: