CS 450: COMPUTER GRAPHICS REVIEW: STATE, ATTRIBUTES, AND OBJECTS SPRING 2015 DR. MICHAEL J. REALE

Transcription

1 CS 450: COMPUTER GRAPHICS REVIEW: STATE, ATTRIBUTES, AND OBJECTS SPRING 2015 DR. MICHAEL J. REALE

2 OPENGL STATE MACHINE OpenGL state system or state machine Has list of all current state values called state variables or state parameters When rendering check current state OpenGL context = contains all of the state information associated with an instance of OpenGL State variables or state parameters = determine rendering behavior Either enabled/disabled OR take value/array of values Attributes = specific information on how to render primitives E.g., color, position, etc.

3 OBJECTS AND STATE OpenGL Object Contains state information of its own To use/modify an object, you must bind the object to a context: Object state copied into context Changes to context state will change bound object s state

4 CREATION, DATA, AND DESTRUCTION OF AN OBJECT Create object generate object name Object name = unsigned integer ID (GREATER THAN 0) for object Function: void glgen*(glsizei n, GLuint *objects); NOTE: Does NOT necessarily create the object s data! Allocating and filling data must bind first, then allocate/copy in data Destroy object destroy data and release object name Unbound from current context Function: void gldelete*(glsizei n, const GLuint *objects); NOTE: May not be IMMEDIATELY destroyed!

5 BINDING To use an object, we must first bind it to the current context: glbind*(glenum target, GLuint object); Binding target Where in the context the object is bound Tells OpenGL how we will use object Once an object is bound: Just created initial state info set Already has state object state copied into context Changes to context state changes object s state Bind a different object to a target previously bound object is unbound

6 ID ZERO ID zero = non-existent object glgen* will NEVER give you an ID of 0

7 EXAMPLE OF OBJECT: GL_ARRAY_BUFFER GLuint vertex_buffer; glgenbuffers(1, &vertex_buffer); glbindbuffer(gl_array_buffer, vertex_buffer); glbufferdata(gl_array_buffer, sizeof(glfloat) * 3 * 2, vertices, GL_STATIC_DRAW);

8 EXAMPLE OF OBJECT: GL_ARRAY_BUFFER glbindbuffer(gl_array_buffer, vertex_buffer);

9 ONE BOUND AT A TIME glbindbuffer(gl_array_buffer, color_buffer);

10 TYPES OF OPENGL OBJECTS There are two basic types of OpenGL objects: Regular Container Buffer Objects Regular object Vertex Array Objects Container object

11 BUFFER OBJECT Buffer object = Stores an array of unformatted memory Used to store: Vertex data Vertex Buffer Object (VBO) Index arrays Etc. Create buffer void glgenbuffers(glsizei n, GLuint * buffers ); Bind to target in context void glbindbuffer(glenum target, GLuint buffer ); Destroy buffer void gldeletebuffers(glsizei n, const GLuint * buffers );

12 COMMON BUFFER TARGETS GL_ARRAY_BUFFER = use buffer for vertex data NOT connected to attribute array until after glvertexattribpointer() is called GL_ELEMENT_ARRAY_BUFFER = use buffer for list of indices Used when drawing with gldrawelements()

13 BUFFER DATA No data allocated after glgenbuffers() Must bind and allocate data Kinds of data: Immutable storage = size of buffer will NOT change void glbufferstorage(glenum target, GLsizeiptr size, const GLvoid * data, GLbitfield flags ); Mutable storage = size of buffer CAN change (but doesn t have to) void glbufferdata(glenum target, GLsizeiptr size, const GLvoid * data, GLenum usage ); glbufferdata() always reallocates memory!

14 GLBUFFERDATA() void glbufferdata(glenum target, GLsizeiptr size, const GLvoid * data, GLenum usage ); Always reallocates memory! Usage flags does not FORCE usage; only tries to make things optimal GL_ + + STATIC = set once and not changed DYNAMIC = changed occasionally STREAM = changed after (almost) every use DRAW = write only READ = read only COPY = neither (used internally in OpenGL)

15 COPY IN DATA ONLY glbufferdata() copies in data AND reallocates inefficient if data is always the same size glbuffersubdata() ONLY copies in data void glbuffersubdata(glenum target, GLintptr offset, GLsizeiptr size, const GLvoid * data );

16 VERTEX BUFFER OBJECT (VBO) Vertex Buffer Object (VBO) = buffer object used for vertex array data Must be connected to an vertex attribute array to use Vertex attributes = user-defined inputs into a vertex shader Used to pass in per-vertex position, color, normals, etc. Vertex attribute array = array of these inputs Has index (from 0 to GL_MAX_VERTEX_ATTRIBS - 1) Link to buffer stored in VAO state information

17 USING A VERTEX BUFFER OBJECT (VBO) glbindbuffer(gl_array_buffer, vertex_buffer); glvertexattribpointer( 0, // buffer will be used by Attribute Array 0 3, // Number of components (3 = 3D point) GL_FLOAT, // Type for each component GL_FALSE, // Normalized? 0, // Stride (set to 0 if tightly packed) (void*)0 // Array buffer offset );

18 USING A VERTEX BUFFER OBJECT (VBO) glbindbuffer() in CONTEXT, vertex_buffer is now the current GL_ARRAY_BUFFER GL_ARRAY_BUFFER is NOT a state in the VAO! glvertexattribpointer() Looks at current GL_ARRAY_BUFFER vertex_buffer Stores vertex_buffer in VAO for Attribute Array N Can then bind another buffer to GL_ARRAY_BUFFER will NOT affect Attribute Array N in VAO!

19 VERTEX ARRAY OBJECT (VAO) Vertex Array Object (VAO) Container OpenGL object Stores all of the state needed to supply vertex data EXCEPT for GL_ARRAY_BUFFER Format of the vertex data Buffer objects for vertex data arrays GL_ELEMENT_ARRAY_BUFFER (used for index buffer) Etc. Does NOT store contents of referenced buffers Change data in buffers seen by VAO Cannot be shared between OpenGL contexts Need to create and bind a VAO before you can use attribute arrays or an element (index) array

20 CREATING/BINDING/DESTROY Create void glgenvertexarrays(glsizei n, GLuint *arrays ); Bind void glbindvertexarray(gluint array ); Destroy void gldeletevertexarrays(glsizei n, const GLuint *arrays ); Note there is only one target for a VAO GLuint VertexArrayID; glgenvertexarrays(1, &VertexArrayID); glbindvertexarray(vertexarrayid);

21 ENABLING/DISABLING ATTRIBUTE ARRAYS By default, ALL attribute arrays in a VAO are disabled Before you can use a given attribute array in a VAO, you have to enable it: void glenablevertexattribarray(gluint index ); To disable: void gldisablevertexattribarray(gluint index ); Reading from a disabled array will return a constant value GL_MAX_VERTEX_ATTRIBS = maximum number of attribute arrays Example: enabling attribute arrays 0 and 1 glenablevertexattribarray(0); glenablevertexattribarray(1);

22 STATE OF THE UNION