This page explains what overdraw is, how to diagnose it, and ways to eliminate or mitigate it.
When an app draws the same pixel more than once within a single frame, this is called overdraw. Overdraw is usually unnecessary, and it's best to eliminate it. Overdraw becomes a performance problem when it wastes GPU time to render pixels that don't contribute to what the user sees on the screen.
About overdraw
Overdraw refers to the system drawing a pixel on the screen multiple times in a single frame of rendering. For example, if you have a stack of UI cards, each card hides a portion of the one below it. However, the system still needs to draw the hidden portions of the cards in the stack. This is because stacked cards are rendered according to the painter's algorithm—that is, in back-to-front order. This sequence of rendering lets the system apply proper alpha blending to translucent objects, such as shadows.
Find overdraw problems
The platform offers the following tools to help you determine if overdraw is affecting your app's performance.