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Komputer Grafik 2 (AK045206)
Pemetaan Tekstur
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Outline
• Memetakan tekstur ke permukaan datar
• Contoh-contoh dengan OpenGL
• Pemetaan ‘Bump’
• MIPMAPS
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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
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Texture Maps• Images applied to polygons to enhance
the visual effect of a scene– Rectangular arrays of data
• Color, luminance, alpha• Components of array called texels
– We’ve also had volumetric voxels
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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
U
V
S
T
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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 aliasing– Watch for many to one mappings– Watch for perspective foreshortening effects
and linear interpolation
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Example Texture Map
Applied to tilted polygon
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Example Texture Map
glVertex3d (s, s, s)glTexCoord2d(1,1);
glVertex3d (-s, -s, -s)glTexCoord2d(1,1);
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Example Texture Map
glVertex3d (s, s, s)glTexCoord2d(5, 5);
glVertex3d (s, s, s)glTexCoord2d(1, 1);
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Texture Coordinates• Every polygon has object coordinates and
texture coordinates– Object coordinates describe where polygon
vertices are on the screen– Texture coordinates describe texel
coordinates of each vertex (usually 0 -> 1)– Texture coordinates are interpolated along
vertex-vertex edges
• glTexCoord{1234}{sifd}(TYPE coords)
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Komputer Grafik 2 (AK045206)
Textures• Texture Object
– An 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 texture
– You can prioritize textures to keep in memory– OpenGL uses least recently used (LRU) if no priority
is assigned
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Example use of Texture
• Read .bmp from file– Use 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
– Note• malloc() Image->data to appropriate size• .bmp stores color in bgr order and we convert to rgb
order
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Step 2 – create Texture Objects
• glGenTextures(1,
&texture[texture_num]); – First 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 creates
• texture[] is of type GLuint
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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]);
• Textures can be 1D and 3D as well
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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 object
– // 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);
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Step 5 – Assign image data– 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))
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glTexImage2D – Arg 1• GLenum target
– GL_TEXTURE_2D– GL_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
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glTexImage2D – Arg 2• GLint level
– Used for Level of Detail (LOD)– LOD stores multiple versions of texture that
can be used at runtime (set of sizes)– Runtime algorithms select appropriate version
of texture• Pixel size of polygon used to select best texture• Eliminates need for error-prone filtering algorithms
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glTexImage2D – Arg 3• GLint internalFormat
– Describes which of R, G, B, and A are used in internal representation of texels
– Provides 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 hardware
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glTexImage2D – Args 4-6• GLsizei width• GLsizei height
– Dimensions of texture image• Must be 2m + 2b (b=0 or 1 depending on border) • min, 64 x 64
• GLint border– Width of border (1 or 0)
• Border allows linear blending between overlapping textures• Useful when manually tiling textures
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glTexImage2D – Args 7 & 8• GLenum format
– Describe how texture data is stored in input array
• GL_RGB, GL_RGBA, GL_BLUE…
• GLenum type – Data size of array components
• GL_SHORT, GL_BYTE, GL_INT…
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glTexImage2D – Arg 9
• Const GLvoid *texels– Pointer to data describing texture map
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Komputer Grafik 2 (AK045206)
Step 6 – Apply texture• Before defining geometry
– glEnable(GL_TEXTURE_2D);– glBindTexture(GL_TEXTURE_2D, texture[0]);
– glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
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glTexEnv()
GL_TEXTURE_ENV_MODE GL_DECAL
GL_REPLACE
GL_MODULATE
GL_BLEND
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
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gluScaleImage()• Alters the size of an image to meet the 2m size
requirement of OpenGL– Scaling performed by linear and box filtering
glCopyTexImage2D()
• Use current frame buffer contents as texture
• Copy frame buffer to named texture location
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glTexSubImage2D()• Replace a region of current working
texture with a smaller texture• SubImage need not adhere to 2m size
limitation• This is how you add data from your
system’s camera to GL environment• glCopyTexSubImage2D
– Frame buffer cut and paste possible too
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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 surface
• Partial derivative of bump map specifies change to surface normal
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Komputer Grafik 2 (AK045206)
Bump Mapping
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Komputer Grafik 2 (AK045206)
Displacement Mapping• Bump mapped normals are inconsistent with
actual geometry. Problems arise (shadows).• Displacement mapping actually affects the
surface geometry
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Komputer Grafik 2 (AK045206)
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
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MIPMAPS• With versus without MIPMAP
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MIPMAPS• Arrange different versions into one block
of memory
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gluBuild2DMipmaps• Automatically constructs a family of
textures from original texture size down to 1x1
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Advanced Mipmaps
• You can specify additional mipmap levels on the fly– MIN_LOD may reduce popping– MAX_LOD may reduce over compression
• You can specify min mipmap level– Useful for mosaicing (Alphabet on a
texture)
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Komputer Grafik 2 (AK045206)
Referensi• 1: , 4:GraphicsSlides07.pdf , 9:Lecture14
• Buku Teks : 1. F.S.Hill, Jr., COMPUTER GRAPHICS – Using Open GL, Second Edition, Prentice Hall, 20012. Foley, van Dam, Feiner, Hughes, and Philips, Introduction to Computer Graphics, Addison Wesley,
2000• Lecture Notes / Slide-Presentation / Referensi lain yang diperoleh melalui internet :
3. Andries van Dam, Introduction to Computer Graphics, Slide-Presentation, Brown University, 2003, (folder : brownUni)
4. _______________, Interactive Computer Graphic, Slide-Presentation, (folder : Lect_IC_AC_UK)5. Michael McCool, CS 488/688 :Introduction to Computer Graphics, Lecture Notes, University of
Waterloo, 2003 (lecturenotes.pdf)6. _______________, Computer Science 559, Slide-Presentation, Wisconsin University,(folder :
Lect_Wisc_EDU)7. http://graphics.lcs.mit.edu/classses/6.837/F98/Lecture4/Slide23.html , Slide-Presentation, MIT,
(folder : MIT_CourseNote)8. _______________, CS 319 : Advance Topic in Computer Graphics, Slide-Presentation, (folder :
uiuc_cs)9. _______________, CS 445/645 : Introduction to Computer Graphics, Slide-Presentation,
(folder :COMP_GRAFIK)10. Gladimir V.G. Baranoski, CS 488 : Introduction to Computer Graphics , Waterloo University11. Prof. Peter Panfilov , http://cse.yeditepe.edu.tr/~osertel/courses/CSE484/index.html12. http://www.cl.cam.ac.uk/Teaching/1998/AGraphics/l3a.html