OpenGL Vertex Arrays OpenGL vertex arrays store vertex properties such as coordinates, normal vectors, color values and texture coordinates. These properties can be specified indirectly using their positions in the arrays, instead of calling glVertex*(), glNormal*(), glColor*() and glTexCoord*(). Procedures to use vertex arrays: Enable vertex arrays. Specify data for the arrays. Dereference the arrays and render primitives. Enable Vertex Arrays void glEnableClientState(GLenum array)
void glDisableClientState(GLenum array) Disable vertex array array Meaning GL_VERTEX_ARRAY Array with vertex coordinates GL_COLOR_ARRAY Array with color values GL_NORMAL_ARRAY Array with normal vectors GL_TEXTURE_COORD_ARRAY Array with texture coordinates When using multitexture, glEnableClientState() and glDisableClientState() only affect current client texture unit specified by glClientActiveTexture() void glClientActiveTexture(GLenum texUnit) Select current client texture unit. texUnit is the texture unit identifier. Specify Data for the Arrays
void glVertexPointer(GLint size, GLenum type, GLsizei stride, const GLvoid *pointer) Specify array with vertex coordinates size:Number of coordinates per vertex. Must be 2, 3 or 4. type:Data type for vertex coordinates. GL_SHORT, GL_INT, GL_FLOAT, GL_DOUBLE. stride:Byte offset between consecutive vertices. If stride = 0, the vertices are assumed to be tightly packed in the array. pointer:Pointer to the array that stores vertex coordinates. void glColorPointer(GLint size, GLenum type, GLsizei stride, const GLvoid *pointer) void glNormalPointer(GLenum type, GLsizei stride, const GLvoid *pointer) void glTexCoordPointer(GLint size, GLenum type, GLsizei stride, const GLvoid *pointer) Command size type glVertexPointer (Vertex coordinates) 2, 3, 4 GL_SHORT, GL_INT, GL_FLOAT, GL_DOUBLE glColorPointer (Color values) 3, 4 GL_BYTE, GL_UNSIGNED_BYTE, GL_SHORT, GL_UNSIGNED_SHORT, GL_INT, GL_UNSIGNED_INT, glNormalPointer (Normal vectors) 3 GL_BYTE, GL_SHORT, GL_INT, glTexCoordPointer (Texture coordinates) 1, 2, 3, 4 When using multitexture, glTexCoordPointer() only affects current client texture unit specified by glClientActiveTexture() Dereference the Arrays and Render Primitives
De-reference a Single Array Element void glArrayElement(GLint i) Specify vertex properties for the ith vertex for all enabled arrays. This command is usually called between glBegin( ) and glEnd( ). This is equivalent to: if (color array is enabled) glColor*v(cpointer + i * cstride); if (normal array is enabled) glNormal*v(npointer + i * nstride); if (texture coordinate array is enabled) glTexCoord*v(tpointer + i * tstride); if (vertex array is enabled) glVertex*v(vpointer + i * vstride); De-reference a List of Array Elements
void glDrawElements(GLenum mode, GLsizei count, GLenum type, GLvoid *indices) Draw a sequence of primitives using array elements referenced by an index array. mode:Primitive type. Same as the argument in glBegin(). count:Number of elements to specify. type:Data type for index array indices. GL_UNSIGNED_BYTE, GL_UNSIGNED_SHORT, GL_UNSIGNED_INT. indices:Pointer to the index array. Equivalent to: glBegin (mode); for (i = 0; i < count; ++i) glArrayElement (indices[i]); glEnd(); De-reference a Sequence of Array Elements
void glDrawArrays(GLenum mode, GLint first, GLsizei count) Draw a sequence of primitives using array elements starting from first and ending at first + count 1. mode:Primitive type. Same as the argument in glBegin(). first:Starting element index. count:Number of elements to specify. Equivalent to: glBegin (mode); for (i = 0; i < count; ++i) glArrayElement (first + i); glEnd(); Example y 8 9 10 11 4 5 6 7 x 1 2 3 Vertex Index Coordinates
1 2 3 Vertex Index Coordinates Texture Coordinates (0.0, 0.0, 0.0) (0.0, 0.0) 1 (5.0, 0.0, 0.0) (1.0, 0.0) 2 (10.0, 0.0, 0.0) (2.0, 0.0) 3 (15.0, 0.0, 0.0) (3.0, 0.0) 4 (0.0, 4.0, 0.0) (0.0, 0.5) 5 (5.0, 4.0, 0.0) (1.0, 0.5) Vertex Index Coordinates Texture Coordinates 6 (10.0, 4.0, 0.0) (2.0, 0.5) 7 (15.0, 4.0, 0.0) (3.0, 0.5) 8 (0.0, 8.0, 0.0) (0.0, 1.0) 9 (5.0, 8.0, 0.0) (1.0, 1.0) 10 (10.0, 8.0, 0.0) (2.0, 1.0) 11 (15.0, 8.0, 0.0) (3.0, 1.0) struct VERTEX { float x[3]; // position float t[2];// texture coordinates }; static VERTEX v[12] = { {0.0,0.0, 0.0, 0.0, 0.0}, {5.0,0.0, 0.0, 1.0, 0.0}, {10.0, 0.0, 0.0, 2.0, 0.0}, {15.0, 0.0, 0.0, 3.0, 0.0}, {0.0,4.0, 0.0, 0.0, 0.5}, {5.0,4.0, 0.0, 1.0, 0.5}, {10.0, 4.0, 0.0, 2.0, 0.5}, {15.0, 4.0, 0.0, 3.0, 0.5}, {0.0,8.0, 0.0, 0.0, 1.0}, {5.0,8.0, 0.0, 1.0, 1.0}, {10.0, 8.0, 0.0, 2.0, 1.0}, {15.0, 8.0, 0.0, 3.0, 1.0}}; glEnableClientState(GL_VERTEX_ARRAY); glEnableClientState(GL_TEXTURE_COORD_ARRAY); glVertexPointer(3, GL_FLOAT, 5*sizeof(VERTEX), v[0].x); glTexCoordPointer(2, GL_FLOAT, 5*sizeof(VERTEX), v[0].t); static unsigned int vindex[24]={
