Digital Image File Types

Number of colors #2

A few display even more. Since the eye has trouble distinguishing between similar colors, 24 bit or 16 million colors is often called TrueColor.

Lossy vs. Lossless Compression

A lossless compression algorithm discards no information. It looks for more efficient ways to represent an image, while making no compromises in accuracy. In contrast, lossy algorithms accept some degradation in the image in order to achieve smaller file size. A lossless algorithm might, for example, look for a recurring pattern in the file, and replace each occurrence with a short abbreviation, thereby cutting the file size.

TIFF #2

A more important use of TIFF is as the working storage format as you edit and manipulate digital images.

File Size Comparisons

Below are comparisons of the same image saved in several popular file types. (Note that there is no reason to view more than one of the TIFFs or the PNG. Since all are lossless formats, their appearance is identical.)

GIF #3

Better algorithms search the image to find an optimum set of 256 colors. Sometimes GIF uses the nearest color to represent each pixel, and sometimes it uses "error diffusion" to adjust the color of nearby pixels to correct for the error in each pixel. GIF achieves compression in two ways. First, it reduces the number of colors of color-rich images, thereby reducing the number of bits needed per pixel, as just described.

JPG #2

Digital cameras save in a JPG format by default. Switching to TIFF or RAW improves quality in principle, but the difference is difficult to see. Shooting in TIFF has two disadvantages compared to JPG: fewer photos per memory card, and a longer wait between photographs as the image transfers to the card.

PNG #2

If you have an image with large areas of exactly uniform color, but contains more than 256 colors, PNG is your choice. Its strategy is similar to that of GIF, but it supports 16 million colors, not just 256.

PNG #3

If you want to display a photograph exactly without loss on the web, PNG is your choice. Later generation web browsers support PNG, and PNG is the only lossless format that web browsers support.

GIF #2

If your image has fewer than 256 colors and contains large areas of uniform color, GIF is your choice. The files will be small yet perfect. Here is an example of an image well-suited for GIF: Do NOT use GIF for photographic images, since it can contain only 256 colors per image.

Number of colors

Images start with differing numbers of colors in them. The simplest images may contain only two colors, such as black and white, and will need only 1 bit to represent each pixel. Many early PC video cards would support only 16 fixed colors. Later cards would display 256 simultaneously, any of which could be chosen from a pool of 224, or 16 million colors.

PNG #4

It produces smaller files and allows more colors. PNG also supports partial transparency. Partial transparency can be used for many useful purposes, such as fades and antialiasing of text. Unfortunately, Microsoft's Internet Explorer does not properly support PNG transparency, so for now web authors must avoid using transparency in PNG images.

PSP

PNG is supported by most of the latest generation browsers. TIFF is not widely supported by web browsers, and should be avoided for web use. PNG does everything GIF does, and better, so expect to see PNG replace GIF in the future.

Digital Image File Types Explained

Part of the reason for the plethora of file types is the need for compression. Image files can be quite large, and larger file types mean more disk usage and slower downloads. Compression is a term used to describe ways of cutting the size of the file. Compression schemes can by lossy or lossless.

GIF #4

Second, it replaces commonly occurring patterns (especially large areas of uniform color) with a short abbreviation: instead of storing "white, white, white, white, white," it stores "5 white." Thus, GIF is "lossless" only for images with 256 colors or less. For a rich, true color image, GIF may "lose" 99.998% of the colors.

PSD

These packages use layers, for example, to build complex images, and layer information may be lost in the nonproprietary formats such as TIFF and JPG.

Other Formats

When using graphics software such as Photoshop or Paint Shop Pro, working files should be in the proprietary format of the software. Save final results in TIFF, PNG, or JPG. Use RAW only for in-camera storage, and copy or convert to TIFF, PNG, or JPG as soon as you transfer to your PC.

Other Formats #3

Will you be able to read a RAW file in five years? In twenty? JPG is the format most likely to be readable in 50 years.Thus, it is appropriate to use RAW to store images in the camera and perhaps for temporary lossless storage on your PC, but be sure to create a TIFF, or better still a PNG or JPG, for archival storage.

Other Formats #2

You do not want your image archives to be in a proprietary format. Although several graphics programs can now read the RAW format for many digital cameras, it is unwise to rely on any proprietary format for long term storage.

PNG

a lossless storage format. However, in contrast with common TIFF usage, it looks for patterns in the image that it can use to compress file size.

TIFF

a very flexible format that can be lossless or lossy. The details of the image storage algorithm are included as part of the file.

RAW

an image output option available on some digital cameras.

BMP

an uncompressed proprietary format invented by Microsoft.

GIF

creates a table of up to 256 colors from a pool of 16 million.

JPG

optimized for photographs and similar continuous tone images that contain many, many colors.

JPG #3

rarely shoot in TIFF mode. Never use JPG for line art. On images such as these with areas of uniform color with sharp edges, JPG does a poor job. These are tasks for which GIF and PNG are well suited. See JPG vs. GIF for web images.