A photomosaic is a digital image made up of hundreds of other digital images. Small pieces of the main image are replaced by small complete images that roughly match the color; the end result is a large image made up of lots of tiny images. Photomosaics are commonly found as poster and calendar art.

The idea of constructing a photomosaic is the brainchild of Robert Silvers, who developed the technique while he was a student at MIT. The mosaics became popular, so Silvers went on to found a company devoted to photomosaics (Runaway) and wrote a book about them, called Photomosaics, available from Henry Holt and Company.

The process of making a photomosaic is quite simple, actually. You need two things to start with: an image to base the photomosaic on and a library of additional images with which to construct the photomosaic. Once you have these, construction of a photomosaic follows five basic steps.

1. Make a grid and place it over the image that you're basing the photomosaic on. The actual size of the grid (in cell count, determined by the height and width) is up to you; it mostly depends on what you want to use the photomosaic for when you are done. Another consideration is that the size of the individual cells of the photomosaic match the size of the images in your library in proportion.
2. Examine each individual cell in the grid. This can be arduous. If you are considering making many of these, it is best to write a program to handle the analysis for you.
3. Calculate the average color of each cell. If the image is in bitmap form, you can just average the value of each pixel; more trickery is needed for other image types. If you are merely doing this in Adobe Photoshop or The GIMP, taking a few samples and averaging the color values will work quite nicely.
4. Find the image in your image library with the nearest average color value, scale it down, and place it in the cell. Again, a script is useful if you are constructing many of these. If it's a single project, Adobe Photoshop or The GIMP can handle these tasks quite well.
5. Repeat the process for each cell. Depending on the number of cells, this can take a long time.

The quality of the photomosaic depends on a number of things. Usually, larger photomosaics turn out better; poster-sized mosaics turn out much better than small mosaics. Another important factor to consider is the size of your library; more images are always better so that you can have a lot of variety in your mosaic. Since the mosaic will require so many images, a good photomosaic will start off with a library of five to ten thousand images to draw from.

Photomosaics are nice pieces of visual art that are easy to construct with digital tools and clever programming. Many great examples of photomosaic art can be found; look through the poster bin at your local department store for some good poster-size examples.

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