Image File Formats - JPG, TIF, PNG, GIF
Which to use?
The three most common image file formats, the most important for printing, scanning and internet use, are JPG, TIF, and GIF.
- Digital cameras normally use JPG files - compresses smaller, to not fill the memory cards, but JPG uses lossy compression, and image quality can suffer.
- TIF is lossless (including LZW compression), and is considered the high quality format for commerical work. Internet browsers and web pages cannot show TIF files, nor can they show RAW files from DSLR cameras.
- GIF is lossless too, but is the older format, the first one for images on the internet. GIF is an indexed file (256 colors maximum), NOT best for photos today, still good for web graphics, but PNG is its replacement today.
- JPG is 24 bit color only. GIF is 8 bit indexed only. PNG and TIF can be anything, whatever you have, 1 bit to 48 bit color.
All photo editor programs like Adobe Photoshop or Adobe Elements support these file formats, which will generally support and store images in the following color modes:
| Color data mode Bits per pixel |
|
| TIF | RGB - 24 bits or 48 bits.
24 bits is called 8 bit color (RGB 256x256x256 = 16.7 million colors), or 48 bits is called 16 bit color (65536x65536x65536 = trillions of colors conceptually, but probably not practically.) Grayscale - 8 or 16 bits, Indexed color - 1 to 8 bits, Line Art (bilevel)- 1 bit For TIF files, most programs allow either no compression or LZW compression (lossless, but is less effective for 24 bit color 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). |
| PNG | RGB - 24 or 48 bits (called 8 bit or 16 bit "color"), Grayscale - 8 or 16 bits, Indexed color - 1 to 8 bits, Line Art (bilevel) - 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 verstile and royalty free, back when the LZW patent was disputed. |
| JPG | RGB - 24 bits (8 bit color), 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. |
| GIF | Indexed color - 1 to 8 bits (8 bit indexes, limiting to only 256 colors maximum.) GIF uses lossless LZW compression, effective on indexed color. GIF files contain no dpi information for printing purposes. |
Note that if your image size is say 3000x2000 pixels, then this is 3000x2000 = 6 million pixels (6 megapixels). Assuming this 6 megapixel image data is RGB color and 24 bits (or 3 bytes per pixel of RGB color information), then the size of this image data is 6 million x 3 bytes RGB = 18 million bytes. That is simply how large your image data is (see more). Then file compression like JPG or LZW can make the file smaller, but when you open the image in computer memory for use, the JPG may not still have the same image quality, but it is always still 3000x2000 pixels and 18 million bytes. This is simply how large your 6 megapixel RGB image data is (megapixels x 3 bytes per pixel).
Best file types for these general purposes:
| Photographic Images | Graphics, including
Logos or Line art |
|
| Properties | Photos are continuous tones, 24 bit color or 8 bit Gray, no text, few lines and edges | Graphics are often solid colors, up to 256 colors, with text or lines and sharp edges |
| For Unquestionable 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 be decent. | TIF LZW or GIF or PNG (graphics/logos without gradients normally permit indexed color of 2 to 16 colors for smallest file size) |
| Maximum Compatibility
(PC, Mac, Unix) |
TIF or JPG | TIF or GIF |
| Worst Choice | 256 color GIF is very limited color, and is a larger file than 24 bit JPG | JPG compression adds artifacts, smears text and lines and edges |
These are not the only choices, but they are good and reasonable choices.
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, 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, TIF file format is the undisputed leader when best quality is required (when less than maximum quality is not a consideration). So TIF is very common in commercial or professional printing environments. High Quality JPG can be pretty good too, but don't 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. Adobe RGB color space may be OK for your home printer and profiles, but if you send your pictures out to be printed, the mass market printing labs normally only accept JPG files and only process sRGB color space.
Difference in photo and graphics images
Photo images have continuous tones, meaning that adjacent pixels often have very similar colors, for example, a blue sky might have many shades of blue in it. Normally this is 24 bit RGB color, or 8 bit grayscale, and a typical color photo may contain perhaps 100,000 colors, out of the possible set of 16 million colors in 24 bit RGB color.
Graphic images are normally not continuous tone (gradients are possible in graphics, but are not seen very often). Graphics are drawings, not photos, and they use relatively few colors, perhaps less than 16 colors in the entire image. In a color graphic cartoon, the entire sky will be only one shade of blue where a photo might have dozens of shades. Or a map for example is graphics, maybe 4 or 5 map colors plus 2 or 3 colors of text, plus blue water and white paper, often less than 16 colors overall. These few colors are well suited for Indexed Color. Normally the edges in graphics do not use anti-alaising - which would add numerous shades (graphics use high resolution to smooth jaggies instead). Scanners have three modes to create the image: color (for all color work), grayscale (like B&W photos), and lineart. Line art is a special case, only two colors (black or white, with no gray), for example clip art, fax, and of course text. However low resolution line art (like cartoons on the web) is often better as grayscale, to add aliasing to hide the jaggies.
JPG files are very small files for continuous tone photo images, but JPG is poor for graphics. JPG requires 24 bit color or 8 bit grayscale, and the JPG artifacts are most noticeable in the hard edges of graphics or text. GIF files (and other indexed color files) are good for graphics, but are poor for photos (too few colors possible). However, graphics are normally not 24 bit color anyway. Formats like TIF and PNG can be used either way, 24 bit or indexed color - these file types have different internal modes to accommodate either type optimally.
Note again that if you are scanning text based documents (black text on white paper, two colors) in the optimum line art mode for this, but then save them as JPG, they necessarily will become 8 bit grayscale images, and those JPG files will be much larger than if you had known to select TIF with LZW compression. The opposite is of course true (about file size) if scanning 24 bit color photos.
The three most common image file formats, the most important for general purposes today, are TIF, JPG and GIF. PNG format is excellent too. These are not the only choices of course, but they are good and reasonable choices for general purposes. Newer formats like JPG2000 in Adobe programs never acquired popular usage, and are not supported by web browsers, and so are not the most compatible choice.
RAW files are popular, from most DSLR cameras. When we take any digital picture, the camera has a RAW sensor, but processes and outputs the image as a JPG file. But often we can choose to output the original RAW image instead. We cannot view or use that RAW file any way other than to process it in computer software and then output a final TIF or JPG image. Postponing this processing offers a few serious advantages... we can bypass all JPG artifacts until the one final output Save. We can defer White Balance decisions until later, when we can see the image first, and judge any trial results. The 12 bit RAW file offers greater range for any of our adjustments.
The Next button will browse through the descriptions on the next pages, or you can use these shortcut links directly:
| PNG Format | TIF Format | JPG Format | GIF Format |
Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.
