Graphics File Formats
You will find a very wide variety of graphics formats in use. Each has a specific purpose and its own strengths and weaknesses. The Web has added complexity to file formats, with the need for small file sizes and the ability to include draw- and paint-style graphics and fancy formatting for fonts and graphic effects, including transparency and animation.
- Draw, Vector, or Object-Based Graphics
- Scalable without loss of resolution. Mathematical descriptions of the shapes, points, lines, curves, fills, strokes, and other attributes. Can contain text. Can usually contain bitmaps also.
- Paint, Raster, or Bitmapped Graphics
- Fixed-resolution rendering of each dot in the image. Scaling can produce “jaggies” (obvious or uneven dots). Editing the image permanently modifies it. Photos and other scanned images are bitmapped. Some programs can trace a bitmapped image to produce a vector graphic, with varying degrees of success.
Below is a list of some popular programs to create graphics. There are many more available.
This section needs revision/updating.
- Drawing Programs
- Macromedia FreeHand (Win, Mac)
- Adobe Illustrator (Win, Mac)
- Corel Draw (Win, Mac)
- and many others
- Painting Programs
- Jasc Paint Shop Pro (Win, non-Mac)
- Adobe Photoshop (Win, Mac)
- and many others
- alpha channel
- An image channel that is used for special effects; usually for transparency or masking.
- To reduce jaggedness or roughness in an image by smoothing (averaging) adjoining pixels. Generally used to smooth diagonal lines and curves, especially for text in an image.
- aspect ratio
- The ratio of the horizontal and vertical size of an image. If the aspect ratio of an image changes in one direction but not the other, the proportions of the image are no longer true to the original.
- color channel
- Any color image can be divided into channels, each of which has only percentages of one of the primary colors, such as red, green, or blue for RGB colors, or cyan, magenta, yellow, and black for four-color CMYK printing.
- Cutting or masking the image to show only the desired portion, usually to fit a certain space or show off the most interesting areas.
- To capture extra data and average it (or apply other statistical methods to it) to reduce error in the sampling process and maximize detail.
- A bitmapped (raster) image has only a limited number of points and so diagonal lines and curves often are noticeably dotted or jagged.
- Separating a solid image, such as a photo, into a pattern of dots for printing, so that the ink does not run together. A halftone dot screen is placed over the original to produce a new, halftoned duplicate of the original.
- To create new data points by taking the average between two or more nearby points.
- A specific image, often including text, that identifies a company, group, or brand.
- A form of compression that does not reduce or lose data (image) quality.
- A form of compression that does reduce or lose some data (image) quality.
- Blocking out portions of an image to provide special color or transparency or animation effects for the unblocked parts of the image.
- progressive rendering
- Showing an image in stages, with a low quality image first, which is gradually filled in by a successively higher quality image.
- To reduce or enlarge, generally by averaging.
- Providing an image with transparency so the background shows through. Transparency is unavailable in some formats; on/off in 1-bit (completely solid or transparent) in other formats; or using varying degrees of transparency from solid through translucent to totally transparent in still other formats.
Standard Graphics File Formats
|Standard File Formats|
|GIF||Graphics Interchange File.||Web Bitmap. Note: limited to 256 or fewer colors. Can set transparency on/off for one color.|
|JPG, JPEG||Joint Photographic Experts Group.||Photo/Bitmap. Web/Publish/Draw/Paint. A compressed, “lossy” format. If your software offers a choice of image quality, choose 100%. If your software offers a choice of progressive rendering, choose Yes. No transparency. Note: solid colors may appear blotchy on-screen.|
|PNG||Portable Network Graphics.||Web Vector/Bitmap format. Some Web/Publish/Draw. New “lossless” format, supported by Macromedia FreeHand 8, Microsoft Internet Explorer 4. May be supported by Netscape Navigator 4 or 5. Offers lossless compression, transparency, and progressive rendering.|
|TIF, TIFF||Tagged Image File Format.||Bitmap (Paint). Publish/Draw/Paint.|
|EPS||Encapsulated PostScript (preferrably) with TIFF preview.||Drawing (Vectors). Publish/Draw. EPS files can contain vector drawings as well as bitmaps. They can even contain editable text. EPS is used by professional publishing and drawing programs. Your local print shop, if they work from clients’ data disks, will prefer EPS or TIFF files.|
|SVG||Scalable Vector Graphics.||Proposed Web Vector format. When the SVG format becomes available, there will be the usual lag time before browsers and other application programs support it. The features will make it tremendously powerful and advantageous for everyone.|
Spot vs. Process
- Spot Color
- A pre-mixed solid color. Spot color is used in jobs that use tints and shades of one or two colors, often including Black, or when there is small text in a specific color, or when a company needs its logo or corporate color scheme to be used exactly. Printing using one or two spot colors can save you money.
- Process Color
- A set of primary color inks, from which all other colors may be printed. Full-color photos require process color printing. One or two spot colors may sometimes be used together with process printing, for the special uses listed at spot color.
