1.2 Multimedia (Graphics and sound) and 1.3 Compression Flashcards
What is a pixel?
Stands for picture element - a minute area of illumination on a display screen, one of many from which an image is composed. Can be in the shape of a square or a circle. At least 8 bits (1 byte) per pixel are needed to code a coloured image (256 possible colours of rgb)
What is the difference between bit depth and colour depth?
The number of bits used to represent a pixel is called the bit depth. The bit depth will determine the colour depth (number of possible colours which can be represented) of that pixel.
For example, a bit depth of 8 bits per pixel allows 256 (2^8) different
colours (the colour depth) to be represented.
What is the difference between image resolution and screen resolution?
• Image Resolution refers to the number of pixels that make up an image; for
example, an image could contain 4096 x 3192 pixels (12,738,656 pixels in total).
• Screen Resolution refers to the number of horizontal and vertical pixels that make up a screen display (for example, if the screen resolution is smaller than the image resolution then the whole image cannot be shown on the screen or the image will be cropped, rotated or made to be lower quality).
What is pixel density?
The resolution can be varied on many cameras before taking, for example, a digital photograph. When magnifying an image, the number of pixels that makes up the image remains the same but the area they cover is now increased. This means some of the sharpness could be lost. This is known as the pixel density and is key when scaling up photographs. It is either measured in ppi or ppcm.
Calculate the bit density for an Apple iPhone 8 (5.5 inch screen size) which has a screen resolution of 1920 x 1080 pixels
- Add together the squares of the resolution size (1920^2 + 1080^2) = (3,686,400 + 16,640) = 4,852,800
- Find the square root √4852800 = 2202.907
- Divide by screen size (2,202.907 / 5.5 = 401)
This gives us the pixel density of 401 pixels per square inch (ppi) (which is the same as the published figure from the manufacturer).
What advantages and disadvantages are there to using a high-resolution image over a low resolution image?
- Image will be more realistic and true to how it looks in real life
- The file size is increased; as the number of pixels used to represent the image are increased, the size of the file will also increase. This impacts how many images can be stored on, for example, a hard drive. It also impacts the time to download an image from the internet or the time to transfer an image between devices.
- Bit-map images rely on certain properties of the human eye and, up to a point, the amount of file compression used. The eye can tolerate a certain amount of resolution reduction before the loss of quality becomes significantly noticeable.
Calculate the full size image (1920x1080 pixels, with a bit depth of 24 bits) to estimate the file size needed to store a bit map image
1920 x 1080 x 24 bits = 49,766,400 bits for the full screen image
Dividing by 8 gives us 6,220,800 bytes (equivalent to 6.222 MB using SI units or 5.933 MiB using IEE units). An image which does not occupy the full screen will obviously result in a smaller file size.
What does the file header contain?
- The command used for each object that makes up the vector graphic image
- The attributes that define the properties that make up each object (for example consider the ellipse of the robots mouth – this will need the position of the two centres, the radius from centres, the thickness and style of each line, the line colour and any fill colour)
- The relative position of each object will also need to be included
- The dimensions of each object are not defined, but the relative positions of objects to each other in the final graphic need to be defined; this means that scaling up the vector graphic image will result in no loss of quality
Explain what vector graphics are
Vector graphics are images that use 2D points to describe lines and curves and their properties that are grouped to form geometric shapes. Vector graphics can be designed using Computer Aided Design (CAD) software or using an application which sues a drawing canvas on the screen.
Explain the differences between vector and bitmap
- Made up of geometric shapes which require definition/attributes; made up of tiny pixels of different colours
- to alter/edit the design, it is necessary to change each of the geometric shapes; possible to alter/edit each individual pixel to change the design of an image
- they do not require large file sizes since they are made up of simple geometric shapes; because of the use of pixels (which give accurate designs), the file size is large
- because the number of geometric shapes is limited, vector graphics are not usually very realistic; since images are built pixel by pixel, the final image is usually very realistic
- file formats are usually .svg .cgm .odg; file formats are usually .jpeg .bmp .png
How is sound transmitted?
Sound requires a medium in which to travel through (it cannot travel in a vacuum). This is because it is transmitted by causing oscillations of particles within the medium.
The human ear picks up these oscillations (changes in air pressure) and interpret them as sound. Each sound wave has a frequency and wavelength; the amplitude (height of a sound wave) specifies the loudness of the sound.
How does an ADC convert analogue sound to digital to be stored in a computer?
If the sound is to be used as a music file, it is often filtered first to remove higher frequencies and lower frequencies which are outside the range of human hearing.
To convert the analogue data to digital, the sound waves are sampled at a given time rate. The amplitude of the sound cannot be measured precisely, so approximate values are stored.
What is the difference between sampling rate and sampling resolution?
Increasing the number of possible values used to represent sound amplitude also increases the accuracy of the sampled sound (for example, using a range of 0 to 127 gives a much more accurate representation of the sound sample than using a range of, for example, 0 to 10). This is known as sampling resolution (also known as the bit depth)
Sampling rate is the number of sound samples taken per second. The higher the sampling rate and/or sampling resolution, the greater the file size. For example, a 16-bit sampling resolution is used when recording CDs to give better sound quality
How is sampling used to record a sound clip?
- The amplitude of the sound wave is first determined at set time intervals (the sampling rate).
- This gives an approximate representation of the sound wave.
- The sound wave is then encoded as a series of binary digits.
What do people use sound software for?
- Edit the start/stop times and duration of a sample
- Extract and save (or delete) part of a sample
- Alter the frequency and amplitude of a sample
- Fade in and fade out
- Mix and /or merge multiple sound tracks or sources
- Combine various sound sources together and alter their properties
- Remove ‘noise’ to enhance one sound wave in a multiple of waves (for example, to identify and exact one person’s voice out of a group of people
- Convert between different audio formats
