Review of OpenGL Basics Chap. 1 of Orange Book

Contents OpenGL History Execution model Framebuffer State Processing Pipeline Drawing geometry Drawing images Coordinate transform Texture map

History of OpenGL Revisions Version 1.0 (1992): initial release Version 1.1 (1997): vertex arrays and texture objects. Version 1.2 (1998): 3D textures and an optional set of imaging functionality. Version 1.3 (2001): cube map textures, compressed textures, multitextures, … Version 1.4 (2002): automatic mipmap generation, additional blending functions, internal texture formats for storing depth values for use in shadow computations, support for drawing multiple vertex arrays with a single command, more control over point rasterization, control over stencil wrapping behavior, and various additions to texturing capabilities. Version 1.5 (2003): vertex buffer objects, shadow comparison functions, occlusion queries, and non-power-of-2 textures.

OpenGL 2.0 (Sept.2004) opened up the processing pipeline for user control by providing programmability for both vertex processing and fragment processing OpenGL 2.0 is completely backward compatible with OpenGL 1.5 Other features: multiple render targets (rendering to multiple buffers simultaneously), non-power-of-2 textures, point sprites, and separate stencil functionality for front- and back-facing surfaces.

Execution Model: client-server An application program (the client) issues OpenGL commands that are interpreted and processed by an OpenGL implementation (the server). The application program and the OpenGL implementation can execute on a single computer or on two different computers. Some OpenGL state is stored in the address space of the client, but the majority of it is stored in the address space of the server. OpenGL commands are always processed in the order in which they are received by the server

State Machine OpenGL was designed as a state machine. Cumulatively, the state settings define the behavior of the OpenGL rendering pipeline and the way in which primitives are transformed into pixels on the display device. OpenGL state is collected into a data structure called a graphics context. Window-system-specific functions create and delete graphics contexts. Quite a few server-side state values in OpenGL have just two states: on or off, controlled by glEnable and glDisable. Client-side state values (such as pointers that define vertex arrays) are controlled by glEnableClientState and glDisableClientState. OpenGL maintains a server-side stack for pushing and popping any or all of the defined state values. This stack can be manipulated with glPushAttrib and glPopAttrib. Similarly, client state can be manipulated on a second stack with glPushClientAttrib and glPopClientAttrib. glGet is a generic function that can query many of the components of a graphics context.

Fixed Functionality of OpenGL

Drawing Geometry Sending geometry Per-vertex operations: T&L T&L : transformation and lighting Sending geometry vertex-at-a-time Vertex array Display list new in 1.5: VBO, server-side vertex array Per-vertex operations: T&L vertex positions are transformed by the modelview and projection matrices normals are transformed by the inverse transpose of the upper leftmost 3 x 3 matrix taken from the modelview matrix texture coordinates are transformed by the texture matrices lighting calculations are applied to modify the base color texture coordinates may be automatically generated color material state is applied point sizes are computed [how]

Drawing Geometry Primitive assembly Primitive processing Rasterization vertex data is collected into complete primitives Primitive processing clipping, culling, perspective-divide Rasterization A fragment comprises a window coordinate, depth, and other associated attributes such as color, texture coordinates, and so on. Size,width,stipple, … Fragment processing Texture mapping, fog Per-fragment operations Scissor, depth, stencil, … Framebuffer operations Clear, masking, buffer

Images – (known as)pixel rectangles Pixel unpacking Data of pixel rectangles transferred from memory to OpenGL Pixel transfer glDrawPixels, … glReadPixels; glCopyPixels Rasterize and placement glRasterPos; glPixelZoom Read control glReadBuffer

Coordinate Transformation Light positions are stored within OpenGL as eye coordinate light positions and surface normals must be in the same coordinate system. glLoadIdentity; glMultMatrix; gluLookAt glTranslate; glScale; glRotate glPushMatrix; … glOrtho; gluPerspective; glViewport

Texture Mapping Texture Mapping = Texture access + texture application Texture unit: the underlying piece of graphics hardware that performs the various texturing operations. A texture unit can be enabled for 1D, 2D, 3D, or cube map texturing Texture object: a name and a texture target (1D,2D,3D,cubemap) associated to a texture unit Texture parameters are then assigned (filters, wrap, …) Texture environment and texgen are not stored in texture objects

Texture Mapping Texture environment: Multitexture TexGen the manner in which a texture value is applied to a graphics primitive (replace, modulate, …) Multitexture serially connect texture units glActiveTexture, glMultiTexCoord, … TexGen Procedurally generate texture coordinates

Excursions Vertex array New OpenGL features Reflection models Explanation, spec, example, tutorial, … sought Vertex array What’s the significance of “client-side”? glEnableClientState [how they are used to specify geometry] [vertex array object: server-side vertex array] New OpenGL features Secondary color Compressed texture Reflection models Blinn-Phong: what’s the halfVector in a light? Hemispheric lighting Cook-Torrance, Oren-Nayar, Ward, Ashikhmin-Shirley High dynamic range lighting