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Off-screen video frame composition, decomposition

10 Jan 2014  | Sharath Bhat

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In the case of video streams with a frame rate of 30 fps, these operations have to be carried out at every 1/30s, i.e., a 33.33-ms frame period. And if the frame's pixel resolutions are higher, i.e., 720p, 1080p, then it becomes a highly CPU intensive task to process such frames. The CPU loading results in a poor user interface due to less I/O interactivity. It also results in dropping of frames during frame composition if the processor is unable to process a frame within the frame period.

Using platform specific resizer/scaler
Some embedded processors provide platform-specific resizer hardware implementation. These are poly-phase filters implemented in the hardware with programmable filter coefficients for different scaling ratios.

In order to expose this platform-specific feature for the application, a custom device driver will be provided by the CPU chip vendor. The application can use this resizer through the resizer device driver for composition functionality. The application resizes one input frame at a time on to the output frame with different input/output crop parameters. The resizer interrupts the CPU core once it completes resizing of one input frame onto an output frame.

For the application to compose N input frames on to one output frame, it has to perform N resize operations in one frame period. These resizers are very efficient and perform resize operations within acceptable time limits. The output frame rate of the composited frames depends on the resize time taken for every single resize operation.

For N to 1 composition, involving input/output frames of same pixel resolution,

In the case of composition involving input/output frames of different pixel resolution,

The above formulas are also applicable to a decompositor with one input and N output channels.

Though using this method provides an efficient solution, it fails to address portability. These hardware features are specific to an embedded processor. Either it will not be available on a different embedded processor or it will be implemented in a different way with different feature list.

The interfaces and the features provided by the device drivers controlling this custom hardware are different on each embedded processor. Also, in general, some features like rotation, alpha blending etc., cannot be implemented using such resizer hardware.

About the author
Sharath Bhat is lead engineer for Multimedia Systems, System On Chip IP Business Unit at Ittiam Systems Pvt. Ltd, Bangalore, India.

To download the PDF version of this article, click here.

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