GAM666 – Introduction To Game Programming Introduction to OpenGL ● OpenGL is an alternative to Direct3D for 3D graphics rendering ● Originally developed by Silicon Graphics Inc (SGI), turned over to multi-vendor group (OpenGL Architecture Review Board) in 1992 ● Unlike DirectX, OpenGL is platform independent, with implementations in Linux, Unix and Mac as well as Windows ● Can be mixed and matched with non-DirectX Graphics parts of DirectX such as DirectInput and DirectX Audio GAM666 – Introduction To Game Programming Introduction to OpenGL ● Even though OpenGL is platform independent, the framework from which you call it is very much platform determined ● GLUT provides a platform independent framework from which you can call OpenGL, but isn't intended to be full featured ● GLUT should not be confused with GLU, which are some helper functions to assist with using OpenGL, similar to Direct3D's D3DX functions ● The Windows functions wglCreateContext() and wglMakeCurrent() get OpenGL started in a normal Windows framework GAM666 – Introduction To Game Programming Introduction to OpenGL ● OpenGL has its own data types, e.g. ● GLfloat (float) ● GLint (int) ● GLuint (unsigned int) ● OpenGL functions begin with gl, e.g. ● glClear() ● glDrawArrays() ● GLU functions begin with glu, e.g. ● gluLookAt() ● gluPerspective() GAM666 – Introduction To Game Programming Introduction to OpenGL ● The end of function names often have meaning as well: ● A name ending with f usually has GLfloat parameters, while the same name only, ending with i, has GLint parameters ● A name ending with, e.g., f may take a number of GLfloat parameters while the same name, only ending with fv, will take an array (vector) of GLfloat values GAM666 – Introduction To Game Programming Introduction to OpenGL The OpenGL view is similar but different from the Direct3D view: ● The z axis comes out from the screen rather than going into it ● A point or vector is represented with a 4x1 matrix rather than a 1x4, and all matrix operations are reversed ● Matrices are stored in column major format rather than the regular C-style row major format (i.e. matrices are transposed) GAM666 – Introduction To Game Programming Introduction to OpenGL The OpenGL view is similar but different from the Direct3D view: ● Vertex information is stored as a series of parallel arrays in system memory rather than an array of structures somewhere else ● Triangle culling normally disabled by default, and if you turn it on, it draws the counter-clockwise side rather than the clockwise side by default GAM666 – Introduction To Game Programming Introduction to OpenGL OpenGL is state-based, so you usually set a few states then do something, rather than pass a whole bunch of parameters with each operation, e.g. ● There are two transformation matrices rather than three (Projection and Model/View), and to change one: ● first set the matrix state, e.g. glMatrixMode(GL_PROJECTION) ● then change that matrix, e.g. glLoadMatrixf(array) GAM666 – Introduction To Game Programming Introduction to OpenGL ● Each type of transformation matrix has a stack of matrices, where the top of the stack is the current active one ● glPushMatrix() duplicates the current active matrix, putting the copy onto the top of the stack ● glPopMatrix() gets rid of the current active matrix, restoring the previous one to active duty ● The GL_PROJECTION stack is at least 2 deep, the GL_MODELVIEW stack is at least 32 deep GAM666 – Introduction To Game Programming Introduction to OpenGL ● Since the Model/View matrix replaces the purpose of both the View and World matrices of Direct3D, the usual drawing technique is: ● Set the Model/View to the view matrix, then for each object ● Push a copy of the Model/View matrix onto the top of the stack ● Multiply by the object's world matrix ● Draw the object ● Pop the stack (restoring the view matrix) thereby getting ready for the next object.