COMPUTER GRAPHICS

Image Size

Colour Depth

IMAGE SIZE

• Can refer to the number of pixels in a bitmapped image

• The amount of space it will take to store the image on the disk

• The display size for a particular output device.

In this lesson we will use image size to refer to the number of pixels in a bitmapped image.

The image size of a bitmapped image can be described by the horizontal (H) and vertical (V) pixel count.

The total number of pixels in an image is found by multiplying the horizontal and vertical pixel counts H x V = total pixel count

640 pixels wide (V) and 320 pixels high.

Calculate the total number of pixels

Each pixel of the image contains unique colour information. The amount of colour information, colour depth, is described in the unit of bits.

b = number of bits2b = number of possible display colours

COLOUR DEPTH

2 BITS 24 BITS

An 8-bit image uses 8 places of binary code to code for the colours.

28 = 2*2*2*2*2*2*2*2 = 256

In a one bit image, each pixel has either a 0 or a 1 to code colour so only two colours-black or white-- are used. An 8-bit image uses 8 places of binary code to code for the colours.

That allows a palette of 28 (2 to the eighth power) = 256 colours or 256 shades of gray.

A color depth of 4 bits would be 2 times itself 4 times:

2 x 2 x 2 x 2 = 16 colours

A color depth of of 8 bits would be 2 times itself 8 times:

2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 colours.

A 24-bit colour image works with a palette of over 16.7 million colours.

The image must set aside the full 24 bits of computer memory for each pixel to give it the flexibility of displaying any of those possible colours.

Most images you see either have a colour depth of either 8 or 24 bits.