Digital imaging notes


Until very recently, photos and motion pictures were taken with light-sensitive celluloid film in analog cameras.  With the advent of digital cameras, especially in the 21st century, once-mighty companies like Kodak and Fuji and Agfa have disappeared.  Even so, many of the ideas and foundational concepts live on.  Cameras still have shutters, apertures, and ISO sensitivity ratings.

The basics:   

PIXEL: “Picture element.” The smallest single thing of one color that you can see on your computer screen or television. The total number of pixels limits the detail that can be seen on a screen. Most computer monitors have a standard screen resolution of 72 pixels per inch, which corresponds with the ancient typesetting system of 72 points per inch. Also, most typical television sets have less than half a million pixels, while HDTV may have two million or more.


In the early days of digital imaging, we had computers with only 8-bit  processors and the images had only 256 colors. We all know that a bit has only two positions: on or off.  At the simplest level, a pixel with only 1 bit could be either black or white.  (Or whatever two options you have indexed it for).

All eight bits only gives you 256 possibilities because 2 to the 8th power is 256:
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2  = 256

These early images were called Graphic Image Format (gif) and were developed by a long-forgotten online company called Compuserv, so you might see these called Compuserv gif formats. Because you can reduce the colors in a gif, they are still used for maps and cartoons.

When computers became more complex in the mid-1990s, we had 32 bit processors (or dual processing with 64 bits, like Nintendo 64).   These could handle three or four channels of image information in what is called RGB mode (Red Green Blue).

A standard three channel RGB mode format is the jpg (or jpeg).  The name stands for Joint Photographic Experts Group of the International Telecommunications Union, which is a United Nations entity.  JPGs can have 16.8 million colors.  (256 x 256 x 256)

Sometimes RGB has a fourth channel called an “alpha” channel, and a format that can use the fourth channel is called a png (Portable Network Graphics).

Another mode of color is the CMYK mode.  This is for printing.  It has four channels:   cyan – magenta – yellow – black (K is for black, so it doesnt get confused with blue).  The main format used with CMYK mode is a tiff (and also a pict), and the good thing about a tiff or a pict is that they dont compress.

Usually jpgs are not used for printing.  Why? Because they do compress, which is what you want for the web but not for printing.


Format — channel — mode — typical use

GIF   — 1 channel — Indexed mode  — maps, cartoons

JPG — 3 channels — RGB mode — photos to be compressed for the web

PNG — 4 channels — RGB mode — photos to be compressed for the web

TIFF, PICT — 4 channels — CMYK mode — large file size photos for printing

PSD — Photoshop original file format