0,1,5,4, 1,2,6,5, 2,3,7,6, 4,5,9,8, 5,6,10, 9, 6,7,11, 10}; glEnable(GL_TEXTURE_2D); glBindTexture(GL_TEXTURE_2D, tex_obj); glDrawElements(GL_QUADS, 24, GL_UNSIGNED_INT, vindex); Environment Mapping Environment mapping is used to render reflective surface. It assumes that the environment is much larger than the object and far away from the object. Viewer N R V Environment Map Object Surface NUnit Surface Normal VUnit Direction Vector to Viewer RUnit Reflection Vector Environment Mapping Procedure
1. Generate Environment Map Place a camera at the center of the object and take a panorama snapshot of the surrounding environment. Store the image as a texture. This texture is called Environment Map. 2. Obtain Reflection Vector 3. Generate Texture Coordinates for Sampling in the Environment Map Using Reflection Vector R Sphere Environment Mapping
Sphere Environment Mapped Object The surrounding environment is projected onto an enclosing sphere
The surrounding environment is projected onto an enclosing sphere. The image on the sphere surface is then mapped into a circular area on a 2D texture. Unit Reflection Vector x R W z Texture Coordinates: Cube Environment Mapped Object
Cube Environment Mapping Cube Environment Map Cube Environment Mapped Object The surrounding environment is projected onto an enclosing cube.
The images on the six sides of the cube are then stored as six 2D textures. y z x x z y x y z z x z x x z x z y y y y z y x w = max (|Rx|, |Ry|, |Rz|);
Cube Map Face Local x Local y x z y x z y x z R w = max (|Rx|, |Ry|, |Rz|); if (w == |Rx| and Rx > 0) {Choose +x face; s = Rz / w; t = Ry / w;} if (w == |Rx| and Rx < 0) {Choose x face; s = Rz / w; t = Ry / w;} if (w == |Ry| and Ry > 0) {Choose +y face; s = Rx / w; t = Rz / w;} if (w == |Ry| and Ry < 0) {Choose y face; s = Rx / w; t = Rz / w;} if (w == |Rz| and Rz > 0) {Choose +z face; s = Rx / w; t = Ry / w;} if (w == |Rz| and Rz < 0) {Choose z face; s = Rx / w; t = Ry / w;} s = 0.5(s + 1); t = 0.5(t + 1); Light Map Light map: Texture that stores the lighting intensities of a surface. Scene with base textures Light Maps = Textured scene modulated by light maps For static scene under static lighting, light maps can be pre-computed. Light map was first introduced in Quake engine.
Without Light Map With Light Map Light Map Bump Mapping + Texture Mapped Object Height Map = Bump Mapped Object Normal texture mapping creates a flat looking surface.
Light Source Viewer Bump mapping applies a perturbation to surface normal and uses the perturbed normal for lighting calculation. Thus creates an illusion of surface bumpiness. Light Source Viewer Create Bump Map from Height Map
Height Map h(x, y): A 2D texture that stores the height perturbation of a surface. Bump Map Nb(x, y) :A 2D texture that stores perturbed normal vectors of a surface. Calculate perturbed normal: Use finite difference Normalization: Change component range from [1, 1] to [0, 1]: Store nx, ny, nz in R, G, B components of the bump map. To retrieve perturbed normal, do the inverse: Transform Vectors from Object Space to Texture Space
Bump map stores perturbed surface normals in texture space. Thus transformation from object space to texture space is needed. Obtain base vectors for texture space N NUnit normal vector TUnit tangent vector BUnit binormal vector B T For curved surface with parametric equation P = P(u, v)
Example: Sphere z N B T P r y x For a point P with texture coordinates (s, t) on the triangle surface
For triangle surface with texture coordinates specified for each vertex N3 B3 Vertex Texture Coordinates P1 P2 P3 (s1, t1) (s2, t2) (s3, t3) P3 T3 N1 B1 N2 P T1 P1 B2 P2 T2 For a point P with texture coordinates (s, t) on the triangle surface Replace P with P2 and P3: Solve the above equations to obtain T1 and B1:
T1 and B1 are then normalized to unit vectors T2 , B2 , T3 , B3 can be obtained similarly. Transform vectors from object space to local texture space For direction to light vector L For direction to viewer vector V Lighting Calculation Use Nb, L' and V' for lighting calculation. Displacement Mapping Bump Mapping only perturbs surface in N direction and only uses perturbed normal for lighting calculations. Displacement Mapping perturbs surface in N, T, B directions. It not only uses perturbed normal for lighting calculations, but also displaces surface position for real. Parallax Occlusion Mapping Water Surface Animation and Rendering
Plane Wave Equation Surface PointP = (x, y, z) AAmplitude ffrequency LxWave length in x direction LyWave length in y direction Surface Normal Reflection Rendering Transparent Effect z xy plane
Camera N N: Surface normal V R Water Surface xy plane P Direction to camera vector Reflection vector Use R to sample environment map to obtain reflection color Transparent Effect Use color blending