Part 1: What is a bitmap graphic?
A bitmap graphic (you may occasionally hear "raster" graphic) is basically a large grid - think of a huge checkerboard, or a screen door, or any grid with a lot of little squares. If you put a different color in each little square, you can build an image square by square. When you stand far enough away from the grid, the individual grid squares blend together and you see a complete photographic image. (See Illustration 1.)
This is how computer monitors and televisions work - they both are a series of little squares (pixels) in a big grid (the screen). When you specify your monitor's resolution (800x600, 1024x768 etc), you are telling the computer how many dots across by how many dots tall, and thus you define the grid that gets filled in with dots to show the image. (See Illustration 2.)
This is also how the pixel or megapixel count of a digital camera is derived: If you have a 6.3 megapixel camera, that means the files produced by the sensor (digital film) will be something like 3072x2048. Just do the math - 3072 X(times) 2048 = 6,291,456. A megapixel is a million pixels, so we get 6.29 megapixels, which is close enough to 6.3 for the marketing people.
Hopefully this is not huge news so far, but the important thing to get out of this is that the grid is finite, as in "of limited size." You can make a grid any size you want, but it will still be a defined size.
The quality and appearance of a bitmap image is determined by (1) its size and (2) its resolution. In some instances, as when printing images to actual paper, "size" can refer to the number of pixels (like 800 x 600) or the physical dimensions (like 8 by 10 inches).
Resolution refers to a couple of different things in different contexts. Sometimes it references the density of pixels, and is usually expressed as Dots Per Inch (DPI). Graphics have different resolutions for different purposes, and it's important to know how you will be using the graphic. If you don't have high enough resolution and you try to print an image, it will look blocky or fuzzy. If your image is very high resolution but you use it on the web, it will take longer than necessary to download. So what resolution, or DPI, should you choose? The traditional rule of thumb is 72 DPI for images that will only appear online and 300 dpi for images that can be commercially printed. If you are printing images at home, 200 DPI or so should give you acceptable results. Resolutions higher than 300 DPI generally will NOT give better printed results - you just get unnecessarily large file sizes. The term DPI also is seen in specifications for printer and scanners - how many dots per inch the device is capable of scanning or printing.
The term resolution is also used when referring to the overall size of a bitmap graphic, as in:
So, for best results you may need to know the number of pixels, the resolution, the physical size, and how the image is to be used. Especially when the images are to be printed by a commercial printer, you have to know dimensions and the DPI to ensure a quality result.
with and resizing bitmaps:
To avoid this damaging interpolation, one needs to think ahead and create a document as large or larger than will ever be needed. That way you can shrink the graphic to any size you need, and it will always look great. If you have files from a digital camera, start with the largest size and work down, then keep the larger original someplace where you can come back to it
One word of caution - if you make an image smaller in Photoshop, that information is gone. You cannot resize the image back to its original size and get your original image. (See Illustration 3.) For instance, you have a 250x250 image and you use "Image Size" to drop the size to 75x75. Since you are making the graphic smaller, Photoshop dumps the information that is no longer needed. ((250 X 250) - (75 X 75) = (62,500 - 5,625) = 56,875 discarded pixels). If you then change your mind and use "Image Size" to put the size back to 250x250, Photoshop DOES NOT go back to what you started with - it will take that 5,625 pixels you're left with and guess (interpolate) the other 56,875, leaving you with a badly degraded image. With any luck, you didn't save and close the file, so you can "Revert" to get back to the original or go back in your history palette to a state before you resized the first time.
advantages of bitmap graphics:
- Bitmaps are quite universal - almost any program can work with simple bitmap file formats, and they require little system overhead to properly display.
disadvantages of bitmap:
- Bitmap file sizes can get quite large, especially if not compressed in some way. Depending on the resolution of the bitmap graphic, it can require lots of information to record the contents of the grid. Each little square in the grid has an X and Y coordinate, and the computer has to record which of the thousands or millions of color choices is present for each square. Illustration 4 has 16 little squares, but a typical bitmap can have millions of squares to record. This can chew up a lot of information.
- Bitmaps do not maintain editability. You cannot open a typical (non-Photoshop format) bitmap file and modify it as though you were the originator - there is no layer or other information in most bitmap files (TIFF, PICT, JPEG, etc...).