Russell Taylor Week 3

Image File Formats - TIF, JPG, PNG, GIF - which to use? The three most common and important image file formats for for printing, scanning and internet use, are TIF, JPG and GIF. However, TIF cannot be used yet in native internet browsers (but can be “shelled-out” to helper programs). All editor programs like Adobe Photoshop or Adobe Elements support these file formats, which will generally support and store images in the following colour modes with RGB (Red Green Blue) colours:

Colour Modes – TIFF File typeColour data mode - Bits per pixel TIFF (Tagged Image File Format) RGB - 24 or 48 bits, Grayscale - 8 or 16 bits, Indexed colour - 1 to 8 bits, Line Art (bilevel compression) - 1 bit for TIF files Most programs allow either no compression or LZW compression (lossless, but is less effective for 24 bit colour images). Adobe Photoshop also provides JPG or ZIP compression too (but which greatly reduces third party compatibility of TIF files). "Document programs" allow ITCC G3 or G4 compression for 1 bit text (Fax is G3 or G4 TIF files), which is lossless and tremendously effective (small).

Colour Modes - PNG File typeColour data mode - Bits per pixel PNG (Portable Network Graphic), pronounced ‘Ping’ RGB - 24 or 48 bits, Grayscale - 8 or 16 bits, Indexed color - 1 to 8 bits, Line Art (bilevel compression) - 1 bit PNG uses ZIP compression which is lossless, and slightly more effective than LZW (slightly smaller files). PNG is a newer format, designed to be both versatile and royalty free, back when the LZW patent was disputed.

Colour Modes - JPG File TypeColour data mode - Bits per pixel JPG/JPEG (Joint Photo-graphic Experts Group) RGB - 24 bits, Grayscale - 8 bits JPEG always uses lossy JPG compression, but its degree is selectable, for higher quality and larger files, or lower quality and smaller files.

Colour Modes - GIF File Type GIF (Graphic Image File) Indexed colour - 1 to 8 bits GIF uses lossless LZW compression, effective on indexed colour. GIF files contain no dots per inch (dpi) information for printing purposes.

Memory Cost of Images E.g Image size is 3000 x 2000 pixels, this equals 6 million pixels (6 megapixels). If this 6 megapixel image data is RGB colour (if 24 bits, or 3 bytes per pixel of RGB colour information), then the size of this image data is 6 million x 3 bytes RGB = 18 million bytes. File compression like JPG or LZW can make the file smaller, but when you open the image on a PC for use, the JPG may not still have the same image quality due to lossy compression, but it is always still 3000 x2000 pixels and 18 million bytes.

Memory Cost of Images (2) Large images consume large memory and make our computers struggle. Memory cost for an image is computed from the image size. For a 6x4 inch image at 150 dots per inch (dpi), the image size is calculated as: (6 inches × 150 dpi) × (4 inches × 150 dpi) = 900 × 600 pixels 900 × 600 pixels is 900 × 600 = 540,000 pixels. The memory cost for this RGB colour image is: 900 × 600 × 3 = 1.6 million bytes. The last "× 3" is for 3 bytes of RGB colour information per pixel for 24 bit colour (3 RGB values per pixel, one 8-bit byte for each RGB value, which totals 24 bit colour).

Appropriate File Types PropertiesPhotographic Images Graphics, including Logos or Line art Image Types: Photos are continuous tones, 24 bit colour or 8 bit Gray, no text, few lines and edges Graphics are often solid colours, up to 256 colours, with text or lines and sharp edges For Very Best Quality TIF or PNG (lossless compression and no JPG artifacts) PNG or TIF (lossless compression, and no JPG artifacts) Smallest File Size JPG with a higher Quality factor can still be a small file size. TIF LZW or GIF or PNG (graphics/logos without gradients normally permit indexed colour of 2 to 16 colours for smallest file size) Max.Compatibility (PC, Mac, Unix) TIFF or JPGTIFF or GIF Worst Choice 256 colour GIF is very limited, and is a larger file than 24 bit JPG JPG compression adds artefacts, smears text and lines and edges

Image Application Web pages require JPG or GIF or PNG image types, because that is all that browsers can show. On the web, JPG is the best choice (smallest file, with quality being less important than size) for photo images, and GIF is common for graphic images. GIF was designed for modems by CompuServe in 1987, for earliest 8 bit video, and so GIF contains no printing dpi information, and is out of date for 24 bit photos now, but GIF still works quite well for video graphics on the internet. Other than the web, the TIF file format is the undisputed leader when best quality is required so TIF is very common in commercial or professional printing environments. High Quality JPG are good too, but do not ruin them by making them too small. If the goal is high quality, then only consider making JPG large instead, and plan your work so you can only save them one or two times (as quality is lost every time you edit/save them).