Texture mapping in OPENGL

1. BY:
Manas U Nayak
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2.  What is Texture?
The feel, appearance, or consistency of a surface or a
substance.
Give (a surface) a rough or raised texture.
Well-defined textures are very important
for rendering realistic 3-D images.
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3. Texture
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4. 1- 3D Model Without Textures
2-3D Model With Textures
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5. Texture Mapping
Texture mapping is a graphic design process in
which a two-dimensional (2-D) surface, called
a texture map, is "wrapped around" a three-
dimensional (3-D)object.Thus, the 3-D object acquires
a surface texture similar to that of the 2-D surface.
Its application to 3D graphics was pioneered
by Edwin Catmull in 1974.
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6. Example for Texture Mapping
 Three identical squares, each covered randomly with
dots, are directly mapped onto the three visible facets
of a 3-D cube.This distorts the size sand shapes of the
dots on the top and right-hand facets.In this mapping,
the texture map covers the cube with no apparent
discontinuities because of the way the dots are
arranged on the squares.
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7. Mapping a 2D surface onto a 3D alters the size and
shape of the image elements
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8. Texture Mapping
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s
t
x
y
z
image
geometry display

9. Texels
 A texel, texture element, or texture pixel is the
fundamental unit of texture space, used in computer
graphics. Textures are represented by arrays of texels,
just as pictures are represented by arrays of pixels.
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10. Texel Vs Pixel…
 When a 3-D texture-mapped object appears close to the
viewer so that the texture elements appear relatively large,
there may be several pixels in each texel and the pattern of
the texture map is easy to see. When the same 3-D object is
removed to increasing distances, the texture-map pattern
appears smaller and smaller. Eventually, each texel can
become smaller than a pixel. Then an averaging process
must be used; several texels are combined to form each
pixel. If the object becomes distant enough, or if one of its
facets appears at a sharp angle with respect to the viewer,
the texels may become so small that the essence of the
pattern is lost in the observed image.
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12. Texture Representation
Bitmap (pixel map) textures (supported by OpenGL)
 Procedural textures (used in advanced rendering
programs)
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Bitmap texture:
 A 2D image - represented by 2D array
texture[height][width]
 Each pixel (or called texel ) by a unique
pair texture coordinate (s, t)
 The s and t are usually normalized to
a [0,1] range
 For any given (s,t) in the normalized range,
there is a unique image value (i.e.,
a unique [red, green, blue] set )
s
t
(0,0)
(1,1)

13. Map textures to surfaces
 Establish mapping from texture to surfaces (polygons):
- Application program needs to specify texture
coordinates for each corner of the polygon
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The polygon can be
in an arbitrary size
(0,0) (1,0)
(1,0) (1,1)

14. Map textures to surfaces
 Texture mapping is performed in rasterization
(backward mapping)
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(0,0) (1,0)
(0,1) (1,1)  For each pixel that is to be painted, its
texture coordinates (s, t) are determined
(interpolated) based on the corners’ texture
coordinates
 The interpolated texture coordinates are
then used to perform texture lookup

15. OpenGL texture mapping
 Steps in your program
1) Specify texture
- read or generate image
- Assign to texture
2) Specify texture mapping parameters
- Wrapping, filtering, etc.
3) Enable GL texture mapping (GL_TEXTURE_2D)
4) Assign texture coordinates to vertices
5) Draw your objects
6) Disable GL texture mapping (if you don’t need to
perform texture mapping any more)
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16. Specifying a Texture
 Define a texture image from an array of texels (texture
elements) in CPU 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.
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17. Define Texture as an Image
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
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);
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18. Converting A Texture Image
 OpenGL requires texture dimensions to be powers of 2.
 If dimensions of image are not powers of 2, perform
scaling:
• 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.
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19. Texture mapping parameters
 What happen if the given texture coordinates (s,t) are outside [0,1]
range?
 Example: glTexParameteri(GL_TEXTAURE_2D,
GL_TEXTURE_WRAP_S, GL_CLAMP)
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(0,0)
(1,1)
texture GL_Repeat
(0,0)
(2,2)
(0,0)
(2,2)
GL_Clamp
If (s >1) s = 1
If (t >1) t = 1

20. Tiling
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x
y
Texture Space
(0,0)
(1,1)

21. Clamping
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x
y
Texture Space
(0,0)
(1,1)

22. Mapping a Texture
 Based on parametric texture coordinates.
 glTexCoord*() specified at each vertex.
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s
t
1, 1
0, 1
0, 0 1, 0
(s, t) = (0.2, 0.8)
(0.4, 0.2)
(0.8, 0.4)
A
B C
a
b
c
Texture Space Object Space

23. Texture Mapping
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v0
v1
v2
t2
t0
t1
(1,1)
(0,0)
x
y
Texture Space Triangle (in any space)

24. Typical CodeglBegin(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();
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Note that we can use vertex arrays to increase efficiency.

25. Enable (Disable) Textures
 Enable texture – glEnable(GL_TEXTURE_2D)
 Disable texture – glDisable(GL_TEXTURE_2D)
Remember to disable texture mapping when you
draw non-textured polygons
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26. Specify texture coordinates
 Give texture coordinates before defining each
vertex
glBegin(GL_QUADS);
glTexCoord2D(0,0);
glVertex3f(-0.5, 0, 0.5);
…
glEnd();
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27. Interpolation
 OpenGL uses interpolation to find texels from texture
coordinates. Distortions can occur.
 Point sampling and linear filtering.
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Good selection of
texture coordinates.
Poor selection of
texture coordinates.
Texture stretched over
trapezoid shows effects
of bilinear interpolation.

28. Magnification and
Minification
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Texture Polygon
Magnification Minification
PolygonTexture
More than one texel can cover a pixel (minification) or more than one
pixel can cover a texel (magnification).
Can use point sampling (nearest texel) or linear filtering ( 2 x 2 filter)
to obtain texture values.

29. Filtering
 In computer graphics, texture filtering or texture
smoothing is the method used to determine the
texture color for a texture mapped pixel, using the
colors of nearby texels.
 Filtering methods
 Nearest-neighbor interpolation
 Nearest-neighbor with mipmapping
 Bilinear filtering
 Trilinear filtering
 Anisotropic filtering
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30. Mip-Mapping
 When undersampling(pixels only sparsely sample the
texels) are sparsely we use mippmapping
 Mipmapping allows for pre-filtered texture maps of
decreasing resolutions.
 Lessens interpolation errors for smaller textured objects.
 Resample image at lower resolution
 Create a “pyramid” of textures.
 Invoke mipmaps automatically:
glTexParameteri(GL_TEXTURE_2D,GL_TEXURE_MIN_FILTER
, GL_NEAREST_MIPMAP_NEAREST);
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31. Texture Pyramid
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128x128
64x64
32x32
1x1...

32. Other Texture Features
 Environment Mapping
 Bump Mapping
 Orthographic Mapping
 Perspective Mapping
 Displacement Mapping
 Spherical Mapping
 Cylindrical Mapping
 Cube Mapping
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