CS 445 / 645 Introduction to Computer Graphics Lecture 19 Texture Maps Lecture 19 Texture Maps

Texture Mapping Limited ability to generate complex surfaces with geometry Images can convey the illusion of geometry Images painted onto polygons is called texture mapping Limited ability to generate complex surfaces with geometry Images can convey the illusion of geometry Images painted onto polygons is called texture mapping

Texture Maps Chapter 9 of Open GL Programming Guide (Red Book) Images applied to polygons to enhance the visual effect of a scene Rectangular arrays of dataRectangular arrays of data –Color, luminance, alpha –Components of array called texels  Or in 3D, volumetric voxels Chapter 9 of Open GL Programming Guide (Red Book) Images applied to polygons to enhance the visual effect of a scene Rectangular arrays of dataRectangular arrays of data –Color, luminance, alpha –Components of array called texels  Or in 3D, volumetric voxels

Texture Mapping Texture map is an image, two-dimensional array of color values (texels) Texels are specified by texture’s (u,v) space At each screen pixel, texel can be used to substitute a polygon’s surface property (color) We must map (u,v) space to polygon’s (s, t) space Texture map is an image, two-dimensional array of color values (texels) Texels are specified by texture’s (u,v) space At each screen pixel, texel can be used to substitute a polygon’s surface property (color) We must map (u,v) space to polygon’s (s, t) space U V S T

Texture Mapping (u,v) to (s,t) mapping can be explicitly set at vertices by storing texture coordinates with each vertex How do we compute (u,v) to (s,t) mapping for points in between Watch for aliasingWatch for aliasing Watch for many to one mappingsWatch for many to one mappings Watch for perspective foreshortening effects and linear interpolationWatch for perspective foreshortening effects and linear interpolation (u,v) to (s,t) mapping can be explicitly set at vertices by storing texture coordinates with each vertex How do we compute (u,v) to (s,t) mapping for points in between Watch for aliasingWatch for aliasing Watch for many to one mappingsWatch for many to one mappings Watch for perspective foreshortening effects and linear interpolationWatch for perspective foreshortening effects and linear interpolation

Example Texture Map Applied to tilted polygon

Example Texture Map glVertex3d (s, s, s) glTexCoord2d(1,1); glVertex3d (-s, -s, -s) glTexCoord2d(0, 0);

Example Texture Map glVertex3d (s, s, s) glTexCoord2d(5, 5); glVertex3d (s, s, s) glTexCoord2d(1, 1); Repeating textures vs. Clamped textures

The Art of 3D Computer Animation and Effects Isaac Kerlow

Texture Coordinates Every polygon can have object coordinates and texture coordinates Object coordinates describe where polygon vertices are on the screenObject coordinates describe where polygon vertices are on the screen Texture coordinates describe texel coordinates of each vertex (usually 0 -> 1)Texture coordinates describe texel coordinates of each vertex (usually 0 -> 1) Texture coordinates are interpolated along vertex-vertex edgesTexture coordinates are interpolated along vertex-vertex edges glTexCoord{1234}{sifd}(TYPE coords) Every polygon can have object coordinates and texture coordinates Object coordinates describe where polygon vertices are on the screenObject coordinates describe where polygon vertices are on the screen Texture coordinates describe texel coordinates of each vertex (usually 0 -> 1)Texture coordinates describe texel coordinates of each vertex (usually 0 -> 1) Texture coordinates are interpolated along vertex-vertex edgesTexture coordinates are interpolated along vertex-vertex edges glTexCoord{1234}{sifd}(TYPE coords) Why 1  4 coords? s, t, r, and q (for homogeneous coordinates, an advanced topic)

