Representing Images Flashcards

All 14 cards in this deck

What is the formula for the file size of a bitmap image, in bits?

width (pixels) $\times$ height (pixels) $\times$ colour depth (bits)

True or false? Width $\times$ height $\times$ colour depth gives a bitmap's file size in bytes.

False. It gives the size in bits; divide by 8 to get bytes.

How is a bitmap image represented in a computer?

As a grid of pixels, with the colour of each pixel stored as a binary value.

What is colour depth?

The number of bits used to represent each pixel.

What is the image size of a bitmap?

Its width $\times$ height, measured in pixels.

What is the effect of increasing the colour depth of an image?

More colours can be stored so the quality improves, but the file size increases.

How many different colours can an image with a colour depth of $n$ bits contain?

$2^n$ colours.
e.g. a colour depth of 1 bit allows just 2 colours, such as black and white.

What is the effect of increasing the image size (the number of pixels) of a bitmap?

The image has more pixels, so it can show more detail, but the file size increases.

What is a pixel?

A single point in an image: the smallest part of an image that can be given a colour. The word is short for picture element.

How do pixels relate to the way an image is displayed?

The image is shown as a grid of pixels, and each pixel is displayed as one colour. Together the pixels make up the picture.

How do you find the minimum colour depth for an image with a given number of colours?
e.g. an image with 5 colours

Find the smallest number of bits $n$ where $2^n$ is at least the number of colours.
e.g. 2 bits give only 4 colours, so 5 colours need 3 bits.

How is a black and white bitmap stored as binary?

With 1 bit per pixel: one value (eg 1) stands for black and the other (eg 0) for white. The bits are stored in order, row by row.

When converting a bitmap image into binary, what does each pixel become?

The binary code for its colour. Each colour has its own unique code, and every code has the same number of bits: the colour depth.

When turning binary data back into a bitmap, how do you know how many bits belong to each pixel?

From the colour depth. With a colour depth of 2, for example, each group of 2 bits is one pixel.