Knockout vs. Overprint
- To cause the overlapping portion of one item to print, while the item below it is removed (knocked out).
- To cause the overlapping portions of two or more items to print on top of each other. To print one ink on top of another, causing the bottom ink to shine through the top ink.
Generally, most drawing programs allow you separate control over properties of lines (strokes) and fills, and sometimes over individual colors or inks, including the knockout or overprint status of elements.
- A process-color method for printing using a set of four primary color inks: cyan (blue), magenta (red/pink), yellow, and black, from which all other colors may be printed. Also called four-color or just process color printing.
- A model for on-screen display using three primary colors of light: red, green, and blue, from which all other colors may be shown.
- HLS or HSB
- Hue, Lightness, and Saturation; Hue, Saturation, and Brightness.
- A process-color method for printing using a set of six primary color inks: orange, green, cyan (blue), magenta (red/pink), yellow, and black, from which all other colors may be printed. A new method developed by Pantone to increase the gamut (range) of printable colors.
A short list to get the best quality scans.
- ≥ 32 bit color
- This can mean one of two things. At 32 bits, there is room for 24 bit color plus an 8 bit alpha channel reserved for transparency, masking, or special effects. Or, to downsample by capturing extra data and then average it in order to reduce error and maximize detail. Many scanners scan at 32 or more bits per color and then reduce to 24 bit color in order to ensure accurate color.
- 24 bit color (16.7 million colors)
- True photo-quality color. Recommended for color scans.
- 16 bit color (65536 colors)
- Near photo-quality color.
- 8 bit color (256 colors)
- Clip art quality color.
- 8 bit grayscale (256 grays)
- True black & white photo quality. Recommended for grayscale scans. At low resolution, can show higher detail than 1 bit scans, but may be problematic for conversion of text or line art.
- 1 bit monochrome (black or white only)
- Recommended for line art (no grays) or black-and-white text scans.
Resolution (Horizontal and Vertical)
- Rules of Thumb:
- Scanning resolution is determined most by the resolution at which you wish to output (print out or view) the image.
- If you need to convert the text into editable text (by OCR) or to convert a bitmap to vector graphics, then resolution is determined by the color depth and type of art. See the table below.
- Either: Scan at twice the resolution of the printer on which the image will be printed.
- Or: Scan at twice the value of the desired halftone line screen (if known) that will be used when the piece is printed.
- Avoid rotating, resizing, or cropping an image in your page layout program. Save a new copy of the image to a new file using Save As, then transform it in Photoshop or PaintShop Pro or a similar program, then save the modified image. Reason: Saves processing, displaying, and printing time.
- Formula to find scanning resolution:
- ( final-image-height / original-image-height ) * screen-ruling * 2 = desired-image-resolution = scan-resolution;
- Example: ( 6 in. / 4 in. ) * 120 lpi * 2 = 360 dpi;
- Formula for optimizing number of colors or grays:
- This refers to how many levels or shades of a single ink, such as black, can be generated, not the total number of colors available. The maximum of 256 means 1/256 (less than 0.5%) change between each level. This results in the smoothest transition from one color of ink to another.
- ( output-resolution / screen-ruling ) 2 + 1 = shades-of-color; // grays or colors
- ( 300 dpi / 60 lpi ) 2 + 1 = 26 grays;
- ( 600 dpi / 85 lpi ) 2 + 1 = 50 grays;
- ( 600 dpi / 65 lpi ) 2 + 1 = 86 grays;
- ( 1200 dpi / 105 lpi ) 2 + 1 = 132 grays;
- ( 1200 dpi / 85 lpi ) 2 + 1 = 200 grays;
- ( 1200 dpi / 75 lpi ) 2 + 1 = 257 grays; // but only 256 gray levels can be used.
Note: In the table below, the halftone resolution is listed as half (1/2) of the scanning resolution. This is recommended.
|72 dpi or 96 dpi||— lpi||On-Screen: Web, Video, etc.; Halftone is irrelevant here;|
|130 to 170 dpi||65 to 85 lpi||In-Print: Newsletters, newspapers, or coupons on absorbent, uncoated stock;|
|180 to 266 dpi||90 to 133 lpi||In-Print: Business publications, newspaper coupons on coated stock;|
|266 to 300 dpi||133 to 150 lpi||In-Print: medium to high quality on coated stocks;|
|Above 300 dpi||Above 150 lpi||In-Print: highest quality on coated stock and highly accurate presses;|
|At Resolution (dpi)||Use Screen Ruling (lpi)||To Print This Many Grays*|
|300||53 to 60||33 to 26|
|600||71 to 85||72 to 51|
|1200 or 1270||65 to 128||256 to 89|
|2400 or 2540||90 to 150||256|
* This refers to how many levels or shades of a single ink, such as black, can be generated, not the total number of colors available. The maximum of 256 means 1/256 (less than 0.5%) change between each level. This results in the smoothest transition from one color of ink to another.