Textures Texture Object An OpenGL data type that keeps textures resident in memory and provides identifiers to easily access themAn OpenGL data type that keeps textures resident in memory and provides identifiers to easily access them Provides efficiency gains over having to repeatedly load and reload a textureProvides efficiency gains over having to repeatedly load and reload a texture You can prioritize textures to keep in memoryYou can prioritize textures to keep in memory OpenGL uses least recently used (LRU) if no priority is assignedOpenGL uses least recently used (LRU) if no priority is assigned Texture Object An OpenGL data type that keeps textures resident in memory and provides identifiers to easily access themAn OpenGL data type that keeps textures resident in memory and provides identifiers to easily access them Provides efficiency gains over having to repeatedly load and reload a textureProvides efficiency gains over having to repeatedly load and reload a texture You can prioritize textures to keep in memoryYou can prioritize textures to keep in memory OpenGL uses least recently used (LRU) if no priority is assignedOpenGL uses least recently used (LRU) if no priority is assigned

Example use of Texture Read.bmp from file Use Image data typeUse Image data type –getc() and fseek() to read image x & y size –fread() fills the Image  data memory with actual red/green/blue values from.bmp NoteNote –malloc() Image->data to appropriate size –.bmp stores color in bgr order and we convert to rgb order Read.bmp from file Use Image data typeUse Image data type –getc() and fseek() to read image x & y size –fread() fills the Image  data memory with actual red/green/blue values from.bmp NoteNote –malloc() Image->data to appropriate size –.bmp stores color in bgr order and we convert to rgb order

Step 2 – create Texture Objects glGenTextures(1, &texture[texture_num]); First argument tells GL how many Texture Objects to createFirst argument tells GL how many Texture Objects to create Second argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the Texture Objects it createsSecond argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the Texture Objects it creates –texture[ ] is of type GLuint glGenTextures(1, &texture[texture_num]); First argument tells GL how many Texture Objects to createFirst argument tells GL how many Texture Objects to create Second argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the Texture Objects it createsSecond argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the Texture Objects it creates –texture[ ] is of type GLuint

Step 3 – Specify which texture object is about to be defined Tell OpenGL that you are going to define the specifics of the Texture Object it created glBindTexture(GL_TEXTURE_2D, texture[texture_num]);glBindTexture(GL_TEXTURE_2D, texture[texture_num]); –Textures can be 1D and 3D as well Tell OpenGL that you are going to define the specifics of the Texture Object it created glBindTexture(GL_TEXTURE_2D, texture[texture_num]);glBindTexture(GL_TEXTURE_2D, texture[texture_num]); –Textures can be 1D and 3D as well

Step 4 – Begin defining texture glTexParameter() Sets various parameters that control how a texture is treated as it’s applied to a fragment or stored in a texture objectSets various parameters that control how a texture is treated as it’s applied to a fragment or stored in a texture object // scale linearly when image bigger than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);// scale linearly when image bigger than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); // scale linearly when image smaller than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); // scale linearly when image smaller than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);glTexParameter() Sets various parameters that control how a texture is treated as it’s applied to a fragment or stored in a texture objectSets various parameters that control how a texture is treated as it’s applied to a fragment or stored in a texture object // scale linearly when image bigger than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);// scale linearly when image bigger than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); // scale linearly when image smaller than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); // scale linearly when image smaller than texture glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

Step 5 – Assign image data glTexImage2D();glTexImage2D(); GL_TEXTURE_2D (2D Texture) 0 (level of detail 0) 3 (3 components, RGB) image1->sizeX(size) image1->sizeY(size) 0 (no border pixel) GL_RGB (RGB color order) GL_UNSIGNED_BYTE (unsigned byte data) image1->data (pointer to the data))

glTexImage2D – Arg 1 GLenum target GL_TEXTURE_{1|2|3}DGL_TEXTURE_{1|2|3}D GL_PROXY_TEXTURE_2DGL_PROXY_TEXTURE_2D –Provides queries for texture resources –Proceed with hypothetical texture use (GL won’t apply the texture) –After query, call GLGetTexLevelParamter to verify presence of required system components –Doesn’t check possibility of multiple texture interference GLenum target GL_TEXTURE_{1|2|3}DGL_TEXTURE_{1|2|3}D GL_PROXY_TEXTURE_2DGL_PROXY_TEXTURE_2D –Provides queries for texture resources –Proceed with hypothetical texture use (GL won’t apply the texture) –After query, call GLGetTexLevelParamter to verify presence of required system components –Doesn’t check possibility of multiple texture interference

