← ACARA Digital Technologies Years 7–8

Raster (bitmap) images

A picture made of a grid of tiny pixels, each storing its colour as numbers. Also called a bitmap. Photos are raster images and look blocky (pixelated) when zoomed in.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 31

Understanding Raster (bitmap) images in ACARA Digital Technologies Years 7–8

What it does: One pixel in a raster image stores colour numbers. Here the red, green and blue values make an orange pixel. Raster images are also called bitmaps because they are a map of pixels stored as bits. Lines and shapes from formulas are vector graphics. Raster images map the picture pixel by pixel. A photo is a raster image with a fixed number of pixels. Zooming in shows the blocky pixels instead of keeping it smooth.

Common mistakes: The number of pixels in a grid is width multiplied by height (4 x 3 = 12), not added. It should be pixels = width * height. Photos are raster images, so the value should be True. With False, the message about pixels would never print.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

🖨 Print worksheet + answer key Add Raster (bitmap) images to a class →

This module's key terms are also in a printable crossword (Modules 128–152) and word search (Modules 69–152).

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Learning
Duo
A photo taken on a phone is stored as a raster image. What is a raster image made of?
A grid of tiny pixels, each storing colour numbersLines and shapes described by maths formulas
Matches
A raster image is stored as lines and shapes drawn from maths formulas
↓
This describes a vector image. A raster image is stored as a grid of pixels, each with colour numbers.
Applying
Meanings
Term
raster image
Meaning
A picture made of a grid of tiny pixels, each storing its colour as numbers. Also called a bitmap. Photos are raster images and look blocky (pixelated) when zoomed in.
Practice
<div style="display:grid;grid-template-columns:40px 40px">
Testing
Gaps
<img src="photo." alt="A raster image">
Check Answer
Spots
3height = 3
4pixels = width + height

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Quick Questions

1. A photo taken on a phone is stored as a raster image. What is a raster image made of?
A. Lines and shapes described by maths formulas
B. A grid of tiny pixels, each storing colour numbers ✓
A raster image maps a picture pixel by pixel. Each pixel stores the numbers for its colour.
2. Another name for a raster image is a...
A. Vector
B. