-------------------------------------------------------------------------------- OPENGL NOTES from 4.1 core profile specifications (20100725) Copyright (C) 2011 Sylvain BERTRAND (digital.ragnarok@gmail.com). Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". -------------------------------------------------------------------------------- 1. VERTEX 1.1 GENERAL o 1 vertex is a sequence of N attributes. o (N - 1) maximum is MAX_VERTEX_ATTRIBS (carefull it is a index from 0). +-------------------------+ --\ | attribute index 0 | | +-------------------------+ | | attribute index 1 | | +-------------------------+ |--MAX_VERTEX_ATTRIBS + 1 | ... | | +-------------------------+ | | attribute index (N - 1) | | +-------------------------+ --/ 1.2 ATTRIBUTE o 1 attribute is a sequence of 4 components. o an attribute which is said "generic", has its components named x,y,z and w. o by default a "generic" attribute has the value (0.0,0.0,0.0,1.0). "generic" attribute: +---+ | x |<--component, name "x" +---+ | y |<--component, name "y" +---+ | z |<--component, name "z" +---+ | w |<--component, name "w" +---+ 2. VERTEX ARRAY o it is an array of vertex "generic" attributes. o each pack of bits defining the value of a "generic" attribute, is named a array element. o 1 "generic" attribute in an vertex array has from 1 to 4 component values. o the binary structure the components are stored can vary. For instance, they can be packed, namely the bits of all 4 components can be spread over word of 32 bits. o there is a maximum or MAX_VERTEX_ATTRIBS + 1 arrays. Because there is a 1 to 1 mapping between vertex arrays and vertex attributes. The array of index J will map to the attribute index J of the vertices. vertex arrays of vertex "generic" attributes and a maximum index of M: array of 4 components memory ----------------------------> +----+----+----+----+ stride | array element-->| x0 | y0 | z0 | w0 |<------> | +----+----+----+----+ | +----+----+----+----+ | array element-->| x0 | y0 | z0 | w0 |<------> | memory +----+----+----+----+ | ... | +----+----+----+----+ | array element-->| xM | yM | zM | wM |<------> | +----+----+----+----+ V array of 3 components memory ------------------------> +----+----+----+ stride | array element-->| x0 | y0 | z0 |<------> | +----+----+----+ | +----+----+----+ | array element-->| x0 | y0 | z0 |<------> | memory +----+----+----+ | ... | +----+----+----+ | array element-->| xM | yM | zM |<------> | +----+----+----+ V same transformation from an array of 3 components to 2 components, and from an array fo 2 components to 1 component. 3. PRIMITIVE o primitives are basic geometric shaped build from vertices. o root primitives are: point, line and triangle. o the GL automaton parses the vertices in order to build the primitives. 4. COORDINATE SYSTEMS o all coord systems are homogenous:(x,y,z,w). But from clip coord system, we are on the screen. o object coords:raw coords of the primitive (3D modelisation coords). Position not set in world (eye) coord system. o eye coords:world coords of the positionned primitive. eye in (0,0,0) looking along the negative z axis, right handed. o clip coords:perspective projected eye coords. Coords are on the "screen". eye in (0,0,0) looking along z axis towards +z, left handed, all axis coords adjusted to [-wc,wc]. Clipping is done in this coord system of primitive going outside the box defined by [-wc,wc]. wc is called *the* clip coordinate. o normalized device coords:clip coords adjusted to [-1,1] on all axis (division by wc). o window coords:scaled and translated normalized device coords to the view port. 5. TEXTURE AND SAMPLER o a texture is an array of 2D arrays of "texels". Array of 3 layers: +------------+ | | +--------+ | +-------+ | 1| | 2 | |texture|--+ +-----------+ | +-------+ | | | | || | |---------->O-----||-----+ | | | || |------->O--------+| | | 0 | +----->O-----------+ o a texture object hold the components of a texture. o a sample is the result of a texture lookup for window pixel (part of a fragment). A sample defines a location in the pixel/fragment which will be used to locate the proper texture texel. o a sampler object defines how many and which samples will be used for a texture texel lookup. A sampler can be attached to a texture object. o a texture has borders, then internal coordinates. Depth border too. 6. BUFFER OBJECT o a buffer object is a description of a buffer. The actual buffer is called a data store. o the data store of a buffer object can be mapped in memory and answer to a flushing mecanism. 7. OBJECT NAMES GL uses unique identifiers for its objects. Most of the time, you will need to query GL for unused names first, then bind an anonymous object with an unused name (or instanciate or whatever). Carefull because binding is also used for state variable assignment. Those unique identifiers are usually *non 0* 32 bits unsigned integers. 