Computer Graphics Assignment (08bit057)

8/3/2019 Computer Graphics Assignment (08bit057)

http://slidepdf.com/reader/full/computer-graphics-assignment-08bit057 1/23

JPEG & DIB FILEFORMATS FORSTILL ANDMOTION IMAGES

DONE BY:

BHUPESH JAWA (08BIT057)Slot: Morning



The JPEG Standard

JPEG is an image compression standard whichwas accepted as an international standard in1992.

Developed by the Joint Photographic ExpertGroup of the ISO/IEC

For coding and compression of color/gray scale

images

Yields acceptable compression in the 10:1range



The JPEG Standard

JPEG is a lossy compression technique

Based on the DCT

JPEG is a general image compression techniqueindependent of

● Image resolution

● Image and pixel aspect ratio

●

Color system● Image compexity

A scheme for video compression based on JPEGcalled Motion JPEG (MJPEG) exists



The JPEG Standard

JPEG is effective because of the following threeobservations

Image data usually changes slowly across animage, especially within an 8x8 block

● Therefore images contain much redundancy

Experiments indicate that humans are not very

sensitive to the high frequency data images● Therefore we can remove much of this data using

transform coding



The JPEG Standard

Humans are much more sensitive to brightness(luminance) information than to color(chrominance)

JPEG uses chroma subsampling (4:2:0)

The following slide gives an overview of thevarious steps in JPEG compression



JPEG Encoding Overview

The main steps in JPEG encoding are thefollowing

Transform RGB to YUV or YIQ and subsamplecolor

DCT on 8x8 image blocks

Quantization

Zig-zag ordering and run-length encoding

Entropy coding



DCT on Image Blocks

The image is divided up into 8x8 blocks

2D DCT is performed on each block

The DCT is performed independently for eachblock

This is why, when a high degree of compressionis requested, JPEG gives a “blocky” image result



Quantization

Quantization in JPEG aims at reducing the totalnumber of bits in the compressed image

Divide each entry in the frequency space blockby an integer, then round

Use a quantization matrix Q(u, v )



Quantization

Use larger entries in Q for the higher spatialfrequencies

These are entries to the lower right part of thematrix

The following slide shows the default Q(u, v )values for luminance and chrominance

●

Based on psychophysical studies intended tomaximize compression ratios while minimizingperceptual distortion

● Since after division the entries are smaller, we canuse fewer bits to encode them



Quantization

Multiple quantization matrices can be used(perhaps by scaling the defaults), allowing theuser to choose how much compression to use

Trades off quality vs. compression ratio

More compression means larger entries in Q

An example of JPEG coding and decoding onone image block is shown next



Preparation for EntropyCodingWe have seen two main steps in JPEG coding:DCT and quantization

The remaining steps all lead up to entropycoding of the quantized DCT coefficients

These additional data compression steps arelossless

Most of the lossiness is in the quantization step



Run-Length Coding

We now do run-length coding

The AC and DC components are treateddifferently

Since after quantization we have many 0 ACcomponents, RLC is a good idea

Note that most of the zero components are

towards the lower right corner (high spatialfrequencies)

To take advantage of this, use zigzag scanningto create a 64-vector



Zigzag Scan in JPEG



JPEG Modes

JPEG supports several different modes

Sequential Mode

Progresssive ModeSequential is the default mode

Each image component is encoded in a singleleft-to-right, top-to-bottom scan

● This is the mode we have been describing



Progressive Mode

Progressive mode delivers low-quality versionsof the image quickly, and then fills in thedetails in successive passes

This is useful for web browsers, where theimage download might take a long time

The user gets an approximate image quickly

Can be done by sending the DC coefficient and a fewAC coefficients first

Next send some more (low spatial resolution) ACcoefficients, and continue in this way until all of thecoefficients have been sent



Sequential vs. Progressive

Click to edit Master text stylesSecond level● Third level● Fourth level● Fifth level

Click to edit Master text styles

Second level● Third level● Fourth level● Fifth level



JPEG Bitstream

The JPEG hierarchical organization isdescribed in the next slide

Frame is a pictureScan is a picture component

Segment is a group of blocks

Frame header inlcudes

● Bits per pixel

● Size of image

● Quantization table etc.

Scan header includes●



JPEG Bitstream



DIB File Format

The BMP File Format, also known as BitmapImage File or Device Independent Bitmap(DIB) file format or simply a Bitmap, is

a Raster graphicsimage file format used tostore bitmap digital images, independently of the display device (such as a graphics adapter),especially on Microsoft

Windowsand OS/2 operating systems.

The BMP File Format is capable of storing2D digital images of arbitrary width, height, andresolution, both monochrome and color, invarious color depths, and optionally with data



DIBs in memory

A Bitmap Image File loaded into memorybecomes a DIB data structure - an importantcomponent of the Windows GDI API. The in-

memory DIB data structure is almost the sameas the BMP file format, but it does not containthe 14-byte Bitmap File Header and begins withthe DIB Header. For DIBs loaded in memory, the

Color Table can also consist of 16 bit entries,that constitute indexes to the currently realizedpalette(an additional level of indirection),instead of explicit RGB color definitions. In all

cases, the Pixel Array must begin at a memory-



When the size of Gap1 and Gap2 is zero, the in-memory DIB data structure is customarily

referred to as "packed DIB" and can bereferred to by a single pointer pointing to thebeginning of the DIB Header. In all cases, thePixel Array must begin at a memory address

that is a multiple of 4 bytes. In some cases itmay be necessary to adjust the number of entries in the Color Table in order to force thememory address of the Pixel Array to a multipleof 4 bytes. For "packed DIBs" loaded inmemory, the optional Color Profile data shouldimmediately follow the Pixel Array, as depictedin Diag.1 (with Gap1=0 and Gap2=0)."Packed DIBs" are required

by Windows clipboard API functions as well as



Bitmap file header

Click to edit Master text stylesSecond level● Third level●

Fourth level● Fifth level



DIB Header (BitmapInformation Header)

This block of bytes tells the application detailedinformation about the image, which will be usedto display the image on the screen. The block

also matches the header used internally byWindows and OS/2 and has several differentvariants. All of them contain a dword (32 bit)field, specifying their size, so that an application

can easily determine which header is used inthe image. The reason that there are differentheaders is that Microsoft extended the DIBformat several times. The new extended

headers can be used with some GDI functions

Documents

Computer Graphics Assignment (08bit057)