Digital Technologies Hangarau Matihiko Level 1

Transcription

1 External Assessment Resource Digital Technologies Hangarau Matihiko Level 1 This resource supports external assessment against Achievement Standard (1.11) 1 Standard title: Credits: Resource title: Demonstrate understanding of compression coding for a chosen media type 3 credits Family Wedding Resource reference: Digital Technologies Hangarau Matihiko (1.11)Version 1 This resource provides: models of common assessment tasks. a model response that illustrates key aspects the assessment a schedule Achievement standard: 1.11 Standard title: Credits: Resource title: External Assessment Resource Demonstrate understanding of compression coding for a chosen media type 3 credits Family Wedding Resource reference: Digital Technologies Hangarau Matihiko 1.11A Version 1 Teacher/Kaiako Guidelines The following resources are supplied to enable teachers/kaiako to carry out valid and consistent assessment using this external assessment resource. Teachers/kaiako need to befamiliar with the outcome being assessed by the achievement standard. The achievement criteria and the explanatory notes contain information, definitions, and requirements that are crucial

2 when interpreting the standard and assessing students/ākonga against it. Context/Te Horopaki This assessment activity requires students to respond digitally to the material presented in the common assessment task to demonstrate their understanding of Human computer interaction. The context of this example is an online interface. Conditions/Ngā Tikanga Teaching and learning should take place within a collaborative environment consistent with the Nature of Technology. Candidates should produce an individual response to the assessment stimulus for the purposes of summative assessment. The 3 credits for the achievement standard indicates that approximately 30 hours needs to be allocated for teaching, learning (in and out of the classroom) and assessment in a programme of study. This achievement standard does not assess writing style or format however the response must communicate the candidates understanding. You may give students guidance on appropriate style and format for their response. Assessment specifications for this achievement standard can be found on the NZQA s NCEA subject page Submission of candidate responses Candidate responses must be submitted following the school s authenticity verification process. The schools authenticity process must be informed by Authenticity requirements for non- examination external assessment» NZQA Candidate responses must be uploaded digitally on the specified date The Common Assessment Task Will be made available online at the start of term 3 The formats used in 2018 and following years will be similar to either Assessment Type One or Assessment Type Two. In each year a different media type and scenario will be presented. Resource requirements/ngā Rauemi Additional information/he Kōrero Atu

3 External Assessment Resource Achievement standard: AS Standard title: Credits: Demonstrate understanding of compression coding for a chosen media type 3 credits Resource reference: Digital Technologies Hangarau Matihiko 1.11A Version 1 Introduction/Kupu Arataki Student/Ākonga instructions This assessment activity requires you to respond to the question(s) or real- world scenario provided to you. Your response will be assessed on how comprehensively you demonstrate your understanding of compression. Teacher note: Insert due dates and timeframes Task/Hei Mahi Model Assessment Type One. Respond to the scenario and instructions below. In your response, you should seek to demonstrate comprehensive understanding of compression A family member has recently had their wedding and they asked you to take photos. You used a professional quality digital camera that takes images in an uncompressed format. You have taken over 200 photos. You have now been asked to arrange the following: Format and provide high quality images to be professionally printed, some will be placed in a photo album and other will be printed A3 size to the framed and hung on the wall Format and provide images for a web site that is to be created. There will be thumbnails that are 128 pixels wide that will link to images that are 1920 pixels wide At the wedding reception, a TV will be set up with a USB stick of the photos playing a slideshow of the photos Some sample images need to be sent by FAX for checking and approval As a computer scientist write a report to: explain using examples how each compression method works explain how all three image formats, uncompressed, compressed- lossless, and compressed- lossy allow the wedding photos to be used in the range of ways required. For each use discuss which compression type is best.

4 Task/Hei Mahi Model Assessment Type two. Respond to the scenario and questions below. In your response, you should seek to demonstrate comprehensive understanding of compression A family member has recently had their wedding and they asked you to take photos. You used a professional quality digital camera that takes images in an uncompressed format. You have taken over 200 photos. You have now been asked to arrange the following: Format and provide high quality images to be professionally printed, some will be placed in a photo album and other will be printed A3 size to the framed and hung on the wall Format and provide images for a web site that is to be created. There will be thumbnails that are 128 pixels wide that will link to images that are 1920 pixels wide At the wedding reception a TV will be set up with a USB stick of the photos playing a slideshow of the photos Some sample images need to be sent by FAX for checking and approval As a computer scientist answer the following questions. 1. Why are image files compressed? 2. How can the colours in an uncompressed photo image be presented in bits? 3. How does a lossless image compression method work? 4. From the scenario, what are two uses when lossy compression is appropriate? 5. What are the advantages and disadvantages of lossy and lossless compression methods? 6. How do the algorithms used to compress images work to minimise the perceived loss? 7. For each example of use from the scenario, which is the appropriate form of compression coding? Why?

