SVG

From Open Clip Art Library Wiki

What is SVG?

SVG is an Acronym for "scalable vector graphics" It is a W3C specification for creating vector images. Read more about the SVG format

What is Vector Graphics?

Digital Image formats can be broken up into two major categories: raster and vector. This Open Clip Art Library uses the vector format, and as stated above, specifically the SVG format.

Vector Formats

This project uses the vector format. Popular vector formats include WMF and SVG. The difference between raster and vector formats is significant. If you draw a circle and save it as a raster image the file will describe a bunch of white pixels and a bunch of black pixel. The fact that it represents a circle is lost. If you zoom in on the picture, the computer can only make each dot bigger, and it looks worse.

In vector formats, what gets stored is data describing the shape. If you draw a circle, it'll store bits of info that mean "circle of this radius at this point". If you want to go back and re-edit it, you can use your application's knowledge of editing circles. Also, if you zoom in on the circle, the program redraws it completely instead of just making each dot bigger. Another advantage is that the files are usually quite small despite their clarity. All this makes vector formats great for clipart.

Raster Formats

Popular raster formats include GIF, JPEG, PNG, and TIFF. In these formats, each pixel in the image is represented by a specific bit of data in the file. Storing each pixel of a large image creates a huge file! Many formats try to get around this by compressing the data so that each bit of data can represent several pixels. For example, the GIF format uses a system called 'run length encoding', if you have 100 blue pixels in a row, it would say something to the effect of "blue pixel times 100" which only takes a few bits of data to say. This is why GIF works fine for images that have large areas of similar colors and high amounts of regularity, like line art. This type of encoding is called "lossless", and something similar is used by BMP, PNG, and TIFF formats. They can be saved and converted without ever losing any quality. (Actually, GIFs only use 256 colors, so they can lose some of the color data the first time you save them.)

JPEG, on the other hand, uses a different approach for achieving compression. The method it uses drops some of the data, so it is called "lossy". It does this using some known tricks to how the human eye works. This works well for photographic images, less so for artwork, and extremely poorly for line art. The problem with JPEG, as it relates to clip art, is that JPEGs show artifacts (strange blocks and shadows) when you zoom in on them. You also can't edit them often. When you save the JPEG again, the computer saves the original artifacts and creates more, so the quality goes down. The higher the compression ratio you use, the faster it'll get bad.

PNG (pronounced "ping") is usually the best raster format for saving clip art, but it too has a few tradeoffs. It is a lossless format so it doesn't have the problems that JPEG has, and it uses more colors and better compression than GIF. However, the file size still gets unreasonable for large images, and it is not supported by some applications. This is much less of an issue today than it was a few years ago, but since most Microsoft programs don't display PNGs correctly, many users are still affected.