HA2 - Task 1 Technical Glossary

Pixel and Resolution:

The term "pixel" or "pel" is actually short for "Picture Element." These small little dots are what make up the images on computer displays, whether they are flat-screen (LCD) or tube (CRT) monitors. The screen is divided up into a matrix of thousands or even millions of pixels. Typically, you cannot see the individual pixels, because they are so small. This is a good thing, because most people prefer to look at smooth, clear images rather than blocky, "pixelated" ones. However, if you set your monitor to a low resolution, such as 640x480 and look closely at your screen, you will may be able to see the individual pixels. As you may have guessed, a resolution of 640x480 is comprised of a matrix of 640 by 480 pixels, or 307,200 in all. That's a lot of little dots.

Each pixel can only be one colour at a time. However, since they are so small, pixels often blend together to form various shades and blends of colours. The number of colours each pixel can be is determined by the number of bits used to represent it. For example, 8-bit colour allows for 2 to the 8th, or 256 colours to be displayed. At this colour depth, you may be able to see "graininess," or spotted colours when one colour blends to another. However, at 16, 24, and 32-bit colour depths, the colour blending is smooth and, unless you have some kind of extra-sensory vision capability, you should not see any graininess.

PPI and DPI:

PPI stands for Pixels Per Inch (the number of pixels displayed per inch) and DPI stands for Dots Per Inch (the number of dots displayed per inch). Pixels are only seen on screens, therefore any image you look at on a screen contains pixels. And, the less pixels you have there will be less detail there will be vice versa (as explained above). Dots are what pixels effectively become once a picture is printed. The printer places millions of dots in all sorts of different colours so that we can see what the image is. This works the same way as pixels, the more DPI you have the more detailed the image. 

The standard resolution for a screen is 72 PPI.
The standard resolution for a print is 300 DPI.

The videos below explains in greater detail about PPI and DPI in photography terms. 

Cameras and pixels:

Here is a simple explanation of how how a camera works followed by a more detailed and in depth video.

There are many different types of cameras:

Single use cameras,

Compact lens-shutter cameras,

Bridge cameras,

Rangefinders,

Twin lens reflex,

SLR (single lens reflex),

DSLR (digital single lens reflex)

All of these cameras work in a similar way with slight variations. This is how the most simple camera works.

In film cameras, there are chemicals that change their nature when exposed to light.

Light from the sun and other sources reflects off of objects and is focused by a glass lens, at the end of the lens is an aperture control, a diaphragm that opens and closes to allow more or less light through the lens. At the rear of the camera (just in front of the film or light sensor) is a shutter. The shutter is in the form of several curtains that expose the film/light sensor to a consistent amount of light across the entire film/sensor. The amount of time the shutter is open helps determine how much light will reach the sensor.

If you want to know how a camera works, here:

If you are looking at an image the light bouncing off the object you are photographing, passes into the camera, through a set of lenses and onto a mirror. From there, the light bounces up and into a piece of glass called a pentaprism (a five sided prism). Once light enters the pentaprism, it bounces around in a complicated way until it passes through the eyepiece and enters your eye.

If you are looking at a photo, when you press the button on the camera, the mirror flips up, in front of the pentaprism. Instead of the light from the object bouncing on the pentaprism, the light passes directly to the back of the camera. There, it either hits photographic film or starts a chemical reaction (single use camera), or the light it impacts on a ray of light-sensitive cells that release a tiny electric charge in each activated cell (digital camera).

There are several types of camera.

The old style, using photosensitive film and chemicals. This works by having a light proof box with the film stretched across the inside.

A shutter lets light in, for a brief period, when the shutter release button is pressed. The light passes through a glass lens which focuses a picture onto the back of the inside of the camera, where the film is. The light leaves a chemical change on the photosensitive emulsion, which is invisible until processed.

After the film has been used, it has to be kept dark (in a cassette) until processed with chemicals.

That is the basics. Cameras will vary in complexity on how much adjustment of the functions are allowed, such as shutter speed, aperture, focus and automation.

Digital cameras.

The box remains the same as a film camera. Instead of the film, there is a semiconductor chip, with an open sensitive surface, onto which the picture is focused by the lens. This is commonly a Charge Coupled Device (CCD).

The chip is made up of a grid of photo sensitive pixels, the electrons are knocked off by the light, in varying degrees and leave a charge in that area.

A computer, built within the camera, can access each of the pixels, assigns a digital value, depending on the amount of charge (light) and then write this value to a file, which is then stored in non-volatile memory.

Some cameras have internal memory and others (commonly) have a removable memory card.

The picture files can then be accessed by a cable, or removing the memory card.

Once downloaded onto another computer or printer, the pictures can then be printed using ink jet or laser printers, onto normal or glossy paper.

Vector and Raster Images:

There are two kinds of computer images, Vector and Raster. Vector images; unlike JPEGs, GIFs, and BMP images, vector graphics are not made up of a grid of pixels. Instead, vector graphics are comprised of paths, which are defined by a start and end point, along with other points, curves, and angles along the way. A path can be a line, a square, a triangle, or a curvy shape. These paths can be used to create simple drawings or complex diagrams. Paths are even used to define the characters of specific typefaces.

Because vector-based images are not made up of a specific number of dots, they can be scaled to a larger size and not lose any image quality. If you blow up a raster graphic, it will look blocky, or "pixelated." When you blow up a vector graphic, the edges of each object within the graphic stay smooth and clean. This makes vector graphics ideal for logos, which can be small enough to appear on a business card, but can also be scaled to fill a billboard. Common types of vector graphics include Adobe Illustrator, Macromedia Freehand, and EPS files. Many Flash animations also use vector graphics, since they scale better and typically take up less space than bitmap images.

Raster images; in computer graphics, a raster graphics image, or bitmap, is a dot matrix data structure representing a generally rectangular grid of pixels, or points of colour, viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats.

A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. A bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a colour depth, which determines the number of colours it can represent).

The printing and prepress industries know raster graphics as contones (from "continuous tones"). The opposite to contones is "line work", usually implemented as vector graphics in digital systems.

Images have aspect ratios, for example 4:3, 3:2, 16:9, 1.85:1, 2.39:1. An aspect ratio is the size the image is shown. For example 16:9 (the most commonly used ratio) means the image is 16 inches by 9 inches. However, you pronounce this ratio as 16 to 9 despite it actually meaning 16 by 9. 

There are many different types of image formats, example: GIF Graphics Interchange Format -Images using a fixed colour pallet, 256 colours to be exact. JPEG Joint Photographic Experts Group -Used for photographic (continuous tone) images. TIFF Tagged-Image file format -used by bitmaps only. EPS Encapsulated PostScripts -A file format used fro both vector images and bitmaps. Adobe PSD -Proprietary format used by photoshop documents. Adobe Portable Document Format (PDF) -Used for publishing documents over the internet. 

Colours+Images:

RGB Additive -Red, Green, Blue

These colours are known as the primary colours because they mix together to create different colours; but no other colours can mix together to make them.

This colour model is seen on computer screens.

CMYK Subtractive -Cyan, Magnta, Yellow, Black

Black was given the letter K so that it isn't confused with blue from the other colour model.

 

This colour model is used by printers. 

Subtractive models combine colours to produce black.

Additive models combine colours to produce white.

The following video explains the difference between the two colour models.