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Solution Involves Selection of DiscreteRepresentation Values

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Antialiasing:An Example x 9

y 3

d0 2y x 3

incrE 2y 6

incrNE 2(y x) 12

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Aliasing:Square Pixels

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Antialiasing:Unweighted Area Sampling

Pixel Intensity Proportional toArea of Intersection

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Antialiasing:Unweighted Area Sampling

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Antialiasing:Weighted Sampling Function (Conical)

Desired Line

Weighting Function

Region of Overlap

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Gupta-Sproull Antialiasing

v

1+v

1-v

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Write from Memory to Frame Buffer

Main Memory

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Read from Frame Buffer into Main Memory

Main Memory

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Copy from One Part of Frame Bufferto Another

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OpenGL Functions for Reading, Writing,Copying Pixel Data

glReadPixels( ) - Reads a rectangular array of pixelsfrom the frame buffer and stores in main memory

glDrawPixels( ) - Writes a rectangular array of pixelsinto the frame buffer from data in main memory

glCopyPixels( ) - Copies a rectangular array of pixels fromone part of the frame buffer to another

Issues:

• Kind of frame buffer data• How data is stored in main memory• Data conversions performed during reading, writing, or copying

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Reading Pixel DataFrame Buffer -> Memory

void glReadPixels(GLint x, Glint y, Glsizei width, GLsizei height, GLenum format,

Glenum type, GLvoid *pixels);

x - x coordinate of lower left corner of rectangular subimagey - y coordinate of lower left corner of rectangular subimagewidth - width of subimage in pixelsheight - height of subimage in pixelsformat - type of pixel data elements readtype - storage data type for each element*pixels - array to receive the data

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Pixel Formats

GL_COLOR_INDEX A single color indexGL_RGB A red color component followed by

a green followed by a blue

GL_RGBA Red, green, blue, alpha componentsGL_RED A single red componentGL_GREEN A single green componentGL_BLUE A single blue componentGL_ALPHA A single alpha componentGL_LUMINANCE A single luminance component

GL_LUMINANCE_ALPHA A luminance component followedby an alpha component

GL_STENCIL_INDEX A single stencil indexGL_DEPTH_COMPONENT A single depth component

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Data Types

GL_UNSIGNED_BYTE unsigned 8-bit integer

GL_BYTE signed 8-bit integer

GL_BITMAP single bits in unsigned8-bit integers

GL_UNSIGNED_SHORT unsigned 16-bit integer

GL_SHORT signed 16-bit integer

GL_UNSIGNED_INT unsigned 32-bit integer

GL_INT 32-bit integer

GL_FLOAT single-precision float pt.

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Writing Pixel DataMemory -> Frame Buffer

void glDrawPixels(Glsizei width, GLsizei height, GLenum format,

Glenum type, GLvoid *pixels);

width - width of subimage in pixelsheight - height of subimage in pixelsformat - type of pixel data elements readtype - storage data type for each element*pixels - array holding the data to be drawn

Lower left corner of the subimage is defined by the currentraster position, defined by glRasterPos*().

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Copying Pixel Data

void glCopyPixels(GLint x, Glint y, Glsizei width, GLsizei height,Glenum type);

x - x coordinate of lower left corner of rectangular subimagey - y coordinate of lower left corner of rectangular subimagewidth - width of subimage in pixelsheight - height of subimage in pixelstype - GL_COLOR, GL_DEPTH, or GL_STENCIL

Copied data is drawn at the current raster position.

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Pixel Storage Mode Determination

void glPixelStore{if} (GLenum pname, TYPE param);

Parameter Type DefaultGL_UNPACK_SWAP_BYTES GLboolean FALSEGL_PACK_SWAP_BYTES

GL_UNPACK_LSB_FIRST GLboolear FALSEGL_PACK_LSB_FIRST

GL_UNPACK_ROW_LENGTH GLint 0GL_PACK_ROW_LENGTH

GL_UNPACK_SKIP_ROWS GLint 0GL_PACK_SKIP_ROWS

GL_UNPACK_SKIP_PIXELS GLint 0GL_PACK_SKIP_PIXELS

GL_UNPACK_ALIGNMENT GLint 4GL_PACK_ALIGNMENT

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Reading a Subimage from Memory

ROW_LENGTH

subimage

SKIP_PIXELS

SKIP_ROWS

image in memory

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Pixel Transfer Operations

void glPixelTransfer{if}(GLenum pname, TYPE param);

Parameter Type Default

GL_RED_SCALE GLfloat 1.0GL_GREEN_SCALE GLfloat 1.0GL_BLUE_SCALE GLfloat 1.0

GL_RED_BIAS GLfloat 0.0GL_GREEN_SCALE GLfloat 0.0GL_BLUE_SCALE GLfloat 0.0

... and many more

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Graphics Files

Model Render Display

Model Render Display

Model Render Display

Store

Store

Customary graphics data flow:

Alternative graphics data flows:

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Graphics File Types

“Vector” Bitmap

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Vector File FormatsAdvantages and…….. Disadvantages

0 Efficient representation0 Work well for “line art”0 Well suited to scaling

(zoom)0 Objects can have identity0 Extensible to 3D

0 Limited set of objects that can be represented

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Bitmap File FormatsAdvantages and…….. Disadvantages

0 Represent images with complex variations in colors

0 Storage size- Mitigated by

compression techniques0 Costly to manipulate0 Fixed resolution - problem

with scaling0 Inability to identify “objects”

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Graphics MetafilesPage Description Languages

GraphicsMetafile

Attributes

Vector Data

Image Data

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Bitmap File Formats:Generic Structure

Header

Pixel Data[Optional Compression]

[Color Table]

Fixed FileOffset

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

File Header

Bitmap Data

Bitmap Header

0

14

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

0

Offset Size Name Description

0 2 bfType ASCII “BM”

2 4 bfSize Size of file in bytes

6 2 bfReserved1 Zero

8 2 bfReserved2 Zero

10 4 bfOffBits Byte offset to image

File Header

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

14Offset Size Name Description

14 4 biSize Size of this header

18 4 biWidth Image width (pixels)

22 4 biHeight Image height

26 2 biPlanes #planes (=1)

28 2 biBitCount Bits/pixel (1,4,8 or 24)

30 4 biCompression Compression type

34 4 biSizeImage Size (bytes) of comp.img.