glTexImage2D – Arg 2 GLint level Used for Level of Detail (LOD)Used for Level of Detail (LOD) LOD stores multiple versions of texture that can be used at runtime (set of sizes)LOD stores multiple versions of texture that can be used at runtime (set of sizes) Runtime algorithms select appropriate version of textureRuntime algorithms select appropriate version of texture –Pixel size of polygon used to select best texture –Eliminates need for error-prone filtering algorithms GLint level Used for Level of Detail (LOD)Used for Level of Detail (LOD) LOD stores multiple versions of texture that can be used at runtime (set of sizes)LOD stores multiple versions of texture that can be used at runtime (set of sizes) Runtime algorithms select appropriate version of textureRuntime algorithms select appropriate version of texture –Pixel size of polygon used to select best texture –Eliminates need for error-prone filtering algorithms

glTexImage2D – Arg 3 GLint internalFormat GL defines 38 symbolic constants that describe which of R, G, B, and A are used in internal representation of texelsGL defines 38 symbolic constants that describe which of R, G, B, and A are used in internal representation of texels Provides control over things texture can doProvides control over things texture can do –High bit depth alpha blending –High bit depth intensity mapping –General purpose RGB GL doesn’t guarantee all options are available on given hardwareGL doesn’t guarantee all options are available on given hardware GLint internalFormat GL defines 38 symbolic constants that describe which of R, G, B, and A are used in internal representation of texelsGL defines 38 symbolic constants that describe which of R, G, B, and A are used in internal representation of texels Provides control over things texture can doProvides control over things texture can do –High bit depth alpha blending –High bit depth intensity mapping –General purpose RGB GL doesn’t guarantee all options are available on given hardwareGL doesn’t guarantee all options are available on given hardware

glTexImage2D – Args 4-6 GLsizei width GLsizei height Dimensions of texture imageDimensions of texture image –Must be 2 m + 2b (b=0 or 1 depending on border) –min, 64 x 64 GLint border Width of border (1 or 0)Width of border (1 or 0) –Border allows linear blending between overlapping textures –Useful when manually tiling textures GLsizei width GLsizei height Dimensions of texture imageDimensions of texture image –Must be 2 m + 2b (b=0 or 1 depending on border) –min, 64 x 64 GLint border Width of border (1 or 0)Width of border (1 or 0) –Border allows linear blending between overlapping textures –Useful when manually tiling textures

glTexImage2D – Args 7 & 8 GLenum format Describe how texture data is stored in input arrayDescribe how texture data is stored in input array –GL_RGB, GL_RGBA, GL_BLUE… GLenum type Data size of array componentsData size of array components –GL_SHORT, GL_BYTE, GL_INT… GLenum format Describe how texture data is stored in input arrayDescribe how texture data is stored in input array –GL_RGB, GL_RGBA, GL_BLUE… GLenum type Data size of array componentsData size of array components –GL_SHORT, GL_BYTE, GL_INT…

glTexImage2D – Arg 9 Const GLvoid *texels Pointer to data describing texture mapPointer to data describing texture map Const GLvoid *texels Pointer to data describing texture mapPointer to data describing texture map

Step 6 – Apply texture Before defining geometry glEnable(GL_TEXTURE_2D);glEnable(GL_TEXTURE_2D); glBindTexture(GL_TEXTURE_2D, texture[0]);glBindTexture(GL_TEXTURE_2D, texture[0]); glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE); Before defining geometry glEnable(GL_TEXTURE_2D);glEnable(GL_TEXTURE_2D); glBindTexture(GL_TEXTURE_2D, texture[0]);glBindTexture(GL_TEXTURE_2D, texture[0]); glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

glTexEnv() GL_TEXTURE_ENV_MODE GL_DECAL (alpha blends texture with poly color) GL_REPLACE (straight up replacement) GL_MODULATE (texture application is a function of poly lighting) GL_BLEND (texture controls blending with another color) If GL_BLEND selected, second call to glTexEnv() must specify GL_TEXTURE_ENV_COLOR 4-float array for R,G,B,A blend First argument to function is always GL_TEXTURE_ENV

