1. Computer Graphics Ben-Gurion University of the Negev Fall 2012

2.  Until now, colors and shades determined the vertices color  Texture is the characteristic physical structure given to an object by the size, shape, arrangement of its parts  By texture mapping we apply image data to a geometric primitive 2

3. Three steps to apply a texture 1. Specify the texture  read or generate image  assign to texture  enable texturing 2. Specify texture parameters  wrapping, filtering 3. Assign texture coordinates to vertices  Proper mapping function is left to application 3

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5.  Images and geometry flow through separate pipelines that join at the rasterizer - “complex” textures do not affect geometric complexity 5

6.  Define a texture image from an array of texels (texture elements) in CPU memory Glubyte my_texels[512][512][3];  Enable texture mapping  glEnable(GL_TEXTURE_2D)  OpenGL supports 1-3 dimensional texture maps 6

7.  Define a handle to different available texture states (image and params) glGenTextures(1, &texture[texture_num])  First argument tells OpenGL how many texture objects we would create  Second argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the texture objects  Texture[ ] is of type GLuint 7

8.  To specify the text object that we are going to define its specifics glBindTexture(GL_TEXTURE_2D, texture[texture_num]) 8

9. glTexImage2D( target, level, internalFormat, w, h, border, format, type, texels ) target: type of texture, e.g. GL_TEXTURE_2D level: used for mipmapping (discussed later) internalFormat: elements per texel w, h: width and height of texture image border: used for smoothing (discussed later) format and type: describe texels texels: pointer to texel array glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels); 9

10.  OpenGL requires texture dimensions to be powers of 2  If dimensions of image are not powers of 2  gluScaleImage( format, w_in, h_in,type_in, *data_in, w_out, h_out,type_out, *data_out );  data_in is source image  data_out is for destination image  Image interpolated and filtered during scaling 10

11.  Once a texture is uploaded to the graphics card, its resolution has no importance  Texture is mapped to “texture coordinates” between 0.0 to 1.0 (s,t)  glTexCoord*() specified at each vertex 11 s t 1, 1 0, 1 0, 01, 0 (s, t) = (0.2, 0.8) (0.4, 0.2) (0.8, 0.4) A BC a b c Texture Space Object Space

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13.  OpenGL a variety of parameter that determine how texture is applied  Wrapping parameters determine what happens if s and t are outside the (0,1) range  Filter modes allow us to use area averaging instead of point samples  Mipmapping allows us to use textures at multiple resolutions  Environment parameters determine how texture mapping interacts with shading 13

14. glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP ) glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT ) 14

15. 15 Modes determined by  glTexParameteri( target, type, mode ) glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR);

16. 16 A group of texels can cover a pixel (minification) or One texel can cover a group of pixels (magnification) Can use point sampling (nearest texel) or linear filtering ( 2 x 2 filter) to obtain texture values TexturePolygon MagnificationMinification PolygonTexture

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19.  Texturing far/close objects  Mipmapping allows for pre-filtered texture maps of decreasing resolutions  Lessens interpolation errors for smaller textured objects  Declare mipmap level during texture definition glTexImage2D( GL_TEXTURE_*D, level, … )  GLU mipmap builder routines will build all the textures from a given image gluBuild*DMipmaps( … ) 19

20. 20 N*N N/4*N/4 N/2*N/2 What about memory ?

21.  Controls how texture is applied glTexEnv{fi}( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, param)  GL_TEXTURE_ENV_MODE modes  GL_MODULATE: modulates with computed shade  GL_BLEND: blends with an environmental color  GL_REPLACE: use only texture color  GL_DECAL: alpha value is considered 21

22. void init(void) { glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel(GL_FLAT); glEnable(GL_DEPTH_TEST); makeCheckImages(); glGenTextures(2, texName); glBindTexture(GL_TEXTURE_2D, texName[0]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,GL_NEAREST); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, checkImage); 22

23. glBindTexture(GL_TEXTURE_2D, texName[1]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,GL_NEAREST); glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, otherImage); glEnable(GL_TEXTURE_2D); } 23

24. void display(void) { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glBindTexture(GL_TEXTURE_2D, texName[0]); glBegin(GL_QUADS); glTexCoord2f(0.0, 0.0); glVertex3f(-2.0, -1.0, 0.0); glTexCoord2f(0.0, 1.0); glVertex3f(-2.0, 1.0, 0.0); glTexCoord2f(1.0, 1.0); glVertex3f(0.0, 1.0, 0.0); glTexCoord2f(1.0, 0.0); glVertex3f(0.0, -1.0, 0.0); glEnd(); glBindTexture(GL_TEXTURE_2D, texName[1]); glBegin(GL_QUADS); glTexCoord2f(0.0, 0.0); glVertex3f(1.0, -1.0, 0.0); glTexCoord2f(0.0, 1.0); glVertex3f(1.0, 1.0, 0.0); glTexCoord2f(1.0, 1.0); glVertex3f(2.41421, 1.0, -1.41421); glTexCoord2f(1.0, 0.0); glVertex3f(2.41421, -1.0, -1.41421); glEnd(); glFlush(); } 24

25.  OFF files are standard format to represent 3D model  Starts with number of points, then number of polygons ( usually triangles )  List of points  List of polygons –  Easy to write OFF file reader and draw these models. 25

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