1, 2, 3, code ! - Cycle 3 activities - Lesson 3.1. How to send an image

Summary

Students figure out how to send an image remotely. To do this, they learn that an image can be represented by a pixel grid. The learn about the notion of resolution as they see that the more pixels an image has, the clearer it becomes, but also the slower it is to send.

Key ideas

(see Conceptual scenario)

"Information"

  • An image can be represented by a grid of squares called pixels.

Inquiry-based methods

Observation, experimentation

Equipment

For the class

  • Magnets or blu-tack to attach finished work to the whiteboard.

For each group (four groups, A, B, C and D)

  • Handheld magnifying glass or microscope
  • Newspapers
  • Use large posters if there are no magnifying glasses or microscopes available
  • Photocopy of Image A, Handout 39, for each student in Group A, Image B for Group B, etc.
  • Handout 40 printed or photocopied on a slide or tracing paper and cut into 3 grids. Make sure there are extra grids.

For each student

  • Tracing paper (1/4 of an A4 page) and sharpened pencil, or slide and fine-tipped permanent marker
  • Sticky tape or paper clips

Glossary

Image, pixel, resolution

Duration

1 hour 30 minutes

 

 

Introductory question

The teacher explains that explorers want to photograph their discoveries, and send the photos to their base. “How can they send their photographs across long distances?” Students make suggestions: by courier, carrier pigeon, Facebook, scan or e-mail.

Even if students do not think of digitizing the photograph, the teacher then asks this question: “But what is an image?”

 

Research work: Defining an image (in groups)

The teacher hands out newspapers to each group. They ask the students to think about what an image is made of. The students mention materials: paper, cardboard and ink.  When this work is mentioned, the teacher hands out the magnifying glasses or microscopes. “Can you describe how the ink looks in these pictures? What color is it?”

Very quickly the students will see that newspaper print is made up of thousands of tiny dots, and that the colors of these dots are in fact very limited. The teacher introduces the word ‘pixel’ (“picture element”) and helps draw a conclusion such as the following: “A photograph is made up of tiny colored dots, called pixels. From a distance, we can’t see the pixels, but an image that appears continuous”.

 


Studying magazines through a microscope. Handheld magnifying glasses can be used if the print chosen is of basic quality, such as newspaper. 6th grade class in Paris

 

Teaching notes:

  • Depending on the quality of the microscopes and the print (laser versus inkjet, for example), the pixel overlap may make it hard to see them in magazines and photographs. This is why we recommend using newspaper. However, you can test the microscopes before the lesson to see if the pixels can be seen on a material other than newspaper.
  • If there are no magnifying glasses available, pixels are visible to the naked eye on large-format advertisement posters. Pixels are not visible the further we are away from them.
  • During the following lesson, we will look at the pixels again, but on a computer screen, to conclude that images are made up of tiny, discontinuous spots of various shades (see scientific note below).

Scientific notes:

  • For technical reasons, pixels on screens are rows of tiny squares (to be more precise, on color screens each square pixel is in fact made up of three rectangular sub-pixels: red, green and blue, from left to right. See below). On paper, the colored dots may overlap (the white in the paper is also used in reproducing colors).
  • The pixel colors depend greatly on the device used. On a computer, tablet or smartphone screen, pixels exist in Red, Green and Blue. This is known as RGB printing. For images printed in four colors, these are Cyan, Magenta, Yellow and Black. This is known as CMYB printing. Two-color printing simply requires two complementary ink colors (blue and orange, for example). The combination of these few colors enables a wide variety of colorful creations to be reproduced. 

 

Exercise (in pairs): How many pixels are needed for our image?

The teacher puts the conclusion into context: “To send an image, you need to send all the pixels, one by one.” The teacher suggests an exercise that enables the students to fully grasp this key idea and go further into depth by pixelating an image, i.e. replace it with a pixel grid.

The teacher divides the class into four groups, and each group pixelates one of the four images (A, B, C, or D) on Handout 39.

  • A copy of the image for their group. Don’t forget to remind them that the other groups cannot know which image they have.
  • Grid 1 on Handout 40 printed on a slide or tracing paper
  • Plain tracing paper (a quarter of an A4 page) with a pencil or slides (a quarter of an A4 slide) and fine-tipped permanent markers (in this example, we used tracing paper);
  • Sticky tape or paper clips.

