Android™ Application Development - Bahar Ali Khan

Chapter 11: Advanced Android Development
❑ DiscretePathEffect Similar to the DashPathEffect, but with added randomness. Specifi
es the length of each segment and a degree of deviation from the original path to use when
drawing it.
❑ PathDashPathEffect This effect lets you defi ne a new shape (path) to use as a stamp to outline
the original path.
The following effects let you combine multiple Path Effects to a single Paint.
❑ SumPathEffect Adds two effects to a path in sequence, such that each effect is applied to the
original path and the two results are combined.
❑ ComposePathEffect Compose applies fi rst one effect and then applies the second effect to
the result of the fi rst.
Path Effects that modify the shape of the object being drawn will change the area of the affected shape.
This ensures that any fi ll effects being applied to the same shape are drawn within the new bounds.
Path Effects are applied to Paint objects using the setPathEffect method as shown below:
borderPaint.setPathEffect(new CornerPathEffect(5));
The Path Effects API sample gives an excellent guide to how to apply each of these effects.
Changing the Xfermode
Change a Paint’s Xfermode to affect the way it paints new colors on top of what’s already on the Canvas.
Under normal circumstances, painting on top of an existing drawing will layer the new shape on top. If
the new Paint is fully opaque, it will totally obscure the paint underneath; if it’s partially transparent, it
will tint the colors underneath.
The following Xfermode subclasses let you change this behavior:
❑ AvoidXfermode Specifi es a color and tolerance to force your Paint to avoid drawing over (or
only draw over) it.
❑ PixelXorXfermode Applies a simple pixel XOR operation when covering existing colors.
❑ PorterDuffXfermode This is a very powerful transfer mode with which you can use any of
the 16 Porter-Duff rules for image composition to control how the paint interacts with the existing
canvas image.
To apply transfer modes, use the setXferMode method as shown in the sample below:
AvoidXfermode avoid = new AvoidXfermode(Color.BLUE, 10, AvoidXfermode.Mode.AVOID);
borderPen.setXfermode(avoid);
Improving Paint Quality with Anti-Aliasing
When you create a new Paint object, you can pass in several fl ags that affect the way the Paint will
be rendered. One of the most interesting is the ANTI_ALIAS_FLAG, which ensures that diagonal lines
drawn with this paint are anti-aliased to give a smooth appearance (at the cost of performance).
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