CSC345: Advanced Graphics & Virtual Environments Lecture 7: Textures (2) Patrick Olivier p.l.olivier@ncl.ac.uk 2nd floor in the Devonshire Building 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Textures: the story so far… textures/environment maps/bump maps transformations between three of more coordinates need a backward mapping two-part mapping uses an intermediate object cylinder/box/sphere area averaging can address aliasing problems v 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Textures coordinates (u,v) in range (0.0,1.0) but what if (u,v) >1.0 or <0.0 ? to repeat textures use fractional part (e.g 5.3 -> 0.3) different repeat patterns (repeat, mirror, clamp, border): (0,0) (1,0) (1,1) (0,1) (u0,v0) (u1,v1) (u2,v2) (2,2) (-1,-1) 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Textures in OpenGL specify the texture read or generate image assign to texture enable texturing assign texture coordinates to vertices mapping function is left to application specify texture parameters wrapping filtering 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Example 1: specifying textures (1) define a texture image from an array of texels (texture elements) in memory: Glubyte my_texels[512][512]; define as any other pixel map: scanned image generate by application code enable texture mapping glEnable(GL_TEXTURE_2D); OpenGL supports 1-4 dimensional texture maps 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Example 1: specifying textures (2) glTexImage2D( target, level, components, w, h, border, format, type, texels ); target: type of texture, e.g. GL_TEXTURE_2D level: used for mipmapping (discussed later) components: elements per texel w, h: width and height of texels in pixels border: used for smoothing (discussed later) format and type: describe texels texels: pointer to texel array glTexImage2D( GL_TEXTURE_2D, 0, 3, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels); 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Example 1: texture coordinates (1) Based on parametric texture coordinates glTexCoord*() specified at each vertex t 1, 1 (s, t) = (0.2, 0.8) 0, 1 A a c (0.4, 0.2) b B C s (0.8, 0.4) 0, 0 1, 0 Texture Space Object Space 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Example 1: texture coordinates (2) glBegin(GL_POLYGON); glColor3f(r0, g0, b0); // if no shading used glNormal3f(u0, v0, w0); // if shading used glTexCoord2f(s0, t0); glVertex3f(x0, y0, z0); glColor3f(r1, g1, b1); glNormal3f(u1, v1, w1); glTexCoord2f(s1, t1); glVertex3f(x1, y1, z1); . glEnd(); Note we can use vertex arrays to increase efficiency 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) GLfloat vertices[][3] = {{-1.0,-1.0,-1.0}, {1.0,-1.0,-1.0}, {1.0,1.0,-1.0}, {-1.0,1.0,-1.0}, {-1.0,-1.0,1.0}, {1.0,-1.0,1.0}, {1.0,1.0,1.0}, {-1.0,1.0,1.0}}; GLfloat colors[][4] = { {0.0,0.0,0.0,0.5}, {1.0,0.0,0.0,0.5}, {1.0,1.0,0.0,0.5}, {0.0,1.0,0.0,0.5}, {0.0,0.0,1.0,0.5}, {1.0,0.0,1.0,0.5}, {1.0,1.0,1.0,0.5}, {0.0,1.0,1.0,0.5}}; void polygon(int a,int b,int c,int d) { glBegin(GL_POLYGON); glColor4fv(colors[a]); glTexCoord2f(0.0,0.0); glVertex3fv(vertices[a]); glColor4fv(colors[b]); glTexCoord2f(0.0,1.0); glVertex3fv(vertices[b]); glColor4fv(colors[c]); glTexCoord2f(1.0,1.0); glVertex3fv(vertices[c]); glColor4fv(colors[d]); glTexCoord2f(1.0,0.0); glVertex3fv(vertices[d]); glEnd(); } void colorcube() { /* map vertices to faces */ polygon(0,3,2,1); polygon(2,3,7,6); polygon(0,4,7,3); polygon(1,2,6,5); polygon(4,5,6,7); polygon(0,1,5,4); } GLubyte image[64][64][3]; // in “main()” int i, j, r, c; for(i=0;i<64;i++) for(j=0;j<64;j++) c =((((i&0x8)==0)^((j&0x8))==0))*255; image[i][j][0]= (GLubyte) c; image[i][j][1]= (GLubyte) c; image[i][j][2]= (GLubyte) c; ... glEnable(GL_TEXTURE_2D); glTexImage2D(GL_TEXTURE_2D,0,3,64,64,0, GL_RGB,GL_UNSIGNED_BYTE, image); 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Converting a texture image 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 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Texture parameters OpenGL has a variety of parameters that determine how a 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 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Wrapping mode Clamping: if s,t > 1 use 1, if s,t <0 use 0 Wrapping: use s,t modulo 1 glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP ) glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT ) s t GL_CLAMP GL_REPEAT 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Magnification One texel cover more than one pixel? Box filter (nearest neighbour) leads to “pixelation” (blocky) one texel per pixel (fast) glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST); Bilinear interpolation interpolates neighbouring pixels GL_LINEAR); GL_NEAREST GL_LINEAR 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Bilinear interpolation t(u,v) accesses the texture map b(u,v) filtered texel (u’,v’) = (pu – | pu |, pv – | pv |) b(pu ,pv) = (1 – u’)(1 – v’)t(xl ,yb) + u’(1 – v’)t(xr ,yb) + (1 – u’)v’t(xl ,yt) + u’ v’ t(xr ,yt) Example: (pu,pv) = (81.92,74.24) (xl,yb) = (81,74) (xr,yt) = (82,75) 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Minification One pixel covers many texels? Towards horizon many artefacts e.g. temporal aliasing (object motion) Bilinear filtering (fails > 4 texels) glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR); GL_NEAREST GL_LINEAR 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Mipmapping Multum in parvo (MIP) Image pyramid Half width & height going upwards Average 4 ”parents”  ”child texel” Depending on amount minification, determine image to fetch from Compute d (λ in OpenGL) to give two images: how to use the two images? but how to interpolate and calculate d? 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Mipmapping: filtering Usual filtering options: point sampling (nearest neighbour) linear filtering Interpolate the bilinear values trilinear interpolation Constant time filtering 8 texel accesses Level n+1 Level n GL_NEAREST (u0,v0,d0) d v u GL_LINEAR 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Mipmapping: calculating d Approximate quad with square: A = approximate area of of square b = √ A d = log2 b Gives overblur! Better: anisotropic texture filtering approximate quad with several smaller mipmap samples pixel projected to texture space texel 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Controls how texture is applied: glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param) GL_TEXTURE_ENV_MODE modes GL_MODULATE: modulates with computed shade GL_BLEND: blends with an environmental coloor GL_REPLACE: use only texture coloor GL(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE); Set blend color with GL_TEXTURE_ENV_COLOR 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Perspective correction hint Hints are… Texture coordinate and color interpolation either linearly in screen space or using depth/perspective values (slower) perspective projections are nonlinear w.r.t. depth Noticeable for polygons “on edge” glHint(GL_PERSPECTIVE_CORRECTION_HINT, hint) Where hint is one of: GL_DONT_CARE GL_NICEST GL_FASTEST 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Generating texture coordinates OpenGL can generate texture coordinates automatically: glTexGen{ifd}[v]() Specify a plane & generate texture coordinates based upon distance from the plane Generation modes GL_OBJECT_LINEAR GL_EYE_LINEAR GL_SPHERE_MAP Utah teapot geometry (quads) Same texture coordinates for each quad Automatic texture coordinates (object space) Automatic texture coordinates (eye space) 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Texture Objects Texture is part of the OpenGL state If we have different textures for different objects, OpenGL will be moving large amounts data from processor memory to texture memory Recent versions OpenGL have texture objects one image per texture object texture memory can hold multiple texture objects Significant improvements in performance 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition) Textures and shading Must consider the interaction between texture and shading, various possibilities: modulate texture colour with result of shading (default) blend texture colour with shader colour only use the texture colour To controls how texture is applied glTexEnv{fi}[v](GL_TEXTURE_ENV, prop, param) Some prop = GL_TEXTURE_ENV_MODE modes: GL_MODULATE: modulates with computed shade GL_BLEND: blends with an environmental colour GL_REPLACE: use only texture color 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

More advanced textures… Ripmaps and anisotropic filtering Environment Maps Bump maps Multitexturing 3D textures .... 20/2/2006 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)