The students must place the grid over the image, ensuring the image corners are aligned within the crop marks. They place the tracing paper on top, attach the 3 papers with sticky tape or paper clips, and color in (on the tracing paper) the boxes that the image outline crosses.

 


6th grade class in Paris

 

When they have finished, they write the image letter (A, B, C or D) and the number of the grid (1, in this instance) on their Handout. Each group hands the teacher one or two copies of the pixelated image with 64 pixels, choosing the darkest pencil-colored boxes. The teacher displays the group work in 4 columns (“Image A”, “Image B”, etc.) allowing room for 3 subsequent lines (which will be named “Grid 1”, “Grid 2”, and “Grid 3”). The images pixelated with Grid 1 are unidentifiable.

 


Image A pixelated with Grid 1. Anne-Marie Lebrun’s 4th grade class (Bourg-la-Reine)

 

“How can we make these images clearer so that we can recognize the picture?” The students will come up with two ideas: either by using different shades of gray, instead of just black and white, or by adding more pixels. The first option — if it is suggested spontaneously — should be written on the whiteboard, and studied in further detail at a later stage (Lesson 3.3). 

To explore the second suggestion, the teacher hands out the finer grids in Handout 40

 


A student works on pixelating Image B with Grid 2. Anne-Marie Lebrun’s 4th grade class (Bourg-la-Reine)

 

Teaching notes:

  • Hand out Grids 2 and 3 depending on the speed at which students in the same group work, in order to avoid lengthening the lesson time. Each student should complete the exercise at least once.
  • An alternative method would be to hand out Grids 1, 2 and 3 to the groups from the start, rather than having the whole class use Grid 1. This saves 15 minutes, but the discussion on how to improve the first result will not be possible.
  • Expect to repeat the instructions “color in the boxes fully or leave them blank” and “color in the boxes fully where the image outline passes through” several times. Don’t hesitate to show the students how this should be done, using the whiteboard.

The teacher asks the pairs using Grid 2 to show their result beneath the previous images of their group. If the students from the other three groups appear to identify the picture, the teacher adds a sub-heading with their guess (apple? peach? pear?). Then they ask the pairs using Grid 3 and write down the new guesses.

 


From left to right, Image D pixelated with Grids 1, 2 and 3. Right: image pixelated with a 64x64=4,096-pixel grid (too long to use in the classroom).

 

Group discussion

The teacher asks the students if adding more pixels is an effective solution to the issue encountered (i.e. how to render the image intelligible despite the pixelation). The term “resolution” is introduced. “When we increase the number of pixels, the image resolution is higher, and it is easier to see what the picture is.”

By comparing pixelated images with varying resolutions, the resolution requirement can be explained. Certain images were identifiable when using Grid 2, whereas for others Grid 3 was needed. The teacher must remind the class that all pixels must be sent one by one to the base before the image can be reproduced. They highlight the necessary compromise between resolution and ease of transfer: “If we had limited means, what resolution would be sufficient?” For each pixelated image, the class discusses and chooses a compromise resolution. For example, the lowest-quality resolution which renders at least three of the four images identifiable, or the resolution which at least shows the difference between the four images.

 


Images A, B, C and D pixelated using grids 1, 2 and 3. Anne-Marie Lebrun’s 4th grade class (Bourg-la-Reine)

 

Conclusion and lesson recap activity

The class summarizes together what they learned in this lesson:

  • An image is made up of pixels.
  • To send an image, you need to send all of its pixels one by one.
  • The more pixels we use, the closer the pixelated image will be to the original, but it takes up more memory and takes longer to send.

Students write down these conclusions in their science notebook. The teacher updates the “Information” section of the poster entitled “What is computer science?”

 

Further study

  • Pixelated images can be used for artistic applications. For example, Post-it art (see examples here: http://www.postitwar.com) can be used to create posters or decorate walls with Post-its© that act as pixels. This could be a good opportunity to talk about art history, and pointillism in painting. See also the activity suggested in Cycle 2: Lesson 1.4 and Handout 18.
  • This work can extend to include photomosaics. In photomosaics, the pixel is made of an image. Looking closer, you can see the details of a myriad of miniature photos, whereas from a distance an entirely different image is seen. For example, miniature photos may be pictures of the students, and the overall image could be a panoramic image of the school, or a mythical creature or landscape, etc. Free software, such as AndreaMosaic (http://www.andreaplanet.com/andreamosaic) offers the possibility of creating photomosaics.

 

 


  Sequence III Lesson 3.2 >>

 

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