8. GL AUTOMATON The GL is made of 2 sides: o the server side o the client side Each side maintains a state. A GL state is a set of variables which you can assign values to in order to control the automaton operations. In GL terminology, assigning "something" to a state variable is called "binding" that "something" Basically, the client side sends commands to the server side which is a automaton. Think of a network between the client side and the server side. The automaton is a pipeline with is a sequence of stages. The stages are, in sequence order: o vertex stage, which parses vertices to assemble primitives. o tesselation control stage, optional. o tesselation evaluation stage, optional. o geometry stage, optional, processes primitives from the previous stages and outputs primitives too. (TODO:probably not really true). o fragment stage, which combine primitives and textures to produce 2D data. +------+ |vertex| +------+ | V +--------------+ | tesselation | +--------------+ | +-------+ | | |control| | | +-------+ | | | | | V | | +----------+ | | |evaluation| | | +----------+ | +--------------+ | V +--------+ |geometry| +--------+ | V +--------+ |fragment| +--------+ 8.1 TEXTURE UNITS The GL automaton has texture units. The protocol to apply GL operations to textures is to select a texture unit to perform the work and then bind a texture object the proper target (GL automaton state variable). 9. VERTEX ARRAY OBJECT (this is not a VERTEX ARRAY) o does contain the entire state of the "vertex sub-automaton". o for instance, knows all the buffer objects related to the "vertex sub-automaton". o the "vertex sub-automation" parses vertices in order to build the primitives. 10. RASTERISATION 10.1 MULTISAMPLING In multisampling mode, 1 pixel in the framebuffer has several "sample" locations associated with it. Minimun count of samples per fragment can be enforced with sample shading. Basically multisampling is as special case of supersampling: it's rendering at higher resolution and a method to downsample to the real size of the framebuffer (grossly). 10.2 MATHS x,y,z coords of a point on a primitive. u,v coords of the perspective projected point of the previous point on the window plane (near plane): u/z and v/z are linearly linked-->texture mapping in linear in homogenous coord system. Then it comes down to computing constant scalar slops based on u/z and v/z variables. 11. THE MESS OF SHADERS 11.1 PROGRAM PIPELINE OBJECT This is a 1-to-1 mapping with the GL server side automaton and a bit more. Namely, it has an holder for each stage of the GL server automaton. It holds executable GPU machine pieces of code ("executables" in GL terminology) and a reference *or not* to the program object used to build the GPU machine code (see next section). That reference to the program object is its "name" (see section 5). The state of this objects is made of the following variables: +--o VERTEX_SHADER | o TESS_CONTROL_SHADER GL automation stages-->| o TESS_EVALUATION_SHADER | o GEOMETRY_SHADER +--o FRAGMENT_SHADER o ACTIVE_PROGRAM o status of the last validation attempt. o the log: o the log data (chars...) o lenght of the data *_SHADER* is badly chosen since those are references to program objects and not shader objects. 11.2 PROGRAM OBJECT This object holds what is required to build GPU machine code, namely "executables" in GL terminology. It has a holder for each GL automaton stage, which refers to shader objects and contain the executable for that stage. It has an information log. The linking process is done by this object. It reports to the information log. It can lead to executables which can not be assigned independently to a GL automaton stage or program pipeline stage, they are said to be "not separable". It's possible to get a binary image for the executables in the program object. This binary image can be loaded back in program objects. +--------------------------------------+ | program object | | +----------------------------------+ | | | vertex | | | | +-----+ +----------------------+ | | | | | | | bound shader names | | | | | | | +----------------------+ | | | +----------------------------------+ | | | | | | +----------------------------------+ | | | | | tessellation control | | | | | E | +----------------------+ | | | | | X | | bound shader names | | | | | | E | +----------------------+ | | | +----------------------------------+ | | | C | | | +----------------------------------+ | | | | U | tessellation evaluation | | | | | T | +----------------------+ | | | | | A | | bound shader names | | | | | | B | +----------------------+ | | | +----------------------------------+ | | | L | | | +----------------------------------+ | | | | E | geometry | | | | | S | +----------------------+ | | | | | | | bound shader names | | | | | | | +----------------------+ | | | +----------------------------------+ | | | | | | +----------------------------------+ | | | | | fragment | | | | | | +----------------------+ | | | | | | | bound shader names | | | | | +-----+ +----------------------+ | | | +----------------------------------+ | | | | +----------------------------------+ | | | information log | | | +----------------------------------+ | +--------------------------------------+ 11.3 SHADER OBJECT This object holds a piece of source code, binary data which is compiled source code and an information log. A shader can be related to 0 or many program objects. A shader is specific to one only GL automaton stage. The compilation process in done by this object and reports to the information log. You can load pre-compiled shader source code by-passing the compilation process. A shader outputting primitives is said "flatshaded" when all the output varying variables of this shader are the same and based on the output varying variables of one of the primitive vertices. The picked-up primitive vertex for this operation is either the first one or the last one, depending of the convention set up. This vertex is called the "provoking" vertex. 