File formats for images on the web

CSC 1040 - Computing with Images 1

File formats for images on the web

CSC 1040


What’s a picture?

• We have seen that programs represent pictures as grids of picture elements or pixels

Stephanos with his eraser collection


Pixel encodings

Bitmap

1 bit

Grayscale

8 bits

RGB Color 3 colors: red, green, blue

8 bits/color

24 bits


How much can we encode in 8 bits?

• Let’s walk it through.– If we have one bit, we can represent two patterns:

0 and 1.– If we have two bits, we can represent four patterns:

00, 01, 10, and 11.– If we have three bits, we can represent eight patterns: 000, 001,

010, 011, 100, 101, 110, 111


8 bits = 1 byte

• General rule: In n bits, we can have 2n patterns– In 8 bits, we can have 28 patterns, or 256– If we make one pattern 0, then the highest value

we can represent is 28-1, or 255


RGB

• In RGB, each color has three component colors:– Amount of redness– Amount of greenness– Amount of blueness

• In most computer-based models of RGB, a single byte (8 bits) is used for each– So a complete RGB color is

24 bits, 8 bits of each


Position: (12,9)

x = 12

y = 9


Color:(108,86,142) Position: (12,9)

x = 12

y = 9 red=108 green=86 blue=142


Encoding RGB

• Each component color (red, green, and blue) is encoded as a single byte

• Colors go from (0,0,0) to (255,255,255)– If all three components are the

same, the color is in greyscale• (50,50,50) at (2,2)

– (0,0,0) (at position (1,2) in example) is black

– (255,255,255) is white


Is that enough?

• We’re representing color in 24 (3 * 8) bits.– That’s 16,777,216 (224) possible colors– Our eye can discern millions of colors, so it’s probably

pretty close

• Some graphics systems support 32 bits per pixel– May be more pixels for color, or an additional 8 bits to

represent 256 levels of translucence


Size of images

320 x 240image

640 x 480image

1024 x 768monitor

24 bit color 1,843,200

bytes

7,372,800 bytes

18,874,368

bytes

32 bit color 2,457,600

bytes

9,830,400 bytes

25,165,824 bytes


Compression: to lose or not to lose?

• Goal: reduce redundancy– Send the same information using fewer bits

• Originally developed for fax transmission– Send high quality documents in short calls

• Two basic strategies:– Lossless: can reconstruct exactly– Lossy: can’t reconstruct, but looks the same


Bitmap vs. grayscale

http://www.nga.gov/exhibitions/2007/snapshot/snapshotinfo_fs.shtm


Human Vision

• Closely spaced dots appear solid– But irregularities in diagonal lines can stand out

• High frame rates produce apparent motion– Smooth motion requires about 24 frames/sec

• Visual acuity varies markedly across features– Discontinuities easily seen, absolutes less crucial


Do these colors look the same as ...


... as these?


Not quite


12345678910111213141516171819202122232425262728293031

Opportunity:• Large regions of a single color are common

Approach:•Record # of consecutive pixels for each color

An example of lossless encoding

Uncompressed000000000000000000000111111111111100000000000001111111111111111111111

RLERow 1, 21:0,13:1;13:0;22:1

LZW, etc. use algorithms in addition to RLE

01010101010101010101

Run length encoding


Palette selection

• Opportunity:– No picture uses all 16 million colors– Human eye does not see small differences

• Approach: – Select a palette of 256 colors– Indicate which palette entry to use for each

pixel– Look up each color in the palette


JPEGJoint Photographic Experts Group

Opportunity:• Eye sees sharp lines better than subtle shading

Approach:• Retain detail only for the most important parts

• Accomplished with Discrete Cosine Transform

• Allows user-selectable fidelity

• Results: Typical compression 20:1


JPEGJoint Photographic Experts Group

• Preferred format for scanned photographic images for use over the internet or Web. Not meant for printing.

