Scanning Images
Contents:
Introduction
The initial two decisions to make about any image scan concern the image type (greyscale, color) and the resolution. The more dots per inch, and the more information in each dot, the bigger the file.
These decisions will depend upon the use you intend to make of the image. An item that has long-term viability, such as a scan of an original document that is part of an ongoing project, will need to be scanned at higher settings than an image for a web page. Images for printing will require higher resolution than images for screen.
Currently, the Digital Media Center scanning facilities consist of 2 Macintosh and 3 PC workstations equipped with a color scanner, transparency adapters, and the following software:
- EpsonTwain Pro, for scanning images on the Epson scanners.
- MagicScan, for scanning images on the (big blue) Umax scanner.
Image Types
- 1-bit black and white (each dot can be either black or white)
- There is little reason ever to use 1-bit black and white scanning. The visual quality is poor. The 1-bit file is also not amenable to JPEG, the best of the image compression schemes.
- 8-bit greyscale (each dot can be one of 256 grey shades)
- 8-bit greyscale works well for most non-color images, and gives a good, clear image. This works particularly well for line drawings, with little or no shading. For archival images, scan non-color images in 24-bit color.
- 8-bit color (each dot can be one of 256 colors)
- 8-bit color (color photo) gives a less photo-realistic image than 24-bit color (millions of colors), and can look a little grainy. However, the 8-bit color file will be much smaller than the 24-bit color file in an uncompressed form, and you may be working with viewers and programs that do not allow you to use a 24-bit color image.
- 24-bit color (each dot can be one of 16.8 million colors)
- 24-bit color gives a much more photorealistic image, but results in a much larger file than 8-bit color. However, the JPEG process will reduce the file size dramatically.
Resolution
Practical considerations will often determine the choice of dpi (dots per inch). The higher the dpi number, the more information in the file, and the greater the ability to enlarge a detail from that image. Note, though, that if the original image does not have much detail to enlarge, a high dpi setting may gain you little or even highlight flaws in the original.
Raising the dpi value also increases the file size, sometimes beyond a size which your viewing software can cope with (or you can store). To take an extreme case, a 400 dpi, 24-bit color TIFF image that is as big as the bed of the scanner (8.5x14 inches) would be 55 megabytes in size uncompressed. So, there is a degree of experimentation, and of tailoring the resolution to the purpose of the scan, in choosing a dpi value.
If the image will have its principal life on screen (such as an image for a web page), as opposed to being printed out, and if you do not need to enlarge details from it, there is no reason to scan at better than 100 dpi, because screen resolution is typically lower than 100 dpi anyway. If the image will be printed out on a laser printing, you should choose 300 dpi since that is usually printing resolution for laser printers. If you are creating an archival copy of an image (for future use or to edit ext ensively), you should choose 400 dpi to render full detail and to accomodate magnifying detail, enlarging or decreasing size.
| Compression Scheme for a One Inch Square Image | ||||
|---|---|---|---|---|
| Resolution (dpi) | 400x400 | 300x300 | 200x200 | 100x100 |
| 2-bit black and white | 20K | 11K | 5K | 1K |
| 8-bit greyscale or color | 158K | 89K | 39K | 9K |
| 24-bit colour | 475K | 267K | 118K | 29K |
Image File Formats
At the scanner you are likely to create an image in an uncompressed format such as TIFF (works on all platforms), BMP (MS windows only) or PICT (Mac only). The TIFF file has long-term archival use, but is usually too large as a file to work with effectively, especially if you want an image as part of an HTML document on the Web.Several digital image formats save a file in a compressed form, GIF and JPEG are the most common. GIFs give moderate compression on greyscale or 8-bit color (256 colors). JPEG gives greater compression on greyscale, 8-bit, and 24-bit color images. A JPEG compression does not, however, simply store information in an abbreviated fashion but loses some information.
If you are working with large color files, archive the original 24-bit color images in a rich but large format such as TIFF and work with smaller JPEG versions. At normal size it is difficult to tell a JPEG from a TIFF, even though the former file size m ay be 10-40 times smaller than the latter. You will see, however, that as you begin to enlarge the two files the JPEG image begins to pixelate much sooner than the TIFF.
