1 	\section1 Hardware Acceleration

     2 	

     3 	When designing applications for embedded devices there is often a

     4 	compromise between graphics effects and performance. On most

     5 	devices, you cannot have both simply because the hardware needed

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

     7 	devices that use hardware dedicated to graphics operations there is

     8 	less need to compromise.

     9 	

    10 	In addition to enabling dynamic graphics effects, there are two

    11 	other benefits to using graphics acceleration. One is that graphics

    12 	acceleration hardware is more power efficient than using the CPU.

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

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

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

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

    17 	while a software implementation takes easily more than 20 cycles.

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

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

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

    21 	leave out transformation and lighting from the graphics pipeline

    22 	and only implements rasterization.

    23 	

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

    25 	power saving or to perform other operations in parallel. Often

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

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

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

    29 	experience.

    30 	

    31 	The key to writing good applications for devices is therefore to

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

    33 	and to take advantage of any graphics dedicated hardware. Qt

    34 	provides several ways to both render advanced effects on the screen

    35 	and speed up your application using hardware accelerated graphics.

    36 	

    37 	\tableofcontents 

    38 	

    39 	\section2 Qt for Embedded Graphics pipeline

    40 	

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

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

    43 	QPainter use different paint engines implemented in the QPaintEngine API to

    44 	do the actual painting.

    45 	

    46 	The QPaintEngine API provides paint engines for each window system and

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

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

    49 	as OpenVG and DirectFB(Embedded Linux only).

    50 	

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

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

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

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

    55 	graphics operations that are supported by the target devices hardware

    56 	configuration.

    57 	

    58 	\image platformHWAcc.png

    59 	

    60 	The paint engine will direct all graphics operations supported by the

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

    62 	framebuffer. Unsupported graphics operations falls back to the

    63 	QRasterPaintEngine and are handled by the CPU before sent to the

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

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

    66 	to memory consumption, and should be avoided.

    67 	

    68 	\section2 Hardware configuration requirements

    69 	

    70 	Before implementing any application using hardware acceleration, it is wise

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

    72 	that are available for the target device. 

    73 	

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

    75 	QRasterPaintEngine, which handles the acceleration using software. On

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

    77 	CE), Qt will use the

    78 	respective paint engines to accelerate painting. However, hardware

    79 	configurations that only support a limited set of hardware acceleration

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

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

    82 	engine.

    83 	

    84 	\section3 Different architectures

    85 	

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

    87 	which hardware acceleration techniques to use. There are mainly two

    88 	different architectures on embedded devices. These are devices with a

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

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

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

    92 	region and sometimes not.

    93 	

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

    95 	the different graphics operations supported by their hardware.

    96 	

    97 	\list 1

    98 		\o No support for graphics acceleration.

    99 		\o Support for blitter and alpha blending.

   100 		\o Support for path based 2D vector graphics.

   101 		\o Support for fixed function 3D graphics.

   102 		\o Support for programmable 3D graphics.

   103 	\endlist

   104 	

   105 	Based on these characteristics the table below recommends which paint

   106 	engines to use with the different types of hardware configurations.

   107 	

   108 	\section3 Recommended use of hardware acceleration based on hardware

   109 	

   110 	\table

   111 		\header

   112 			\o Type

   113 			\o UMA

   114 			\o Non-UMA

   115 		\row

   116 			\o \bold {None}

   117 			\o Qt Raster Engine

   118 			\o Qt Raster Engine

   119 		\row

   120 			\o \bold {Blitter}

   121 			\o DirectFB

   122 			\o DirectFB

   123 		\row

   124 			\o \bold {2D Vector}

   125 			\o OpenVG

   126 			\o OpenVG

   127 		\row

   128 			\o \bold {Fixed 3D}

   129 			\o OpenGL (ES) 1.x

   130 			\o OpenGL (ES) 1.x

   131 		\row

   132 			\o \bold {Programmable 3D}

   133 			\o OpenGL (ES) 2.x

   134 			\o OpenGL (ES) 2.x

   135 	\endtable

   136 	

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

   138 	handles most of the operations.

   139 	

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