about image processing

1. IMAGE COMPRESSION
Presented By,
M.Ramya,
MSc(CS&IT),
Nadar Saraswathi college of Arts & Science,
Theni.

2. INTRODUCTION
• Audio, image and video require vast amounts of data.
 320x240x8bits grayscale image: 77Kb
 ‰1100x900x24bits color image: 3MB
 ‰640x480x24x30frames/sec: 27.6 MB/sec
• ‰Low network’s bandwidth doesn't allow for real time video transmission.
• ‰Slow storage devices don't allow for fast playing back ‰Compression
reduces storage requirements.

3. TYPES OF IMAGE
There are 5 main formats in which to store images.
 TIFF
 JPEG
 GIF
 PNG
 Raw image files

4. TIFF
• TIFF stands for Tagged Image File Format.
• TIFF images create very large file sizes.
• TIFF images are uncompressed and thus contain a lot of detailed image
data.
• TIFFs are also extremely flexible in terms of and content.
• TIFF is the most common file type used in photo software as well as page
layout software.
• TIFF contains a lot of image data.
• TIFF file types ending in .tif

5. JPEG
• JPEG stands for Joint Photographic Experts Group, which created this
standard for this type of image formatting.
• JPEG files are images that have been compressed to store a lot of
information in a small-size file.
• A JPEG is compressed in a way that loses some of the image detail during
the compression in order to make the file small.
• JPEG files are bad for line drawings or logos or graphics, as the
compression makes them look “bitmappy”.
• JPEG file types ending in .jpg

6. GIF
• GIF stands for Graphic Interchange Format.
• This format compresses images but, as different from JPEG, the
compression is lossless.
• GIFs also have an extremely limited color range suitable for the web but
not for printing.
• This format is never used for photography, because of the limited number
of colors.
• GIFs can also be used for animations.
• GIF file types ending in .gif

7. PNG
• PNG stands for Portable Network Graphics.
• It also allows for a full range of color and better compression.
• It’s used almost exclusively for web images , never for print images.
• For photographs , PNG is not as good as JPEG , because it creates a larger
file.
• It was created as an open format to replace GIF , because the patent for GIF
was owned by one company and nobody else wanted to pay licensing fees.
• PNG file types ending in .png

8. RAW IMAGE FILES
• Raw image files contain data from a digital camera.
• The files are called raw because they haven’t been processed and therefore
can’t be edited or printed.
• Raw files usually contain a vast amount of data that is uncompressed.
• The size of a raw file is extremely large.
• They are converted to TIFF before editing and color-correcting

9. RECENT COMPRESSION STANDARDS

10. JPEG
JPEG Modes of Operation ‰Sequential
DCT
The image is encoded in one left-to-right, top-to-bottom scan
‰Progressive DCT
The image is encoded in multiple scans
Hierarchical
Encoding at multiple resolutions
Lossless
Exact reproduction

11. JPEG BLOCK DIAGRAMS

12. JPEG ENCODER
Three main blocks
 ‰Forward Discrete Cosine Transform (FDCT)
‰Quantizer
 ‰Entropy Encoder
‰Essentially the sequential JPEG encoder ‰
Main component of progressive, lossless and hierarchical encoders ‰
For gray level and color images

13. JPEG DECODER
The same steps in reverse order
The binary sequences are converted to symbol sequences using the
Huffman tables
‰F’(u,v) = Fq(u,v)Q(u,v)
‰Inverse DCT

14. HIERARCHICAL JPEG
Produces a set of images at multiple resolutions ‰
Begins with small images and continues with larger images
The reduced image is scaled-up to the next resolution and used as
predictor for the higher resolution image

15. MPEG
• The Moving Picture Coding Experts Group (MPEG) is a working group for
the development of standards for compression, decompression, processing,
and coded representation of moving pictures and audio
• ‰MPEG groups are open and have attracted large participation

16. MPEG FEATURES
• Random access
• ‰Fast forward / reverse searches
• ‰Reverse playback
• ‰Audio – visual synchronization
• ‰Robustness to errors
• ‰Auditability
• ‰Cost trade-off

17. MPEG ENCODING
• I frames are JPEG compressed
• ‰P, B frames are encoded in terms of future or previous frames
• ‰Motion vectors are estimated and differences between predicted and actual
blocks are computed ‰
These error terms are DCT encoded
 ‰Entropy encoding produces a compact binary code
‰Special cases: static and intracoded blocks

18. px64
• H.261 (px64) was designed for data rates which are multiples of 64kbit/s
and is sometimes called px64kbit/s (p is in the range 1-30).
• These data rates suit ISDN lines , for which this video code was designed
• Intended for videophone and video conferencing systems
• Compressed data stream.
p*64 Kbits/s , p = (1,……,30)
• Video compression for video conferences
Compression in real-time
Targeted to ISDN

19. H.261 (Px64) RESOLUTION
• Common Intermediate Format (CIF)
• Quarter CIF (QCIF)
CIF QCIF
Lines/frame pixels/line Lines/frame pixels/line
Luminance (Y) 288 352 144 176
Chrominance (Cb) 144 176 72 88
Chrominance (Cr) 144 176 72 88