Android(安卓)Copybit简介

以前做Camera时为了解决VideoCamera取景时，在高分辨率下数码变焦花屏的问题，找到的关于Copybit的资料，目前还没找到出处。对Copybit没有过多的研究，先贴到这儿，以后有时间好好研究一下。

Android HAL consists of several hardware accelerated modules, and copybit is one of them implemented in G1 phone.

While OpenGL ES cannot be used or it's in software implementation, copybit is used. Actually in cupcake branch, SurfaceFlinger always uses copybit instead of GL if copybit is available -- but that's an oversight.

[[ Functions ]]
copybit is just an abstraction for a basic 2D blitter with support for rotation, scaling and color/pixelformat conversions. These features can be supported in full or partially.

If the copybit module is present, then SurfaceFlinger will try to use it instead of using OpenGL ES.

On the G1 copybit is used for all window compositions and video playback (CC and scaling), on this h/w copybit is very fast and provide very good up and down scaling.

[[ Donut Changes ]]
HOWEVER, on the release after Donut, the copybit module won't be used by SurfaceFlinger directly, instead it will be used as a back-end for our software OpenGL ES 1.x and SurfaceFlinger will only use the OpenGL ES API. Eventually, the copybit module will be phased out, instead, we will ask the ISV/OEMs to implement these features on their OpenGL ES drivers, if they think it is necessary.

[[ Usability ]]
If your h/w doesn't have accelerated GL, but has some sort of decent 2D blitter, you can write a copybit HAL module for it, and it will be used automatically by SurfaceFlinger -- at least under cupcake and donut.

[[ Limitations ]]
There are no mechanism to accelerate applications at the moment. Skia is a purely software renderer. It can greatly benefit from newer CPUs though, which have SIMD instructions, often, it'll be able to operate at bus speed in that case. We're working on optimization in that area.

If a device has a GPU, it makes no sense to implement copybit, because

(1) it won't be used by apps and

(2) SurfaceFlinger already uses the GPU for its work.

Now, if the device has an additional 2D part (like the G1), it is useful to implement copybit, this way application won't have to share the GPU with SurfaceFlinger

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