• Not good for images with a lot of solid color, vector drawings, type, or line art or images with “Web-safe” colors.

• JPEG compression is lossy! Save and archive the original before converting to JPEG.


In Photoshop, when you Save as… a JPEG file, you can choose the level of compression and, therefore, the size and quality of the file.


GIFGraphics Interchange Format

Palette selection, then lossless compressionOpportunity:

• Common colors are sent more oftenApproach:

• Use fewer bits to represent common colors. Example:1 Blue 75% 75x1= 75 75x2=15001 White 20% 20x2= 40 20x2= 40001 Red 5% 5x3= 15 5x2= 10 130 200


GIFGraphics Interchange Format

• Industry standard graphic format for on-screen viewing through the Internet and Web.

• Not meant to be used for printing.• The best format for all images except scanned

photographic images (use JPEG for these).• GIF supports lossless LZW compression.


Photoshop Save for Web

details for this option

Toolssettings


JPEG cautions:

• Images with hard edges, high contrasts, angular areas, and text suffer from JPEG compression.

• Scanned “natural” photographs do not lose much, especially at High or Maximum quality.

• Only save finished images as JPEGs, every time you open and save again, even if you don’t edit, you lose quality.

• Always keep the original non-JPEG version (the native .psd format).

So why use JPEG?

• It is the best format for photographic images on the Web.• Its compression ability is very great.


GIF cautions:

• Restricted number of colors – not suitable for photographs• Very good compression for line drawings (uniform color

areas)• Only save finished images as GIFs• Always keep the original non-GIF version (the native .psd

format).

So why use GIF?

• It is a good format for images that have few colors and is widely available

• Can do lossless compression• Its compression ability is great, especially for images with few

colors• Can also be used for animation


http://en.wikipedia.org/wiki/Graphics_file_format_summary

Adobe PhotoshopPhotoshop can save files in many file formats:

.psd — Photoshop Document (“native” format)

.pdf — Photoshop Portable Document Format

.eps — Photoshop Encapsulated PostScript

.tiff — Tagged Image File Format

.gif — Compuserve Graphic Interchange Format

.jpg — JPEG, Joint Photographic Experts Group

.bmp — Windows Bitmap

.png — Portable Network Graphicetc., etc.

A summary of Graphics file formats and uses can be found in:

printing

web


EPSEncapsulated PostScript

• Preferred file format for importing into page layout programs such as QuarkXPress, PageMaker, InDesign, XPress, etc. for subsequent printing.

• An object-oriented format.• Will only print to a postscript printer.• Uses lossy JPEG compression.• Only save your file as EPS if you need to import it

into a page layout program.


TIFFTagged Image File Format

• Widely used cross platform file format also designed for printing.

• A bitmap image format.• TIFF supports lossless LZW compression which also

makes it a good archive format for Photoshop documents.


PDFPortable Document Format

• Developed to transfer and read documents without having to print them—the “paperless office.”

• Cross platform format that can be read with the free download Adobe Acrobat Reader.

• Can represent both vector and bitmap graphics.• Can also contain electronic document search and navigation features

as well as hypertext links.• Can be created from almost any application, but the user cannot edit or

modify the file except with Adobe Acrobat (or other software).• Document formatting, fonts, colors, etc. are maintained and appear

identical across platforms.• Excellent in the “prepress” process — can be sent to the printer, but

can also be placed in other documents.


Some of the slides in this presentation were adapted from:

http://www.entomology.umn.edu/museum/links/coursefiles/ENT 5051/PowerPoint presentations/Lec 7-File formats.ppt

and

http://www.umiacs.umd.edu/~jimmylin/LBSC690-2004-Fall/Week10.ppt

http://www.entomology.umn.edu/museum/links/coursefiles/ENT%205051/PowerPoint%20presentations/Lec%207-File%20formats.ppt

http://www.umiacs.umd.edu/~jimmylin/LBSC690-2004-Fall/Week10.ppt

File formats for images on the web

Documents

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