What makes the What makes the differences…differences…

The most important file types when The most important file types when saving images, are jpeg (or jpg), tif and saving images, are jpeg (or jpg), tif and gif.gif.

Each has advantages and disadvantagesEach has advantages and disadvantages Each has an algorithm used to compress Each has an algorithm used to compress

file size, which affects the image file size, which affects the image integrity.integrity.

File size determines quality of image and File size determines quality of image and uses uses

Example: A picture is converted to a digital file by compression that sees it as an array of dots of specific color and brightness for each dot. If the picture contains an area of the same color, it can be compressed without loss by saying "200 red dots" instead of "red dot, red dot, ...(197 more times)..., red dot."

Digital data can be compressed using algorithms that reduce file sizes: : needed to store itthe bandwidth needed to stream itno loss of information in the original fileTwo major algorithms – Lossy and LosslessUsing lossy compression to make smaller sized files, there are some “down” sides to considerOne major cost is image quality.

Files or data streams often contain more than needed information for a particular purpose Developing lossy compression techniques to match human perception as closely as possible is a complex task. Sometimes the ideal is a file providing exactly the same perception as the original, with as much digital information as possible removedOther times, perceptible loss of quality is a valid trade-off for the reduced data size

For example, a picture may have more detail than the eye can distinguish when reproduced at the largest size intended

Or, an audio file does not need a lot of fine detail during a very loud passage

Looking at an example of this…

This photo of a cat has a compression rate decreasing from left to right.

Lossy compression techniques were made to closely match human perception for a complex task. Sometimes the ideal is a file that provides exactly the same perception as the original, with as much digital information as possible removed; other times, perceptible loss of quality is considered a valid trade off for reduced data.A photo of a cat with the compression rate decreasing from left to right.valid trade-off for the reduced data

"JPEG" - acronym for the Joint Photographic Experts Group which created the standard.

Jpg file options are used when small file size is more important than maximum image quality; for example, Web pages, email, camera memory cards. Usually this file type is good enough, if comparison is not necessary.

Lossy algorithms: common in digital photography

Lossy compression makes smaller sized files, “down” side to consider - cost in image quality

Degree of compression can be adjusted: allows a selectable tradeoff between storage size and image quality

JPEG typically achieves 10:1 compression with little perceptible loss in image quality

Always choose a larger JPEG file size Do Not edit and resave a JPG repeatedly

each time a JPG is edited and resaved it loses a little more of its quality

pixels begin to change colors, to colors they were not to begin with

these pixels are called “artifacts” (pixels that become colors they should not be in an image)

JPEG may not be right for line drawings and other textual or iconic graphics, where sharp contrasts between adjacent pixels can cause noticeable artifacts. These images may be better saved in a lossless graphics format (TIFF, GIF, PNG) format. (The JPEG standard actually includes a lossless coding mode, but that mode is not supported in most products.)

Quality Sizes (byt)

Compression Ration

Comments

Highest quality (Q = 100)

83,261 2.6:1 Extremely minor artifacts

High quality (Q = 50)

15,138 15:1 Initial signs of sub-image artifacts

Medium quality (Q = 25)

9,553 23:1 Stronger artifacts; loss of high frequency information

Low quality (Q = 10)

4,787 46:1 Severe high frequency loss; artifacts on subimage boundaries ("macroblocking") are obvious

Lowest quality (Q = 1)

1,523 144:1 Extreme loss of color and detail; the leaves are nearly unrecognizable

Quality Sizes (byt)

Compression Ration

Comments

Highest quality (Q = 100)

83,261 2.6:1 Extremely minor artifacts

High quality (Q = 50)

15,138 15:1 Initial signs of sub-image artifacts

Medium quality (Q = 25)

9,553 23:1 Stronger artifacts; loss of high frequency information

Low quality (Q = 10)

4,787 46:1 Severe high frequency loss; artifacts on subimage boundaries ("macroblocking") are obvious

Lowest quality (Q = 1)

1,523 144:1 Extreme loss of color and detail; the leaves are nearly unrecognizable

Medium quality photo uses only 4.3% of storage space required for uncompressed images, with little loss of detail or visible artifacts being noticeable.

Once a certain threshold of compression is passed, compressed images show increasingly visible defects

A particular limitation of JPEG in this regard is its non-overlapped 8×8 block transform structure

More modern JPEG 2000 or JPEG XR exhibit more supple degradation of quality as bit usage decreases –

by using transforms with a larger spatial extent for the lower frequency coefficients

by using overlapping transform basis functions

Medium quality photo uses only 4.3% of storage space required for uncompressed images, with little loss of detail or visible artifacts being noticeable.

Once a certain threshold of compression is passed, compressed images show increasingly visible defects

A particular limitation of JPEG in this regard is its non-overlapped 8×8 block transform structure

More modern JPEG 2000 or JPEG XR exhibit more supple degradation of quality as bit usage decreases –

by using transforms with a larger spatial extent for the lower frequency coefficients

by using overlapping transform basis functions

“GIF" - originally designed for video use at “dial-up modem” speeds by CompuServe

Limited for 24-bit color photos GIF is very good (maybe best)

for Web graphics (i.e. using a limited

amount of colors), but should not be used for color photos

Lossless compression algorithm, but is always Indexed (using a maximum256 colors)

For graphics of only 8 to 16 colors, GIFS can be a smaller file size than JPG, with more clear pure color.

The GIF file is 18 times larger in file size than the JPG file, but the JPG still looks better. See the purple pixels that the GIF mixes in the face to try to get the correct average color in this region.

TIF can be used for about anything using 1 bit to 48 bit color, RGB, CMYK, and/or Indexed color.

Most “special” file types (like RAW camera file types) are TTG, using unique proprietary data tags,

They are incompatible without special software

• Lossy algorithm: supporting the highest quality format for commercial work

• Not actually “high quality” per se - just does not suffer from the JPG artifact problem, detracting from an image

• TIF images should not be used for Web pages; they are too large for easily transferring to browsers for displays; JPG, GIF and PNG images are far better for this reason.

• TIFF (which stands for Tagged Image File Format) is considered the standard photographic file format, because it is a highly flexible format that uses a lossless compression algorithm, so there’s no image degradation during compression.

• TIFF files may be edited and re-saved without losing any image quality (unlike JPEG files)

• When using third party image processor software (like Adobe’s Photoshop), access to individual layers of an edited and saved image is retained, so they can be revisited in the future.

The PNG format was developed as a replacement for the GIF format when it appeared GIF images would be subject to a royalty fee.

PNG graphics have a better compression rate than GIF images, resulting in smaller file sizes than the same file saved as a GIF

PNG files offer alpha transparency as well as animation.

BUT - PNG images, like GIFs, are not well suited to photographs

It is possible to get around the banding issue that affects photographs saved as GIF files using true colors, but this can result in very large images

• A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap.

• A bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).

• A bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).

• Raster-based (bitmap) images are composed of millions of pixels

• A raster image displays each individual pixel

• Bitmap images have very large file sizes, due to this pixel storage and display method

• Because of their file size, they are not used in Web sites.