Dr. Dobb's Journal - August 2008 - (Page 22)

State of the Art OPENGL AND MOBILE DEVICES: ROUND 2 continued from page 20 • There is no stencil or accumulation buffer. • There are only two texture units. • The maximum texture size is 1024×1024 (use power of two only). • The maximum space for textures and surfaces is 24MB. • Only 2D textures are supported. • There is no software rendering fallback. Figure 1: OpenGL ES app running on the desktop with the iPhone emulator. The PowerVR chip uses a full floatingpoint pipeline throughout. The OpenGL lighting model is fully hardware accelerated, and there is no need to use fixed-point values for either lighting and material values, or vertex data. For best performance, use directional lights instead of point lights when possible, and try to always use indexed strips for geometry submission. To minimize bandwidth, you can use unsigned byte values for colors, and either unsigned byte or shorts instead of floats for texture coordinates. Mipmaps (optimized collections of bitmap images accompanying main textures that increase rendering speed) were not available with OpenGL ES 1.0, but are with 1.1. On the PowerVR hardware, bilinear filtering is considered “free” but you should , limit the mipmapping mode to GL_LINEAR_MIPMAP_NEAREST for best performance. Loading textures with an internal format of 565 instead of 888 speeds things up, and (if possible) you may want to make use of the PowerVR’s native texture compression formats of either PVRTC2 or PVRTC4. One of the more interesting aspects of the PowerVR chip is that it uses Tile-Based Deferred Rendering (TBDR), in which all rendering commands are queued and no actual rendering occurs until all commands for a given frame are given. The rendering commands are then analyzed, geometry is sorted, and converted into strips. Rendering then occurs. You still need to sort by alpha for transparency, but sorting front-to-back to eliminate overwrite is no longer necessary. (This is somewhat like the “early-z” feature some desktop graphics hardware supports.) While this has many performance advantages, there is one performance gottcha for desktop OpenGL developers coming to this platform—state changes can be very expensive. While reducing state changes has always been a staple of OpenGL performance techniques, many desktop systems only moderately benefit from excessive texture or other state changes. Nongaming applications typically ignore this advice completely, yet are still able to maintain relatively fast and interactive frame rates. 22 Dr. Dobb’s Journal l www.ddj.com l August 2008 http://www.scitools.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - August 2008

Dr. Dobb's Journal - August 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook A Conversation with Christos Papadimitriou OpenGL and Mobile Devices: Round 2 Ellipse Specification Using Vectors Embed Custom GUIs in WPF Building RIAs on J2EE Foundations Disentangling Concepts in Object-Oriented Systems The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - August 2008

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