\section1 Hardware Acceleration

	When designing applications for embedded devices there is often a

	compromise between graphics effects and performance. On most

	devices, you cannot have both simply because the hardware needed

	for such operations just is not there. With a growing number of

	devices that use hardware dedicated to graphics operations there is

	less need to compromise.

	In addition to enabling dynamic graphics effects, there are two

	other benefits to using graphics acceleration. One is that graphics

	acceleration hardware is more power efficient than using the CPU.

	The reason for this is that the CPU might require a clock speed

	that is up to 20 times higher than the GPU, achieving the same

	results. E.g. a typical hardware accelerated mobile graphics unit

	can rasterize one or two bilinear texture fetches in one cycle,

	while a software implementation takes easily more than 20 cycles.

	Typical \e {System-on-a-chip} (SoC) graphics hardware generally have

	a much lower clock speed and memory bandwidth, and different level

	of acceleration than desktop GPUs. One example is that many GPUs

	leave out transformation and lighting from the graphics pipeline

	and only implements rasterization.

	Another reason to use a GPU is to offload the main CPU, either for

	power saving or to perform other operations in parallel. Often

	drawing speed with a GPU is not that much faster than a CPU but

	the clear benefit of using the GPU is to free up the CPU to perform

	other tasks which can be used to create a more responsive use

	experience.

	The key to writing good applications for devices is therefore to

	limit the wow factor down to what the target hardware can handle,

	and to take advantage of any graphics dedicated hardware. Qt

	provides several ways to both render advanced effects on the screen

	and speed up your application using hardware accelerated graphics.

	\tableofcontents 

	\section2 Qt for Embedded Graphics pipeline

	Qt uses QPainter for all graphics operations. By using the same API

	regardless of platform, the code can be reused on different devices.

	QPainter use different paint engines implemented in the QPaintEngine API to

	do the actual painting.

	The QPaintEngine API provides paint engines for each window system and

	painting framework supported by Qt. In regards to Qt for Embedded, this

	also includes implementations for OpenGL ES versions 1.1 and 2.0, as well

	as OpenVG and DirectFB(Embedded Linux only).

	By using one of these paint engines, you will be able to improve the

	graphics performance of your Qt application. However, if the graphics

	operations used are not supported, this might as well be a trap, slowing

	down your application significantly. This all depends on what kind of

	graphics operations that are supported by the target devices hardware

	configuration.

	\image platformHWAcc.png

	The paint engine will direct all graphics operations supported by the

	devices hardware to the GPU, and from there they are sent to the

	framebuffer. Unsupported graphics operations falls back to the

	QRasterPaintEngine and are handled by the CPU before sent to the

	framebuffer. In the end, the operating system sends the paint updates off

	to the screen/display. The fallback operation is quite expensive in regards

	to memory consumption, and should be avoided.

	\section2 Hardware configuration requirements

	Before implementing any application using hardware acceleration, it is wise

	to get an overview of what kind of hardware accelerated graphics operations

	that are available for the target device. 

	\note On devices with no hardware acceleration, Qt will use

	QRasterPaintEngine, which handles the acceleration using software. On

	devices supporting OpenGL ES, OpenVG or DirectFB(not supported by Windows

	CE), Qt will use the

	respective paint engines to accelerate painting. However, hardware

	configurations that only support a limited set of hardware acceleration

	features, might slow the application graphics down rather than speeding it

	up when using unsupported operations that must fall back to the raster

	engine.

	\section3 Different architectures

	Based on the architecture used in a device we can make a recommendation on

	which hardware acceleration techniques to use. There are mainly two

	different architectures on embedded devices. These are devices with a

	Unified Memory Architecture (UMA), and devices with dedicated graphics

	memory. Generally, high-end devices will have dedicated graphics memory.

	Low-end devices will just use system memory, sometimes reserving a memory

	region and sometimes not.

	In addition to this, we can categorize the devices into five types based on

	the different graphics operations supported by their hardware.

	\list 1

		\o No support for graphics acceleration.

		\o Support for blitter and alpha blending.

		\o Support for path based 2D vector graphics.

		\o Support for fixed function 3D graphics.

		\o Support for programmable 3D graphics.

	\endlist

	Based on these characteristics the table below recommends which paint

	engines to use with the different types of hardware configurations.

	\section3 Recommended use of hardware acceleration based on hardware

	\table

		\header

			\o Type

			\o UMA

			\o Non-UMA

		\row

			\o \bold {None}

			\o Qt Raster Engine

			\o Qt Raster Engine

		\row

			\o \bold {Blitter}

			\o DirectFB

			\o DirectFB

		\row

			\o \bold {2D Vector}

			\o OpenVG

			\o OpenVG

		\row

			\o \bold {Fixed 3D}

			\o OpenGL (ES) 1.x

			\o OpenGL (ES) 1.x

		\row

			\o \bold {Programmable 3D}

			\o OpenGL (ES) 2.x

			\o OpenGL (ES) 2.x

	\endtable

	\note Since the DirectFB API is quite primitive, the raster paint engine

	handles most of the operations.

	\note Blitter and Alpha blending is currently not supported on Windows CE.