Image File Formats

CSCI N351

Andy Harris

Indiana University / Purdue University - Indianapolis

Image Questions

• Does a grayscale GIF image take up less space than a color image?
• How about a black and white (2 colors only) image?
• Does storing an image in JPG or PNG change the answer to this question?
• Can an animated gif have more than 256 colors?
• Can an animated gif have more than one transparent color?

More Image Questions

• Can PNG handle animations?
• Why is JPG sometimes called JFIF?
• How does the variable compression of a JPG work?
• Which is the best format for an image with text?

GIF Images

• Graphic Interchange Format
• Standard format
• One of oldest formats
• flexible and well-known
• falling from favor

GIF Bit Depth

• 256 element palette of 24-bit colors
• Only 256 colors in image, but each is defined in 24 bits
• 8 bits per pixel, which refer to color numbers
• 'paint by number' approach
• Non-registered colors are dithered

GIFS and Transparency

• One color can be stored as transparent
• Replaced with background on rendering
• no true alpha

GIF Animation

• Multi-frame images are animated
• Each frame can have an independant palette
• Control blocks determine time between frames
• Automated in The Gimp, Gif Construction Set
• Animation can be optimized (store only changes in frames)

GIF Compression

• LZW (Limpel-Ziv-Welch) Compression
• Based on RLE (Run-Length Encoding)
• LZW demo
• Patent owned by Unisys
• Unisys requires a licensing fee for any software using the LZW codec
• lossless compression

GIF Strengths

• solid color images
• Images with irregular boundaries
• Images needing transparency
• animation

GIF Weaknesses

• limited color depth
• no true alpha
• animated images can be quite large
• legal questions regarding LZW

The JPEG (JPG) scheme

• Joint Photographic Experts Group
• Useful for photos
• No transparency
• Loses data

JPG Background

• Not a file format at all
• Compression algorithm
• Used by Tiff 6.0, Mac Pict
• Version usually called JPG is actually JFIF
• (JPEG File InterChange Format)
• placed in public domain by C-Cube Microsystems

JPG Bit Depth

• 24 bit true color
• 8 bits each Red, Green, Blue
• No Alpha, No Transparency
• No animation or multiple images

JPG Compression Algorithm

• Lossy Compression
• Variable Compression Rate
• Use psychological tricks to emphasise important visual information
• Drop less important info
• "perceptually faithful"

JPG compression pt 1 - YCbCr

• Convert to YCbCr color scheme
• Y = brightness
• Cb = blue chrominance
• Cr = red chrominance
• Human eye responds more to brightness than anything else
• Chrominance values are downsampled (1 value for each 2x2 block)
• Results in 50% of savings in file size

JPG compression pt 2 - DCT

• Discrete Cosine Transform
• Human eye is more sensitive to gradual changes in brightness than sudden ones
• DCT throws out less important data
• Groups image into 8x8 blocks

JPG compression pt 3 - Quantization

• Quantization
• divides each 8x8 block by a Q factor
• This is where variable compression rate occurs
• Quantization is where most visible information loss occurs

JPG compression pt 4 - Huffman Encoding

• Applies another encoding scheme (Huffman run-length encoding
• Provides one more level of compression

JPG Strengths

• Completely free algorithm
• Excellent compression
• High color depth
• Works well with photographic data
• Variable compression to meet exact needs
• Visual loss of detail can be unnoticable

JPG Weaknesses

• No alpha or transparency
• Some image loss inevitable
• No support for animation
• Does not handle sharp edges well
• Text reproduces very badly
• Generates recognizable artifacts
• Repeated storage can degrade quality

PNG (Portable Network Graphics)

• Devised as a replacement for GIF
• Free and unencumbered algorithm (no LZW)
• Can use 24 bit or color table to save space
• Improvement over GIF design

PNG characteristics

• True Alpha support
• No animation support
• Color Depth
• TrueColor
• Full 8-bit Alpha channel
• 32-bit color

PNG Compression Algorithm

• Lossless algorithm
• Works well with solid colors (like GIF)
• No problems with proprietary algorithm

PNG Strengths

• Good replacement for GIF
• Better color scheme than GIF
• Few legal problems
• Best support for transparency

PNG Weaknesses

• No animation (although a version called MBG is in the works)
• Support is increasing but not completely universal
• Cannot compress some images as well as lossy algorithms (JPG)

Native Formats

• General Description
• File formats native to image editors
• XCF for Gimp
• PSD for Photoshop

Native Format Strengths

• Contain complete image data
• Include layers
• Include channel and path data
• Easiest for revising image
• No image loss

NativeFormat Weaknesses

• Very large files
• Not supported by browsers
• Ancillary information (layers, channels, paths) not needed by user