Computer pictures are made up of dots. The number of dots per inch (dpi) is known as "resolution". In general terms, the more dots per inch, the clearer the picture. But it also means the smaller the picture. A picture file large enough to print clearly at a reasonable size on high resoultion printer is also said to be high resolution. All the graphics supplied by Christian Computer Art are high resolution without being small.

If you think about a picture, it is essentially a rectangular grid. Each square in that grid is a dot in the picture. Even a small picture that prints out one-third of an inch square on a 300 dpi laser printer (100x100) will have 10,000 squares, or "cells". These "Picture Cells" are referred to as pixels.

Let us assume for the moment that the picture is simple 2-colour (black and white). We already know that computers work in binary digits ("bits") and each bit can have only one of two states, often referred to as "0" and "1". The computer allocates one bit to each pixel and uses the 0 and 1 to represent black and white. Referring to each cell in this way is known as a "bitmap". A bitmap file contains all this information plus some other computer code which says what sort of picture it is, how big it is and so on. There are several bitmap formats. Most compress the bitmap data as well, so there is no direct mathematical correlation between file size and picture dimensions, especially when comparing different formats. Nevertheless, a bigger file usually means a bigger picture!

All Christian Computer Art disks contain bitmap files.

To put colour to a picture, the computer simply increases the number of bits per pixel. There are four permutations of 1 and 0 with two bits, so you can use four colours. Three bits gives you eight colours and 24 bits allow approximately 16.7 million colours. This is a flexible system, because you can choose which colours you use. For example, a depth of 3 bits gives you eight colours. The computer will recognise these as colours 0 - 7 (computers prefer to start counting from 0 rather than 1). Your graphics software will allow you allocate whatever colour you want to each of the eight colour positions. The colours themselves are identified by numbers. The most common method is the combination of the three primary colours of light: red, green and blue (RGB). Each of the elements usually has 256 stages of strength (numbered 0 through to 255, or often in hexadecimal from 00 to FF). It's just a matter of mixing the amount of each of the primary colours to get colour you need. For example, maximum red plus maximum green and zero blue will produce yellow. Zero all values for black and maximise all values for white.

The RGB system is best for colours on screen. For printing, a system known as CMYK is used. This stands for Cyan, Magenta, Yellow and Keyline Black. These are the colours used on a four-colour printing press and if you are sending files to a printer, he will usually want separations; that is four pictures: each containing one of the four colours.

Common bitmap file formats are: BMP, PCX, PICT, TIFF, JPEG, RLE, IFF-ILBM, IMG, DIB, PNG, GIF.

The other method of storing graphics is called "vector". Rather than store a bit-by-bit representation of the graphic as with bitmap formats, vector formats store instructions to the computer on how to draw the picture. When the picture is resized, the computer simply follows the drawing instructions again making best use of pixels to ensure a well-defined graphic every time. To get good definition on bitmaps the file must be reasonably large. The same picture could often be stored in a smaller file in a vector format. Vector does not handle the very small pictures, such as icons, very well. These are better done as bitmaps.

There are fewer vector formats. Look for EPS, CDR, WMF, CGM

Both systems have their merits and demerits. Both systems have their afficionados and their opponents.

Generally speaking, graphics created in vector format look "flatter" and often have more regular shapes than bitmap pictures. With the right software it is possible to convert between the two systems. The Christian Clip Art CD contains vector pictures created from bitmaps, so they do not look as flat as those created as vectors.

The problem with bitmap files is that they do not lend themselves to increasing in size. Curves and lines that are not horizontal or vertical suffer from a pronounced jagged edge, because the dot pattern is magnified.

Pictures with a lot of data do not lend themselves to vectoring. Colouring of vector images is very basic. Vector cannot support full-colour pictures. So the highly detailed images in the Bible Picture Library are all bitmap.

The simpler Christian Clip Art graphics are provided in both bitmap and vector formats.