How images are stored (pixels, resolution) in Intro to Computing - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a pixel grid with 3 rows and 3 columns.

Intro to Computing

pixels = [[0 for _ in range([1])] for _ in range(3)]

Drag options to blanks, or click blank then click option'

A3

B5

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 1 instead of 3 for the number of columns.

Confusing rows and columns in the loops.

The code creates a 3x3 grid representing pixels. The inner loop runs 3 times to create 3 columns.

2fill in blank

medium

Complete the code to calculate the total number of pixels in an image with width 1920 and height 1080.

Intro to Computing

total_pixels = 1920 [1] 1080

Drag options to blanks, or click blank then click option'

A+

B*

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Adding width and height instead of multiplying.

Dividing or subtracting width and height.

Total pixels in an image is width multiplied by height.

3fill in blank

medium

Fix the error in the code to access the pixel at row 2, column 3 in a 2D pixel list.

Intro to Computing

pixel_value = pixels[[1]][2]

Drag options to blanks, or click blank then click option'

A1

B3

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 or 3 as index which is out of range.

Confusing row and column indices.

Indexing starts at 0, so row 2 is index 1.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps pixel coordinates to their brightness if brightness is greater than 100.

Intro to Computing

bright_pixels = {(x, y): pixels[x][y] for x in range([1]) for y in range([2]) if pixels[x][y] > 100}

Drag options to blanks, or click blank then click option'

Awidth

Bheight

C256

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using fixed numbers instead of variables for dimensions.

Confusing width and height in the loops.

The comprehension loops over height (rows) for x and width (columns) for y to check each pixel's brightness.

5fill in blank

hard

Fill all three blanks to create a dictionary that stores pixel brightness for pixels with brightness above 150.

Intro to Computing

bright_pixels = { [1]: pixels[[2]][[3]] for [2] in range(height) for [3] in range(width) if pixels[[2]][[3]] > 150 }

Drag options to blanks, or click blank then click option'

A(x, y)

Bx

Cy

D(x.upper(), y.upper())

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase keys incorrectly.

Mixing up x and y in loops.

The dictionary uses (x, y) tuples as keys and loops over height and width for coordinates.

Practice

1. What is a pixel in a digital image?

easy

A. A tiny dot that makes up the image

B. A type of file format

C. A color filter used in cameras

D. A tool to edit images

Solution

1. Step 1: Understand the basic unit of an image

   An image is made up of many small dots called pixels.

2. Step 2: Identify what a pixel represents

   A pixel is the smallest part of an image that holds color information.

3. Final Answer:

   A tiny dot that makes up the image -> Option A

4. Quick Check:

   Pixel = tiny dot [OK]

Hint: Pixels are tiny dots forming the image [OK]

Common Mistakes:

• Confusing pixels with file formats
• Thinking pixels are editing tools
• Mixing pixels with camera filters

2. Which of the following correctly describes image resolution?

easy

A. The size of the image file in megabytes

B. The color depth of the image

C. The number of pixels in width and height

D. The type of camera used to take the image

Solution

1. Step 1: Define resolution in terms of pixels

   Resolution means how many pixels are arranged horizontally and vertically.

2. Step 2: Differentiate resolution from file size and color depth

   File size and color depth are different properties; resolution is about pixel count.

3. Final Answer:

   The number of pixels in width and height -> Option C

4. Quick Check:

   Resolution = pixel count width x height [OK]

Hint: Resolution counts pixels horizontally and vertically [OK]

Common Mistakes:

• Confusing resolution with file size
• Mixing resolution with color depth
• Thinking resolution depends on camera type

3. An image has a resolution of 1920 x 1080 pixels. How many pixels does it contain in total?

medium

A. 1920 pixels

B. 200,000 pixels

C. 1080 pixels

D. 2,073,600 pixels

Solution

1. Step 1: Multiply width and height pixels

   Total pixels = 1920 (width) x 1080 (height) = 2,073,600 pixels.

2. Step 2: Confirm calculation

   Multiplying these gives the total number of pixels in the image.

3. Final Answer:

   2,073,600 pixels -> Option D

4. Quick Check:

   1920 x 1080 = 2,073,600 [OK]

Hint: Multiply width by height for total pixels [OK]

Common Mistakes:

• Adding instead of multiplying width and height
• Confusing width or height as total pixels
• Ignoring the multiplication step

4. A student says an image with resolution 800 x 600 has 1400 pixels. What is wrong with this statement?

medium

A. They confused pixels with file size

B. They added width and height instead of multiplying

C. They multiplied width and height correctly

D. They used the wrong units for pixels

Solution

1. Step 1: Calculate correct total pixels

   Total pixels = 800 x 600 = 480,000 pixels, not 1400.

2. Step 2: Identify mistake in calculation

   1400 is close to 800 + 600 = 1400, so they added instead of multiplied.

3. Final Answer:

   They added width and height instead of multiplying -> Option B

4. Quick Check:

   Pixels = width x height, not addition [OK]

Hint: Pixels = width x height, not width + height [OK]

Common Mistakes:

• Adding instead of multiplying pixels
• Confusing pixels with file size
• Misunderstanding resolution units

5. If you double the resolution of an image from 640 x 480 to 1280 x 960, how does the total number of pixels change?

hard

A. It quadruples

B. It stays the same

C. It doubles

D. It halves

Solution

1. Step 1: Calculate original total pixels

   Original pixels = 640 x 480 = 307,200 pixels.

2. Step 2: Calculate new total pixels

   New pixels = 1280 x 960 = 1,228,800 pixels.

3. Step 3: Compare new and original pixels

   1,228,800 ÷ 307,200 = 4, so total pixels quadruple.

4. Final Answer:

   It quadruples -> Option A

5. Quick Check:

   Doubling width and height quadruples pixels [OK]

Hint: Doubling width & height quadruples pixels [OK]

Common Mistakes:

• Thinking doubling resolution doubles pixels
• Ignoring multiplication effect on total pixels
• Confusing resolution with file size