Color and Format

Color and Format: GIF, the most common image file format, is usually used for solid colors and simple images, since it uses a maximum of 256 colors. This does not mean that there are always 256 colors present in a GIF image, only that a pixel (see Size for a discussion of pixels) can be only one of 256 possibilities. In fact, there can be fewer, and the fewer the colors, the smaller the file size. Black and white images can be made to be only those two colors (black and white), making such files quite small. GIFs were the original image file format on the Internet, and they do their reduction of size by simplifying the image - primarily by simplifying color.

JPG's, in contrast, reduce their file size by a mathematical algorithm rather than through a simplified image, hence they allow for more colors - 16 million. Again - this is the maximum, but fewer are possible, and can sometimes reduce file size. Since GIF and JPG do the business of building their images differently (see Image Formats), a JPG and GIF of the same image with the same number of colors will not be the same file size. Which will be bigger will depend on the complexity of the image. 

What does this mean in terms of appearance? It's most noticeable in gradients - colors that smoothly transition from one shade to another. The more colors available, the smother the transition. This is why JPG's are better for photographs, because photographs are full of subtle color transitions. If an image is created with blocks of electronically-produced color, then uniformity of color through the image is guaranteed, and no price is paid for a small color count.

GIF also has the possibility of including transparency. In fact, you can choose any of the 256 colors available to a GIF as its transparent color, although a white background is the most common. Below, the wallpaper is set to yellow in the boxes to display the ability to select transparency color in GIFs. Converting a JPG to a GIF for the purpose of making a portion transparent is usually doomed to failure - the conversion process may reduce color count, but adjacent pixels will still usually be different colors, and the transparency will barely be noticeable.