The command glPolygonStipple defines a 32×32 bit stipple pattern. In OpenGL, screen-door transparency can be implemented in a number of ways one of the simplest uses polygon stippling. If the viewer gets too close to the display, then the individual pixels in the pattern become visible and the effect is lost. This method works because the areas patterned by the screen-door algorithm are spatially integrated by the eye, making it appear as if the weighted sums of colors in Equation 11.4 are being computed, but no read-modify-write blending cycles need to occur in the framebuffer. The percentage of bits in the bitmask which are set to 0 is equivalent to the transparency of the object (Foley et al., 1990). A 1 bit in the bitmask indicates that the transparent object should be rendered at that pixel a 0 bit indicates the transparent object shouldn't be rendered there, allowing the background pixel to show through. A bitmask is used to control which pixels in the object are rasterized. A transparent object is created by rendering only a percentage of the object's pixels. TOM McREYNOLDS, DAVID BLYTHE, in Advanced Graphics Programming Using OpenGL, 2005 11.10 Screen-Door TransparencyĪnother simple transparency technique is screen-door transparency. The third profile design in progress is the Safety Critical profile. The two defined profiles are the Common and Common-Lite profiles. The conformance test is also defined and overseen by the Khronos Group. Similar to desktop OpenGL, OpenGL ES profiles must pass a profile-specific conformance test to be described as an OpenGL ES implementation. Required extensions must be supported by an implementation and optional extensions are at the discretion of the implementation vendor. The set of extensions include those already defined for desktop OpenGL, as well as new extensions created specifically to address additional market-specific needs of the profile's target market.Ī profile includes a strict subset of the desktop OpenGL specification as its base and then adds additional extensions as either required or optional extensions. Like desktop OpenGL implementations, implementations of OpenGL ES profiles may also include vendor-specific extensions. Similar to OpenGL ARB extensions, an OpenGL ES profile specification may include new OES extensions that are standardized versions of extensions useful to the particular embedded market. It defines the exact subset of the OpenGL pipeline included in the profile, detailing the commands, enumerants, and pipeline behavior. From the feature proposals a more detailed specification document is created. The characterization is followed by draft proposals of features from the desktop OpenGL specification that match the market characterization document. Since a profile is intended to address a specific market, the definition of a profile begins with a characterization of the market being addressed, analyzing the demands and constraints of that market. Profile specifications are created by working groups, comprised of Khronos members experienced in creating OpenGL implementations and familiar with the profile's target market. To date, the Khronos Group has completed version 1.0 and 1.1 of the specifications for two profiles ( Blythe, 2003 ) and is in the process of creating the specification for a third. TOM McREYNOLDS, DAVID BLYTHE, in Advanced Graphics Programming Using OpenGL, 2005 8.3.1 Embedded Profiles
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