11.4 HOW THE GL AUTOMATON SELECT EXECUTABLES WHICH ARE ACTUALLY RUNNING The GL automaton maintains holders for the GPU machine code of the exectuables which are actually run by its stages. The objects which are related to what is actually run are tagged "active". The GL automaton also maintain holders, said "current" for each of its stages, which are the objects which *would have been* (and *not* are) used to build the running exectutables. Each "current" holder for a GL automaton stage refers to a program or program pipeline stage. Those objects are used in order to build what is actually run by the GL automaton. *Carefull*, those latter objects *are* actually what is run by the GL automaton *only* if they are tagged "active". The "current" objects are kind of scratch pads to build the executables which the GL automaton will run. Selection of the "current" program for a GL automaton stage, order of precedence (2 and 3 not sure): 1 - "unified" program object set with the "UseProgram" command. 2 - corresponding stage of *the* "bound" program pipeline object. 3 - ACTIVE_PROGRAM of *the* "bound" program pipeline object. 11.5 PROGRAMMATIC USE OF VARIABLES A variable which the compiler/linker find actually used by a executable is said "active". 11.5.1 INPUT/OUTPUT MATCH OF PROGRAMS This concern input and output of shaders which are vertex attributes. It is possible to bind a variable char name (*not* the integer object name) in a program to a specific vertex attributes refered by its index. It gives a sort of flexibility in order to connect the input and output of different programs during linking. 11.5.2 UNIFORMS Those are constants over a primitive for programs. They are grouped in blocks, there are 2 types of blocks, named (char) uniform block and the default uniform block. Named (char) uniform block data is the data store of a buffer object of uniform type. The latter buffer object has to be bound to the program on a uniform buffer binding. Named (char) uniform block are assigned an index. Uniforms access in named (char) uniform block is based on memory structure rules in the data store of the buffer object. Uniforms access in the default block is based on locations (integers) and a set of accessor functions. A "subroutine" uniform can have a context value computed by a "subroutine" per GL automaton stage. A sampler is a special uniform variable used to select texture unit by index. 11.5.3 VARYINGS AND TRANSFORM FEEDBACK Varying variables are outputs of a vertex shader. Transform feedback varying variables are output variables of the closest active GL automaton stage from the GL automaton fragment stage, which will be stored in one or more buffer objects. See transform feed back section. 11.6 TESSELLATION SHADERS mmmmh... to save memory and bandwidth? Specialization of a geometry shaders (see next shader) a fixed-function accelerator for the tessellation geometric operation? 11.7 GEOMETRY SHADERS From 1 primitive, a geometry shader generates 1 and more new primitives. The input primitive is discarded. The geometry shader is executed a programable number of times, and the set of output primitives of a run is assigned an invocation ID (run ID). There is a unique primitiveID which is generated by the GL automaton after the vertex stage for each primitive (re-read how this ID fits with the generated primitives from tessellation stages). Since the geometry shader is the last stage before rasterization/ fragment shader, many geometric operations are done on the output primitives (viewport mapping, clipping etc). Geometry shaders have the same output signature than the vertex shader of their stream, in order to be added/removed seemlessly. Output primitives can be directed to a specific layer of a cube map and a specific framebuffer thanks to built-in variables. 12. ASYNCHRONOUS QUERIES It's a kind of instrumentation framework. It's counter and time based. It revolves around the management of the query object. A query object has counters for each vertex stream. 13. CONDITIONAL RENDERING A block of command can be discarded based an occlusion query object state. There are several ways to deal with the occlusion object state. -->Skipping what is not seen, will speed rendering a frame up. 14. TRANSFORM FEEDBACK It will record processed/tranformed primitive data in data stores of buffer objects, in order to reinject/replay them later in the GL automaton. I guess, it allows to kind of pre-generate primitive data with the geometric part of the GL automaton. The GL automaton is set to perform transform feeback capture with binding TRANSFORM_FEEDBACK state variable to a valid transform feedback object. ================================================================================ GNU Free Documentation License GNU Free Documentation License Version 1.3, 3 November 2008 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. 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