gluScaleImage() Alters the size of an image to meet the 2 m size requirement of OpenGL Scaling performed by linear and box filteringScaling performed by linear and box filtering Alters the size of an image to meet the 2 m size requirement of OpenGL Scaling performed by linear and box filteringScaling performed by linear and box filtering

glCopyTexImage2D() Use current frame buffer contents as texture Copy frame buffer to named texture location Use current frame buffer contents as texture Copy frame buffer to named texture location

glTexSubImage2D() Replace a region of current working texture with a smaller texture SubImage need not adhere to 2 m size limitation This is how you add data from your system’s camera to GL environment glCopyTexSubImage2D Frame buffer cut and paste possible tooFrame buffer cut and paste possible too Replace a region of current working texture with a smaller texture SubImage need not adhere to 2 m size limitation This is how you add data from your system’s camera to GL environment glCopyTexSubImage2D Frame buffer cut and paste possible tooFrame buffer cut and paste possible too

Bump Mapping Use textures to modify surface geometry Use texel values to modify surface normals of polygon Texel values correspond to height field Height field models a rough surfaceHeight field models a rough surface Partial derivative of bump map specifies change to surface normal Use textures to modify surface geometry Use texel values to modify surface normals of polygon Texel values correspond to height field Height field models a rough surfaceHeight field models a rough surface Partial derivative of bump map specifies change to surface normal

Bump Mapping

Displacement Mapping Bump mapped normals are inconsistent with actual geometry. Problems arise (shadows). Displacement mapping actually affects the surface geometry Bump mapped normals are inconsistent with actual geometry. Problems arise (shadows). Displacement mapping actually affects the surface geometry

Mipmaps multum in parvo -- many things in a small place A texture LOD technique Prespecify a series of prefiltered texture maps of decreasing resolutions Requires more texture storage Eliminates shimmering and flashing as objects move (why?) multum in parvo -- many things in a small place A texture LOD technique Prespecify a series of prefiltered texture maps of decreasing resolutions Requires more texture storage Eliminates shimmering and flashing as objects move (why?)

Shimmering in mipmaps How do I reduce the size of this image by half? near far

MIPMAPS With versus without MIPMAP

MIPMAPS Arrange different versions into one block of memory

gluBuild2DMipmaps Automatically constructs a family of textures from original texture size down to 1x1

Advanced Mipmaps You can specify additional mipmap levels on the fly MIN_LOD may reduce poppingMIN_LOD may reduce popping MAX_LOD may reduce over compressionMAX_LOD may reduce over compression You can specify min mipmap level Useful for mosaicing (Alphabet on a texture)Useful for mosaicing (Alphabet on a texture) You can specify additional mipmap levels on the fly MIN_LOD may reduce poppingMIN_LOD may reduce popping MAX_LOD may reduce over compressionMAX_LOD may reduce over compression You can specify min mipmap level Useful for mosaicing (Alphabet on a texture)Useful for mosaicing (Alphabet on a texture)

Filtering OpenGL tries to pick best mipmap level Question: Which texel corresponds to a particular pixel? GL_NEAREST (Point Sampling) Pick the texel with center nearest pixelPick the texel with center nearest pixel GL_LINEAR (Bilinear Sampling) Weighted average of 2x2 closest texelsWeighted average of 2x2 closest texelsGL_NEAREST_MIPMAP_LINEAR Average nearest texels from two mipmap levelsAverage nearest texels from two mipmap levels GL_LINEAR_MIPMAP_LINEAR (Trilinear) Average two averaged texels from two mipmapsAverage two averaged texels from two mipmaps OpenGL tries to pick best mipmap level Question: Which texel corresponds to a particular pixel? GL_NEAREST (Point Sampling) Pick the texel with center nearest pixelPick the texel with center nearest pixel GL_LINEAR (Bilinear Sampling) Weighted average of 2x2 closest texelsWeighted average of 2x2 closest texelsGL_NEAREST_MIPMAP_LINEAR Average nearest texels from two mipmap levelsAverage nearest texels from two mipmap levels GL_LINEAR_MIPMAP_LINEAR (Trilinear) Average two averaged texels from two mipmapsAverage two averaged texels from two mipmaps