The Importance of Surface Texture Objects in the real world have rich detailed surface textures to produce believable scenes we must replicate this detail uniformly colored surfaces only get us so far Generated with Blue Moon Rendering Tools www bmrt org How Do We Model Intricate Surface Detail Approach 1 Explicit geometric representation actual polygons that model all the surface variations up to some finest level of detail may generate a lot of polygons Approach 2 Geometry texture images geometry only describes the general shape of the object paste an image onto the wall to give the appearance of brick Often We Use Simple Patterns Generally useful for skin bricks stucco granite Typically need to repeat texture over the object must make sure there are no seams when texture is tiled Or Given a Model and a Single Texture Sample model from www cyberware com Wrap the Texture onto the Model Sample model from www cyberware com Framework for Texture Mapping The texture itself is just a 2 D raster image acquired from reality hand painted or procedurally generated Establish a correspondence between surface points texture When shading a particular surface point look up the corresponding pixel in the texture image final color of point will be a function of this pixel Texturing Polygonal Models Polygonal models don t have a natural 2 D parameterization we need to create one For each vertex we specify a texture coordinate a u v pair that maps that point into the texture image a triangle on the surface will be mapped to a triangle in texture can interpolate texture coordinates over the triangle note that the size of the triangle may be quite different Texturing with Intermediate Surfaces There are certain objects that are easy to parameterize spheres and cylinders are good examples For a given object we can often establish a mapping to one of these this implicitly provides a parameterization of the surface Texturing and Rasterization During rasterization we traverse the pixels of a triangle at each pixel we interpolate the correct texture coordinate and we retrieve the corresponding texel texture element What do we do with the contents of the texel color use it to fill in the current pixel reflectance coefficient for illumination equation e g kd transparency an alpha value and many others some of which we ll discuss next time Minification Magnification Minification 1 pixel covers multiple texels Magnification 1 texel covers multiple pixels Texture Screen Surface OpenGL Texture Modes Determines how the contents of the texture are interpreted For RGB images GL MODULATE multiply together with surface color GL BLEND use as a t value to blend surface color and a predetermined color GL DECAL and GL REPLACE use texture color directly Texturing with OpenGL First turn on texturing glEnable GL TEXTURE 2D Next pass the actual texture image to OpenGL glTexImage2D GL TEXTURE 2D level channels width height border format type image for now level 0 and border 0 channels is usually 3 RGB with format GL RGB and type GL UNSIGNED BYTE Have lots of options to control texturing behavior see glTexEnvf and glTexParameterf for details texture coordinates clamped to 0 1 or do they wrap around how is the color of the texture applied to the surface Texturing with OpenGL Here s an example setup glEnable GL TEXTURE 2D glTexEnvf GL TEXTURE ENV GL TEXTURE ENV MODE GL MODULATE glTexParameterf GL TEXTURE 2D GL TEXTURE WRAP S GL REPEAT glTexParameterf GL TEXTURE 2D GL TEXTURE WRAP T GL REPEAT glTexParameterf GL TEXTURE 2D GL TEXTURE MAG FILTER GL LINEAR glTexParameterf GL TEXTURE 2D GL TEXTURE MIN FILTER GL LINEAR glTexImage2D GL TEXTURE 2D 0 3 width height 0 GL RGB GL UNSIGNED BYTE image This configures the texturing system to combine modulate the texture color with the surface color wrap texture coordinates around outside unit square linearly average texels when magnifying and minifying Texturing with OpenGL When drawing just assign texture coordinates to vertices glBegin GL TRIANGLES glNormal3fv n1 glTexCoord2f s1 t1 glVertex3fv v1 glNormal3fv n2 glTexCoord2f s2 t2 glVertex3f v2 glNormal3fv n3 glTexCoord2f s3 t3 glVertex3fv v3 glEnd Solid Texture Instead of texture images we can define texture volumes create a 3 D parameterization u v w for the texture map this onto the object the easiest parameterization u v w x y z Turns out to be a powerful technique for procedural generation more readily applies to implicit surfaces and other surfaces without natural 2 D parameterizations Some Texturing Applications First there s the obvious one realistic surface detail paste a fur marble face scan on a surface We can also support illumination precalculation suppose we precompute some expensive lighting effects soft shadows indirect light e g radiosity can hard code this lighting into texture maps Texturing can also be handy for faking objects billboards place image on a polygon which always rotates to face the viewer handy for things like trees sprites used in video games are a similar idea And texturing is useful for level of detail management can decouple resolution of texture from resolution of surface