Basics of Images

Reference

 Most of the material on this page comes from the book This is an excellent reference book for anyone dealing with computer graphics image files; it contains a detailed description of many file formats, as well as several good chapters at the beginning which explain many concepts related to images. It also comes with a CD containing hundreds of sample images, image processing software (for Macintosh, Windows, and UNIX), and detailed image format specifications.

Pixels

 An image consists of a rectangular array of dots called pixels. The size of the image is usually specified as width X height, in numbers of pixels. The physical size of the image, in inches or centimeters or whatever, depends on the resolution of the device on which the image is displayed. Resolution is usually measured in terms of DPI, which stands for dots per inch. An image will appear smaller (and generally sharper) on a device with a higher resolution than on one with a lower resolution.

Note: The term resolution is also sometimes used in the context of an image, in which case it specifies the resolution at which the image is intended to be displayed. Some image file formats allow for the specification of an image resolution. It is important to understand the distinction between this number, which expresses a preferred DPI for displaying the image, and the size of the image, which gives the number of pixels in the image.

Image Depth / Bitplanes

 For each pixel in an image, one needs to know what color to make that pixel when displaying the image. For a black and white image there are only two choices --- each pixel is either black or white --- so one bit of information is all that is needed for each pixel. Such images are sometimes called 1-bit, or monochrome images.

For color images, one needs enough bits per pixel to represent all the colors in the image. The number of bits per pixel is sometimes called the depth of the image, or the number of bitplanes. A number consisting of n bits can have 2^n different values, so an image of depth n can store up to 2^n colors. The most common depths in computer graphics today are probably 8 and 24, although 2-bit and 16-bit images are also common. The human eye can discern between roughly 2^24 different colors (this number obviously varies greatly from person to person and depends a lot on viewing conditions), so a 24-bit image is needed to represent the full range of colors that we can perceive. In many cases, however, the quality of an 8-bit image is very acceptible, and such images are often preferably because they require less storage space.

Color: Direct vs Colormaps

 A color in computer graphics is usually represented as an ordered triple of numbers in one of several color spaces. The most common color space in computer graphics is RGB, which stands for Red, Green, and Blue. If you stare very closely at a computer screen with a magifying class you will see that it consists of lots of tiny red, green, and blue dots which light up in various combintations to create the impression of a range of colors (each individual pixel on the screen usually consists of several of this single-color dots).

The most direct way to store color information in an image is for the data representing each pixel to directly specify the color of that image by giving the values of the red, green, and blue components (or the components in some other color space) of that color. We sometimes use the term RGB image to refer to an image which stores color in a direct RGB representation.

Another way of storing colors is to use the data for each pixel in an image to store an index into a table of colors, rather to store the color value directly. The advantage of this method is that it can drastically reduce the amount of storage space necessary when only a relatively small number of colors appear in the image. The table of colors is called a color table, a color map, a palette, an index map, or a look-up table (lut) (these are all equivalent terms).

Transparency

 It is sometimes useful to combine two images by overlaying one onto the other. Transparency is a technique whereby certain pixels in the image being layed on "top" can be labeled as "transparent", which means that the "bottom" image will show through them. Transparency is usually handled by treating transparency as additional color information; for example by storing a 4th number for each pixel along with its R,G,B values. Simple transparency can be stored in just 1 bit; in this case a pixel is either transparent (or not), which means that pixel will be ignored (or not) when the image is combined with another image.

It's also possible to use varying degrees of transparency, which can allow us to combine images in a way such that parts of the bottom image show through parts of the top image to varying degrees. This requires more than 1 bit of transparency information.

The transparency value of a pixel is sometimes called its alpha value, and the collection of all an image's alpha values is sometimes called its alpha channel. The term RGBA is sometimes used to refer to an RGB image with an alpha channel.

In the context of the Web transparency is often used to get an image to blend in well with the browser's background.

Data Compression

 Several data compression schemes are often used to reduce the amount of space necessary to store an image. It isn't necessary for the purposes of this course to get into the details of these techniques, but it is helpful to know what they are and to have a general idea of what they involve. The above algorithms are all lossless, meaning that the process of compressing and restoring the data does not change the data at all. Some algorithms are lossy, meaning that the the compression/uncompression step may actually alter or destroy some of the data (but hopefully only parts that are in some way expendable anyway).

Bitmap Files

 There are many different bitmap file formats in use today, but they all have more or less a similar structure: Note that it is usually only the pixel data itself that is compressed in an image file; the header, color table, and other parts are usually left uncompressed.

Bitmap File Formats

 There are hundreds, if not thousands, of bitmap file formats. The ones you are most likely to encounter in the process of producing and using materials for the Web (at least at the Geometry Center) are:

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Created: May 31 1996 --- Last modified: Sun Jun 2 20:30:49 1996