Counting the pixels in an image

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Quick Questions

Shade in the A or B circle for each question.

1. How do you work out the total number of pixels in an image?

A width * height

B width + height

2. An image is 1920 x 1080 pixels. What is the total number of pixels?

A 2,073,600

B 3,000

3. Which one is the correct first step when calculating the file size of a bitmap image?

A Find the total pixels: width * height

B Multiply by the sample rate

4. An image is 10 pixels wide and 4 pixels high. How many pixels is it in total?

A 14 pixels

B 40 pixels

5. In the image 1920 x 1080, what does 1920 stand for?

A The number of rows (height)

B The number of pixels in each row (width)

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1pixels = width * height		AThe answer 3000 is just the two numbers added together (1920 + 1080). The product is 2,073,600 pixels.
21920 * 1080 = 2073600		Btotal_pixels = 800 * 600
3The width is the number of pixels in each row, and the height is the number of rows, so multiply them to get the total pixels		CWidth and height are added instead of multiplied. Adding gives only the pixels along two edges, not the total in the whole image. It should be width * height.
4Counting the pixels is the first step before working out the file size of a bitmap image		Dfile_size = total_pixels * colour_depth
5pixels = width + height		EWorks out the total number of pixels in an image by multiplying the pixels per row by the number of rows
61920 * 1080 = 3000		FA 1920 x 1080 image contains 2,073,600 pixels in total

Definitions

pixels = width * height

Fill in the Blanks

Complete each line of code by filling in the missing parts.

pixels = width _ height

______ = width * height

pixels = _____ * height

pixels = width * ______

width = 1920 height = 1080 OUTPUT _____ * height

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A width = 1920

B height = 1080

C pixels = width * height

D print(pixel)

Question 2

A width == 800

B height = 600

C pixels = width * height

D print(pixels)

Question 3

A width = 1920

B height = 1080

C pixels = width + height

D print(pixels)

Question 4

A cols = 640

B rows = 480

C total = cols * cols

D print(total)

Question 5

A width = 1024

B heigth = 768

C pixels = width * height

D print(pixels)

Answer Key

Counting the pixels in an image

Section A: Quick Questions

A — width * height

A — 2,073,600

A — Find the total pixels: width * height

B — 40 pixels

B — The number of pixels in each row (width)

Section B: Match the Pairs

pixels = width * height → Works out the total number of pixels in an image by multiplying the pixels per row by the number of rows

1920 * 1080 = 2073600 → A 1920 x 1080 image contains 2,073,600 pixels in total

The width is the number of pixels in each row, and the height is the number of rows, so multiply them to get the total pixels → total_pixels = 800 * 600

Counting the pixels is the first step before working out the file size of a bitmap image → file_size = total_pixels * colour_depth

pixels = width + height → Width and height are added instead of multiplied. Adding gives only the pixels along two edges, not the total in the whole image. It should be width * height.

1920 * 1080 = 3000 → The answer 3000 is just the two numbers added together (1920 + 1080). The product is 2,073,600 pixels.

Section C: Definitions

pixels = width * height — Total pixels = pixels per row times number of rows. For 1920 x 1080 that is 2,073,600 pixels. This is the first step when working out image file size.

Section D: Fill in the Blanks

pixels = width * height

width = 1920
height = 1080
OUTPUT width * height

Section E: Spot the Error

D — The total was stored in a variable called pixels, but this line prints pixel, which does not exist. It should be print(pixels).

A — A double equals sign compares two values. It does not store one. The width must be assigned with a single equals sign: width = 800.

C — Total pixels is pixels per row times number of rows, so the values must be multiplied, not added. It should be pixels = width * height.

C — The width (pixels per row) is multiplied by itself, so the number of rows is never used. It should be total = cols * rows.

B — The variable name is misspelled. Line 3 uses height, which would not exist, so the program would crash. It should be height = 768.