5 1 Why are image files compressed? Model response The photos from the wedding are being used for a number of purposes including being placed on a website, put on a USB stick and Faxed for checking. Uncompressed images take up quite a bit of space on the devices they will be stored on and are slow to transfer over the Internet. They can also be slow to copy or load from a USB stick. It is therefore good to compress the images so they take up less space I took a sample photo and saved it in 3 formats to demonstrate the types of uncompressed and compressed files that might be used BMP is uncompressed and is just over 9MB PNG is compressed lossless and is just over 5MB JPG is compressed lossy and is just over 1MB I did the maths and 1300/9217 is.14 or 14% of the original size. So you can fit 7 times as many photos in the same amount of space or the photo will download 7 times faster. 2 How can the colours in an uncompressed photo image be presented in bits? There are three primary colours of light. Red, Green and Blue. This is often shortened to RGB All images from Wikipedia These three colours can be combined to make up any colour. On a computer monitor or TV there are groups of these 3 colours together that make up picture elements or pixels. Each of the three colours in a pixel can have 256 different levels of brightness. 256 times 256 times 256 is 16,777,261 so there are over 16 million possible colours that each pixel can show. Computers use zeros and ones to make up numbers, this is called binary. The zeros and ones are combined into groups of 8 zeros and ones which are called a byte I have produce a table below showing how the numbering system works Binary number Decimal Number

6 I went into paint and painted an orange background as orange is my favourite colour. Next I clicked on Edit Colours. Below is a screenshot of what I got You can see in the Edit Colors box it has Red: 255, Green: 127, Blue 39 Next I used the calculator to convert these numbers into binary Colour Decimal Binary Red Green Blue So you can see that in a photo there are millions of pixels, each pixel is red, green and blue and they can have 256 levels of brightness which is over 16 million colours in total. The 256 levels of each colour are one byte of 8 binary characters of zero or one.

7 3 How does a lossless image compress method work? There are a number of lossless compression methods. A lossless compression lets the image be made smaller without any loss of quality when it is un- compressed In the wedding some photos are being faxed. Faxing uses Run Length Encoding to transmit the photo, so I will show how that works, using a picture of a wedding cake I drew the picture of the wedding cake on the left, then changed all of the white squares to 0 and the black square 1 The next step is to count how many times there is a zero until it changes to a one, or how many times there is a one until it changes to a zero ,6, ,1,4,1, ,1,4,1, ,1,4,1, ,12, ,1,10,1,3 You can instead of each row one and five numbers. This is much faster Depending on the type of image sent the efficiently can vary see that, with this example, having to send 18 numbers for you only have to send between

8 4 What are two uses when lossy compression is appropriate? Images are going to be shown from a USB stick onto a TV at the reception. To fit as many photos on as possible and to make sure the photos can load quickly the lossy format JPG should be used On the website the large photos should also be in the lossy JPG format. As lossy JPG is the smallest file size it means the photos will download quickly onto the computer and take up the smallest amount of space on the server With large size photos being displayed on a screen the lossy JPG format is normally the best format to use. JPG has been designed for use with these types of images 5 What are the advantages and disadvantages of lossy and lossless compression methods? Lossy File Type - JPG Lossy images have the advantage of producing much smaller file sizes and faster download speeds. Lossy JPG compression is best suited for large size photographs such as the high- resolution wedding photos to be placed on the web site With large images the 8 by 8 blocks are not seen due to the high resolution of the file. This is because they are too small and there are so many of them that they are not an issue With photographs (as opposed to other image types), like the ones taken for the wedding they are of the natural world that is made of shading with few sharp edges. This means that the eye is unlikely to notice any obvious compression artefacts The disadvantage of lossy JPG images is with small size images or images that are not photographs as the compression artefacts can be easily noticed Below is the letter W. The first is just typed in, the second is snipped and saved as a JPG. You can see the compression algorithm does not work well with the sharp edges found in text W The same problem will also occur in the low resolution thumbnails of the wedding photos to be put on the web site. Because they are only 128 pixels wide it is only 16 8 by 8 blocks wide meaning the compression artefacts will be easily visible. Lossless File Type PNG With lossless file types there is only one real disadvantage and that is the file size. You can see from the table that PNG is 7 times the size of a JPG file It makes no sense to use lossless PNG file for large photos, lossy JPG does a good job The disadvantage of lossless PNG files is the large file size The advantage of lossless PNG files is when there are low resolution photographs such as the ones used for the thumbnails. The file size might be 4 times greater. However, with low resolution images even though the ratio does not change the number of KBs becomes unimportant You can see above that the 128 pixel JPG is 9KB and the 128 pixel is 41KB