As the figures below indicate, you are usually better off scanning at 24-bit and then making a JPEG than scanning at 8-bit. You don't need to keep the 24-bit uncompressed file, but you may want to use it as a stage towards making the JPEG.
| Comparison of Image File Sizes | |
|---|---|
| Image Format | Size |
| 300 dpi 24-bit color | 2.65 x 3.14" |
| TIFF | 2188 K |
| JPEG | 59 K |
| 300 dpi 8-bit color | 2.65 x 3.14" |
| TIFF | 729 K |
| JPEG | 76 K |
| 100 dpi 24-bit color | 2.65 x 3.14" |
| TIFF | 249 K |
| JPEG | 9 K |
| 100 dpi 8-bit color | 2.65 x 3.14" |
| TIFF | 85 K |
| JPEG | 12 K |
Scanning Images
To scan images, the Digital Media Center uses EpsonPro for the Epson scanners and MagicScan on the oversized UMax scanner.
Scanning with EpsonPro
At the scanning station, find PhotoShop on the Program Menu or desktop.
- From the File menu in Adobe PhotoShop, choose "Import," then "Twain_32" to call in the scanning software.
- Select the image Type. 24 Bit Color generally works best on any image, even a black and white image, with shading. 24 Bit Color will create a larger file size, but this will be reduced substantially if you save the image as a jpg or gif.
- Choose the destination: LaserPrinter if you are planning to print out the image, Screen if you plan to use it in a web page.
- Choose the resolution. In general, 72 dots per inch is fine if you are planning to display the image on a web page or PowerPoint presentation at the same size. If you plan to enlarge the image, you might increase the resolution. If you are doing a project that requires you to make archival copies of the image (or if you never want to have to scan the image again, and you can store it on a CD-ROM or a zip disk), you might use 400-600 dpi.
To capture the image:
- Place the image on the scanner's surface, against the top right corner.
- Try to align the image as accurately as possible. You will, however, be able to adjust alignment later.
- Close the lid, if possible, for better clarity.
- Once you have placed your image on the scanner bed, click on Preview button from the Menu image on screen. When the preliminary version of your image is on screen, you can do some basic editing.
- Click in a corner of the image and drag the crosshairs tool to draw a marquee around the part of the image that you want to capture. If you don't like the marquee you've created, you can delete it by hitting the delete key or the first button under the marquee heading on the interface (the one with the blue eraser).
- To zoom in on the area that you've selected in the marquee, click on the top button under preview--the page with a magnifying glass on top of it. You can return to the unzoomed view by clicking the button with red arrow pointing up.
- To automatically correct brightness and contrast, click on the Auto button under "Adjust."
- To manually correct brightness, contrast, highlight, and shadow, click on the tools (the wrench and screwdriver) under the Adjust menu. (Not recommended unless you know what you're doing.)
- To manually adjust the tone, click on the button with the color bars under "Auto" in the "Adjust" menu.
- To manually adjust the color balance, click on pie chart button with the red, green, and blue.
- If, in your attempts at manual adjustments, you've messed things up, click the "reset" button to return to your original scan.
To produce the final scan:
- After you have adjusted the image to your satisfaction, select the Scan button. The image will rescan.
- The image has now been sent to PhotoShop. To save or manipulate the image file, you must EXIT the EpsonTwain program. If you have several images to scan, you might want to scan them all before closing EsponTwain, then manipulate them in PhotoShop. (Don't scan more than 10 or 15 at a time, though, so you won't lose too much work if the program crashes.)
- To save the file, choose "Save As" under the file menu in Photoshop. You may create a folder for yourself in MyDocuments and save your files there, then move them to a floppy or use file transfer protocol to place them in your own directory on the network. If you prefer, you can save directly to a floppy, although sometime PhotoShop will crash when you use this method.
- Determine what file format you would like to save the image in:
- If you want to save the image for archival purposes, or if you want to be able to manipulate it to the fullest extent, save it in the uncompressed TIF format. Warning: the file size will be LARGE.
- If you want to manipulate the image in Photoshop, you might save it in Photoshop's proprietary PSD format.
- If you want to use the image on a webpage , save it as a jpg or gif. (JPG often produces better image quality, but its compression algorithm throws away some information--generally undetectable to the huamn eye at the current image size--for good.) For a PowerPoint presentation, jpgs generally work fine.
- If you plan to print the image, BMPs, PSDs, and TIFs are good, though they produce large image files. Most printers are at least 300 dpi, so scan to a fairly high resolution if you intend to print the image.
Using the UMAX scanner
The scanning interface on the UMAX oversized scanner (Magic Scan) differs from the Epson interface, but core concepts remain the same. For more information on Magic Scan, see http://www.itg.uiuc.edu/help/powerlook/
A Final Word
If you are interested in further editing the image, such as cropping, retouching, changing colors, adding text, or otherwise manipulating it, ask the Digital Media Center staff for assistance with Photoshop, an image editing application that has a vast range of capabilities.
