R 25 Images and Pixels - Reading Images need to be stored and processed using binary. The simplest image format is for an image to be stored as a bitmap image. Bitmap images are made up of picture elements called pixels. These contain a mapping of the colour of each pixel to bits. Black and white images have two colours (black and white) which can be stored with 1 bit per pixel. 0 0 1 1 0 0 0 1 0 1 0 0 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 1 1 0 0 1 0 0 1 0 0 0 0 1 0 0 1 1 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 1 1 0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 1 0 0 0 0 1 1 0 0 0 1 1 0 1 1 1 0 1 1 0 1 0 1 1 1 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 Bitmap image file 0001111000 00100100 0000 00000000 001100 00011001 0000000011 00011111 000110 00000000 01100000 00110000 0000111000 00011000 00000000 00000000 00000000 00000000 00100100 000000000000 Images that have different shades of gray are called grayscale images. We can use more bits to store the level of gray each pixel will have. The number of bits used for each pixel is called the colour depth. Type of image Example Bitmap Grayscale Grayscale Colour depth Total number of colours 1 bit/pixel 2 2 bits/pixel 8 bits/ /pixel 4 256 Full colour images store 8 bits for each of red, green and blue colours (RGB). These correspond to the sub pixels on a computer display. This allows for 2 24 (about 16.8 million) different colours. These can be written as 6 digits of hexadecimal. For example, FFFFFF is white and FF0000 is red. 49
Q 25 1. A bitmap file contains the binary on the left below. 1 is white and 0 is black. Colour in each of the squares. What is the letter that is revealed? 0000 0111 0111 0000 Letter revealed: _ 2. Pixels are named after what? Picture Elements Picture Cells Part Elements Picture Hex Elements 3. A black and white image will require how many bits per pixel? Number of bits: _ 7. As you increase the colour depth what happens to the image quality? It makes no difference It improves It gets worse You cannot change it 8. A bitmap file contains the binary on the left below. 11 is white, is gray, 01 is light gray and 00 is black. Colour in each of the squares. What is the letter that is revealed? 11 11 11 11 11 11 11 11 11 11 11 00 00 00 Letter revealed: _ Images and Pixels - Questions 9. An colour image has a 24 bit colour depth. Its dimensions are 24x768. How much storage space will be taken up with the data for the image? Space required: Megabytes 4. The number of bits per pixel is called what?. A school logo requires 5 different colours. How many bits will be required for each pixel? Bits equired: 5. A grayscale image is stored using the following colour depth. For each, state how many colours (shades of gray) will be available. a) 1 bit _ b) 2 bits _ c) 4 bits _ d) 8 bits _ e) 16 bits _ [5] 6. A grayscale image contains 24 pixels. 4 colours (shades of gray) have been used. How much storage space will the data for this image require? Space required: bytes 11. Computer displays use 3 colours for each pixel. What are they? Colour 1: _ Colour 2: _ Colour 3: _ 12. A web designer wishes to use 24 bit colour for their images. How many colours will be available for them to use? colours 13. What do each of the following colours represent in hexadecimal? a) FFFFFF _ b) 0000FF _ c) 00FF00 _ d) 555555 _ [4] 50
R 26 Image Size - Reading The resolution of an image is the concentration of pixels or the number of pixels per unit. In printed material this is often given as the number of dots per inch (dpi). The following black and white bitmap image on the left is represented by the binary data on the right. 00011110000010010000000000000 00001100 000110000000001001111100011 0000000000 011000000011000000001110000001100 0000000000 0000000000000000000000001001000 000000000000 The problem is that the computer needs more information than just this data. It also needs to know the width and height of the image along with the colour depth (also known as bit depth). This extra data is known as metadata. Meta means describing something. So metadata describe the data that we are storing. Modern digital cameras also store other data about the photographs they take such as the camera that was used, the aperture (how open the lens is to light), the exposure (how long the shutter was open) and the make of camera used. Some cameras will even store the GPS coordinates of the image so that the place it was taken can be located on a map. All this information is also stored as part of the metadata. The above image would store the following metadata: width = 12 pixels height = 20 pixels bit depth = 1 bit In certain file formats such as GIF the metadata also needs to provide a colour map of what colour each binary pattern will represent. Question: Answer: How large, in bytes, will the image shown above be? = width * height * bit depth + metadata = 12 * 20 * 1 + metadata = 240 + metadata (this is in bits) = 240 / 8 + metadata = 30 bytes + metadata We could design an image format that stored meta data the following way: A 2 byte number to store the width A 2 byte number to store the height A 1 byte number to store the bit depth The image above would therefore require: 30+2+2+ +1 bytes = 30+5 bytes = 35 bytes Different file formats will requiree different amounts of metadata so this is only a guide. 51
