• As the field of digital imagery expands, many people are getting deeply involved with computer grafics, with little or no background on the subject.
    While there are many books about grafics on the shelves today, most of them asume that you you know the terminology and the technical material that serves as the foundation of computer grafics. The end result is frustration for the user. This chapter will try to help you fill in some of the gaps you might have in your grafics background.

  • Originaly written by David Huss, this document made as electronic document by me SDA- BullDozer is dedicated to this great man and the time he spent to it.
  • © David Huss © electronic doc by SDA BullDozer

  • When it comes to computer images, there are two types:
    Bitmaps (also called paint) and Vector also called freehand).
    The image file is composed of million pixels (picture elements).
    Bitmap files tend to be much larger than their Vector counterparts and resolution dependent (see resolution chapter later) To work effectively with Bitmap images, it is necessary to understand why they act differently than the object-based images. Let's start defining terms.

  • These are not litlle elf-like creatures that fly through the forest at twillight. Bitmap images are composed of pixels. The term "pixel" is short for PIcture ELement. They are the individual squares that make up an image on a computer screen. One way to understand pixels is to think of a wall mural created with mosaic tiles. When you get close to a mural made of mosaic tiles, it looks like someone had a bad Lego day.
  • This is because you are so close you are looking at individual tiles. But step away a few feet from the mosaic and the individual begin to lose their definition and visually merge. The tiles have not changed their size or number, yet the further back you move, the better the image looks.
  • Pixels in Bitmaps work much the same way. As you zoom in on an image, the individual pixels begin to stand out more and the image they produce becomes less and less evident. Returning to our mosaic tile analogy, there are, of course, major differences between pixels and mosaic tiles. Pixels come in a greater selection of decorator colors (16.777.216 - TC - 24bit), and pixels don't weigh as much as tiles. However pixels and tiles operate in the same way to produce an image.

  • What is color-depth ? This is the number of bits necessary to describe an individual pixel color. If a color image has a depth of 4 bits, then tha means there are 16 possible combinations of 4² to describe the color in each pixel. Another way to say it is, there are 16 possible colors available, or the image has a 16-color palette. There are several different color depths available. They are 1 bit = 2 colors, 4 bit = 16 colors, 8 bit = 256 colors,16 bit = 65.536 colors and 24 bit = 16.777.216 colors.
  • There's also 32-bit color, but it is used for pre-press and essentially only represents TC (true color or 24-bit) using a different type of color model. The greater an image's color depth, the more shades of color it contains, as shown in the table below. The drawback is that as the color depth of an image changes, the file size changes.

    Color Depth Type of image Colors available
    1-bit Black and white 2 colors
    8-bit Greyscale 256 shades of grey
    4-bit Color 16 colors
    8-bit Color 256 colors
    16-bit High-Color 65.536 colors
    16-bit Greyscale 65.536 grey shades
    24-bit RGB or True Color 16.777.216 colors
    32-bit CMYK Color 16.777.216 colors
    48-bit Color 281 billion colors

  • File size as a function of color depth
    Color Depth 32-bit 24-bit 8-bit 1-bit
    16 million16 million256 2 cols
    File Size 793 Kb 632 Kb 395 Kb28 Kb


  • All image file formats have some restrictions regarding the color depth that they can accomodate, so it becomes necessary to know what color depth you are working with in order to recognize what kind of colors and other tools you can use with it.

  • If color depth is new to you, you may be wondering.
    Why do we have all of these different color depths? Why not make all of the images 24-bit and be done with it?
  • There are many reasons for the different image types. One of the major factors of color depth is the physical size of the image file that each type produces. The greater the number of bits associated with each pixel,the larger the file size.
  • If an image has a size of 20Kb as a Black and White image, it will take up more than 480Kb as a True-Color image.
  • If an 8 x 10 inch color photograph is scanned in at 600 DPI (never try this) at a color depth of 24 bits, the resulting 64 Mb file will probably not even fit in your system. Not to mention that every operation with this image will be measured in hours instead of seconds. There are other factors associated with the different color depths. Let's take a closer look at the various types of color depth used.

  • The term "Black and White" has caused some confusion in the past because old movies and television shows are referred to as being black and white.
    They are actually greyscale, not black and white. Don't try to educate
    anyone on this subject.Just remember that the old Griffith and Dick Van Dyke
    shows are really in greyscale, not black and white.
  • In real Black and white images, one bit of information is used per pixel is used to define its color. Because it has only 1 bit, it can only show 1 of 2 states, either black or white. The little pixel is either on or off. It doesn' get any simpler than this.
  • Black and White images are more common than you would imagine. It is common to associate this kind of image with old Victorian woodcuts, but as you can see, there are contemporary examples of Black and White aswell.

  • There is a lot that can be done with Black/White images, also called Line Art. Users of Adobe programs may refer to them as Bitmap images. This can be confusing since most photographic images are referred to as Bitmap. It is possible to use a Black/White to produce photographs that appear to be greyscale. It approximates the greyscale look by a process called dithering.
  • Dithering can be thought of as pseudo-greyscale when it comes to Black/White images. While dithering can simulate greyscale, quality suffers greatly when a dithered image is resized.
  • Besides line art, there are 3 different types of black/white images that can be produced by selecting convert to ...using the dither option. An image can be converted using error diffusion, a dithering process that arranges the the black/white pixels to appear to the viewer's eye as greyscale. Error diffusion is more complex than the other method, ordered diffusion. The difference in speed is almost unnoticeable.
  • While Error diffusion dithering produces the best-looking results, it distorts the most when the image is resized.
  • You may also use line art setting. This method measures the shade value of each pixel in the original image and converts it either to a black or a white dot. The threshold value determines whether it becomes black or white. Any value greater than the threshold becomes white; any below becomes black.