9 Image from Wikipedia In the 128 pixel thumbnails above you can see the PNG on the right is much clearer than the JPG on the left Summary JPG is best for high resolution photographs as it is much smaller and it is hard to tell the difference between lossless and lossy. For images that are not photos or for low resolution images then lossless PNG is better. With small images there is no appreciable space saving and the images are much clearer. Even with high resolution images that are not photos PNG is still the better option 6 How do the algorithms used to compress images work to minimise the perceived loss? In nature most colours shade from one colour to another and humans are used to viewing the world with very few sharp edges. Even when there is a sharp edge like in a window frame or lamppost the way light shines on means there is not normally a perceptible sharp edge JPG compression takes advantage of this by dividing images into blocks of 8 by 8 pixels a total of 64 pixels Image from Wikipedia The compression is based on a Discreet Cosine Transform which makes the block appear close to the original shading so that with a high resolution image it is perceived as the same image as the uncompressed image, while taking a fraction of the original size.

10 7 For each example of use from the scenario, which is the appropriate form of compression coding? Why? The wedding photos are going to be used for a number of reasons. A range of different file types are needed. This includes uncompressed, compressed lossless and compressed lossy Original photos The photos were taken in an uncompressed format on the camera. This is most likely RAW format which is referred to as a digital negative. This is because it contains the unprocessed image data which can still be processed in a similar way to a film negative. Once RAW format images are changed to a compressed format you can no longer make adjustments like the one shown below. Adjustments need to be made before the images are compressed Image from Wikipedia USB Drive at the Reception At the reception a format needs to be used that can be read by the TV and is fast to load. For this reason, the lossy JPG file format is the best choice. The images shown on a TV are high resolution photographs so there is no advantage using PNG format and the TV may not be able to read RAW format files. With the smaller file size, they could be copied onto the USB drive and they would all fit even if the drive was quite small Faxing the Photos When the photos are faxed they will be sent using lossless Run Length Encoding as that is what fax machines use to transmit the data. Because it is a photo being sent this is unlikely to save any time or data as there would not be any white or black areas in a row. The image would be of poor quality and most likely be printed in grey scale Images for the website The large images for use on the website should be saved in lossy JPG format. As they are high resolution photographs there would be little or no difference in the quality of the images displayed compared with lossless PNG. The images would however load considerably faster and take up much less space on the website With the thumbnails lossless PNG should be used. This is because the JPG artefacts would be very obvious on a low resolution photo. Additionally, with low resolution images the file size is very small for both JPG and PNG files. This means lossless PNG files would be much better Photos to be printed When photos are to be printed, especially large A3 size images you want the best possible quality. Because of this even though JPG is the ideal format for large photos it should not be used here as loading time and file size are probably not very important. PNG files would seem to make sense, however PNG files are saved using RGB colours which does not contain all colours. Printing is done in CYMK, which has a bigger range of colours than RGB

11 All images from Wikipedia All images from Wikipedia It would be better to convert the RAW image files to the lossless compressed format of TIFF as that supports CMYK colours that would result in printed picture with better colours

12 External assessment resource Digital Technologies 1.10A V1 for Achievement Standard 1.11 PAGE FOR TEACHER USE Assessment schedule/mahere Aromatawai: Digital Technologies Achievement Standard (1.11) Demonstrate understanding of compression for a selected media type Demonstrate in- depth understanding of compression for a selected media type Demonstrate comprehensive understanding of compression coding for a chosen media type 1. Why are image files compressed? Answers demonstrate understanding of reasons for compression 2. How can the colours in an uncompressed photo image be presented in bits? Answers should to show specific detailed knowlege about the use of bits 3. How does a lossless image compression method work? Answer should show understanding of the mechaims of lossless compression in the specific context of the scenario 4. From the scenario, what are two uses when lossy compression is appropriate? At least two specific examples should be provided See the linked examples 5. What are the advantages and disadvantages of lossy and lossless compression methods? The answer should make plain the advantages and or disadvantages in a specifc situation. 6. How do the algorithms used to compress images work to minimise the perceived loss? The relationship between perception and algorithm should be plain 7. For each example of use from the scenario, which is the appropriate form of compression coding? Why? Each real- world use example should be matched to the correct compression technique. The explanation for appropriateness must be technical. Final grades will be decided using professional judgement based on a holistic examination of the evidence provided against the criteria in the Achievement Standard. This resource is copyright Crown 2017 Page 1 of 1