38 4 biXPelsPerMeter x pixels/meter

42 4 biYPelsPerMeter y pixels/meter

46 4 biClrUsed Number of colors used

50 4 biClrImportant No. of important colors

54 4*N bmiColors Color Map

Bitmap Header

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

54Offset Size Name Description

0 1 rgbBlue Blue value

1 1 rgbGreen Green value

2 1 rgbRed Red value

3 1 rgbReserved Zero

Color Map

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

0 Bits are stored a row at a time starting from the bottom of the image to the top.

0 Each row is padded to a 4-byte boundary0 Significance of bits depends upon the number of bits per

pixel- In case of 1 bit per pixel each pixel is a single bit. High-

order bit is the leftmost pixel- In case of 4 bit per pixel images are packed two pixels per

byte, with the high nibble being the leftmost pixel

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Example Bitmap Format:Windows BMP (Device Independent Bitmap)

Pixel 1

Pixel 2

Pixel 3

Pixel 4

Pixel 5

Pixel 6

Pixel 7

Pixel 8

Pixel 9

Pixel 10

Pixel 1

Pixel 2

8 bits/pixel (color index) 24 bits/pixel

Pixel 3

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Windows BitmapRLE Compression

Repeating groups (2 bytes: count, repeated byte):

05 24 24 24 24 24 24

Literal groups (zero byte, pixel count, literal pixels):

00 05 12 34 56 78 9A 00 12 34 56 78 9A

Special group:

00 00 end of row00 01 end of bitmap00 02 xx yy continuation xx pixels to right and yy pixels down

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Texture Mapping

Texture Map= image Polygon

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Texture Mapping:The Main Steps

0 Specify the texture- Usually a single, two dimensional, rectangular image

=Can be one dimensional- Individual elements: texels- One to four elements per texel (RGBA)- OpenGL 1.x requires dimensions be powers of two

0 Define how the texture is to be applied to each pixel- Decal mode- Replace mode- Modulate mode- Blend mode- Add mode (new with OpenGL 2.0)

0 Enable texture mapping0 Draw the scene, supplying both texture and geometric coordinates

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Texture Mapping:Specifying the Current Texture in OpenGL

void glTexImage2D(GLenum target, GLint level,GLint internalFormat,GLsizei width, GLsizei height, GLint border,GLenum format, GLenum type, GLvoid *texels);

target - future use, must be GL_TEXTURE_2D

level - 0 unless using multiple level of detail

internalFormat - one of 38 symbolic constants

width, height - height and width of texture map in texels

border - size of border in texels

format, type - storage parameters, same as glDrawPixels()

*texels - storage location of the texture map

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Binding Texture Objects

void glBindTexture(GLenum target, GLuint textureName);

target: GL_TEXTURE_1D or GL_TEXTURE_2D

textureName: unique unsigned int identifier

When initially called, binds the default texture state to the identifier.Subsequent calls to texture functions modify the state of thetexture object.

When binding to a previously created texture object, that objectbecomes active.

Unique texture names can be generated by: glGenTextures()

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Texture Mapping:Assigning Texture Coordinates

void glTexCoord2d(GLdouble s, GLdouble t);

s

t

1.00.00.0

1.0

Values outside [0.0, 1.0] can be made to either wrap orclamp to boundary values

Texture Image

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Boundary Wrapping/Clamping

void glTexParameteri(GLenum target, GLenum pname,TYPE param);

target: GL_TEXTURE_2D

pname: GL_TEXTURE_WRAP_S or GL_TEXTURE_WRAP_T

param: GL_CLAMP uses the last pixel value in the s or tdirection for s,t outside [0,1]

GL_REPEAT repeats the texture pattern for s, t outside[0,1]

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Texture Mapping:Magnification and Minification

Texture Polygon PolygonTexture

Magnification Minification

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Texture Mapping:Specifying Filtering for Mag./Min.

glTexParameteri(GL_TEXTURE_2D, filter type, filter)’

filter type - GL_TEXTURE_MAG_FILTER for magnification, GL_TEXTURE_MIN_FILTER for minification

filter - GL_NEAREST selects the texel mapping nearest to the target pixel

GL_LINEAR performs a linear average of the 2x2 arrayof texels around the mapped point

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Texture Mapping:How the Texture is Applied

void glTexEnv*(GLenum target, GLenum pname,GLenum param);

target - must be GL_TEXTURE_ENV

pname - GL_TEXTURE_ENV_MODE

param - one of GL_DECAL, GL_REPLACE,GL_MODULATE, GL_BLEND, GL_ADD

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Texture Mapping:Interpreting glTexEnv() Parameters

From OpenGL 2.0 Specification, J. Leech & P. Brown, ed., Oct. 22, 2004

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Texture Mapping:Interpreting glTexEnv() Parameters

From OpenGL 2.0 SpecificationFrom OpenGL 2.0 Specification, J. Leech & P. Brown, ed., Oct. 22, 2004