Q 26 1. The resolution of an image means what? Fill in one circle. How many colours are available Pixels per unit Additional information about it RGB 2. The resolution of an image can be measure in DPI. What does this stand for? _ 3. Match the following changes to an image on the left with the effects on the right. Additional items of metadata are stored Will increase the number of colours available Image Size - Questions 6. An image format stores metadata as width (1byte), height (1 byte), colour depth (1 byte) followed by the image data. a) How much storage space will be required to store the metadata? _ bytes b) How much storage space will be required to store the image data and metadata if the the image s dimensions are 24x768. _ bytes c) The following binary data is for a file stored in the above format. Use the data to colour the grid and write the character that is revealed below: 1 1 1 1 1 0 0 1 1 0 0 1 1 1 1 1 The resolution of a file is reduced results in larger file size Increasing the colour depth Resultss in a smaller file size [3] 4. Which of the following are parts of the image metadata? Tick as many boxes as required. image width RAM required to display the image image height colour depth 5. How many bits colour depth would be required for each of the following? a) 4 colours _ bits b) 16 colours _ bits c) 256 colours _ bits d) 00 colours _ bits [4] d) How much storage space will the _ bytes Letter revealed: image above in part C require? 7. A digital camera takes images with 4928 width x 3262 height. How many millions of pixels (megapixels) does the sensor have? Megapixels 8. How much RAM is required to store 1 frame of video/graphics for each of the following devices: Device Width Height Pixel depth RAM required Game Boy 89 160 px 144 px 2 bit KB Game Gear 90 160 px 144 px 5 bit KB PSP (2004) 480 px 272 px 24 bit KB iphone 1 (2007) 480 px 320 px 24 bit KB Galaxy S4 (2013) 1920 px 80 px 24 bit MB [5] 52
R 27 Digitising Sound - Reading Sound is input into a computer with a microphone and output from it with a speaker. Sound, unlike say a key stroke, is continuous movement of air and so an analogue signal. Computers need to store things discretely as 1s and 0s. A sound card will contain an analogue-to-digital converter (ADC) to convert the signal from a microphone to digital for the computer. It will have a digital-to-analogue converter (DAC) to convert the computers digital signal to an analogue one for the speaker. This is how it works: 1. Start with a sound wave (the right line shows 1 second of audio) 2. Now sample the data (here we are taking 4 samples per second so 4Hz) This is the vertical lines. This is known as the sample rate. We also choose a bit rate (in this case 2 bits) which is how accurate each sample will be. Notice that because of the low bit rate we need to round up or down if it doesn t cross a horizontal line at the sample point. Sample 1 Sample 2 Sample 3 Sample 4 3. The binary to store the right hand graph will be: 01. Using a higher sample rate this time (7Hz) and higher bit rate (3 bits) we get a more accurate representation, but still not perfect. 1 2 3 4 5 6 7 The binary to store this would be: 1 011 001 0 011 011 011 The amount of storage space to store an uncompressed sound file will be =bit rate * sample rate A CD uses 16 bit per sample * 44.1 KHz sample rate * 2 channels (left and right) = 16*440*2 = 1 411 200 bit/s data rate = (1411200 / 8)/24 = 172.3 Kilobytes per second 53
Q 27 Digitising Sound - Questions 6. Match the words on the left to their meanings on the right. 1. For the following text insert the words underneath into the blank spaces. Sound is an sound wave. To save this on a computer a converts it to an electrical analogue signal. An converts it to a _ which the computer can then save. digital signal microphone analogue analogue-to-digital converter 2. The accuracy of each sample is determined by what? ADC DAC Analogue Digital 7. A sound file is recorded as a 3-bit sound file. The chart to the right is for 1 second of the file. Continuous Analogue to Digital converter Discrete Digital to Analogue converter [4] 3. The quality of sound saved in an audio file is determined by both the bit rate and what? 4. Look at a sound wave to the right. The bit rate is shown on the y-axis and time on the x-axis. What is the value in binary of the sample at the following time intervals? time = 0: time = 1: time = 4: 5. If the bit rate of a sound file is increased what happens to the audio quality? Fill in one circle. The bit rate has no effect It increases It decreases [5] The following data is stored in the file. Plot the sound wave that will be generated by the stored file. 1110100111 8. An audio CD has a bit-rate of 16 bits. It has a frequency (sample rate) of 440Hz for each of the left and right speakers. a) How many bits are required to store 1 second of audio for both speakers? _ bits b) How many bytes are required to store 1 minute of audio? bytes c) A 650MB CD can store how many minutes of audio? _mins. 9. A DVD-Audio disc uses 96KHz sampling at a 24 bit depth for 6 channels (Full surround 5.1). a) What is the bit rate required for this disc? _ Mbit/s b) How much storage space will be required for 1 hour? GB [3] 54