3.1 Basic Image Issues
Generally, the goal when determining what image resolution and compression (file type) to use for presentation images is to find the most appropriate balance between image quality, file size, and file type usability for the type of presentation planned. An image may look great, but be too large use effectively on the internet. Another image might look fine, be small in size, but be an unusual format for which special software is required to view. There are several solutions that offer various combinations of advantages and disadvantages -- below are some guidelines for determining which might best suit your needs.
When a digital image is acquired with a digital camera or scanner, options are available to tell the camera or scanner how much information should be collected. The information is know as “resolution”, and can be described in terms of physical size (ex: 400 pixels x 300 pixels) and "dpi" (dots per inch). Dpi and "ppi" (pixels per inch) can be used interchangeably . The higher the resolution, the more information the image will hold, which increases both quality and file size. A good rule of thumb is to acquire the image at the highest practical resolution that your input device can support, because a high resolution image can always be made smaller, while the reverse is not true.
Microstation typically deals in explicit image resolution, both for specifying Viewport sizes, and in the image rendering and saving utilities.
Type of Image Output Medium
The amount of resolution needed for an image depends on how the image will be presented.
Computer Monitor - PowerPoint, CDROM, Image Viewers
Computer monitors vary in the level of resolution, or dots per inch, they can display, but generally the maximum is around 150 dpi. This means that when scanning images, if it is known that the only display method for the image will be a computer monitor, there is no reason to scan higher than 150 dpi – the file size will increase, and the monitor won't display the extra resolution contained in the image anyway. Note that the way that higher resolution is reflected on a computer monitor is simply by an increased file size.
For a Powerpoint or other computer based presentation, the resolution required will also be driven by the projector or monitor on which the images will be displayed. Most projectors currently display at 1024x768 pixels, which mean a Powerpoint slide will be rendered and displayed at that resolution. Images need be no higher in resolution than the display technology.
Print - Publication or Boards
Printers also vary in the level of resolution, or dots per inch, they can print. But, printer resolution is almost always much greater than monitor resolution. A typical level of resolution for most printing situations is 300 dpi – less will typically cause a reduction in print quality. More will increase file size, but the extra resolution won't be reflected in the print piece anyway.
When preparing images for large boards, (30'x40" for instance) it is unrealistic to work at 300dpi for that final size. An image set up for 300dpi on 11"x17" is usually enough resolution to use for the boards, as viewers will not need to be right next to the image.
The best tool in the effort to obtain a good balance between image quality and file size is called compression, and different compression schemes result in different file types (recognizable by their file extensions such as .jpg. .gif, etc. Compression is a set of standards that imaging software uses to analyze the digital information an image contains, and reduce the amount of information needed to display the image.
The most widely used lossy compression is JPG (pronounced “Jay Peg”). When a JPG image is created, the imaging software analyzes the image data and discards some. The result can be a file size reduction of 90%, which makes JPG images perfect for use on the internet. The problems are that the image quality can be noticeably lessened (depending on the amount of compression used), and once the data is discarded from the JPG, it cannot be recovered. If a JPG image is opened in imaging software, and saved again as a JPG, more data is lost. If this is done continually, the image may degrade to the point that it is no longer usable. It is always advisable to keep an original copy of an image in some form of lossless compression.
Lossless image compression formats include TIF, PCT, and PNG. Lossless compression compresses images without discarding any of the original data which makes them useful for storing original image files, but inefficient for displaying images on the internet.
Common File Formats
- Pros: Offers excellent compression of photographic images. Good for image compression for the internet and other situations where file size or bandwidth is a concern.
- Cons: Lossy compression which discards original image data. Not suitable for repeated editing.
- Pros: Lossless compression that retains all the original image data. Good for images with fewer colors such as logos and cartoon images. Offers smaller file size than TIF.
- Cons: Compression is based on using a more limited color palette. Not suitable for full color photos.
- Pros: Lossless compression that retains all the original data and color palette. Good for archiving images, and for saving images during image editing sessions.
- Cons: Very large file size. Not suitable for internet content.
Image Output and Viewing from Microstation
Setting up viewports and saving images from Microstation is described in Chapter 2 , the Save Image utility is in section 2.3. Image resolution in Microstation is specified in explicit pixel dimensions. Images that need to be higher resolution than display resolution, for print or boards for instance, must be created using the Save Image utility. Images for computer presentation can be created either through Screen Capture or Save Image utilities. Any images can be viewed within Microstation using the Display utilities described in Section 2.6.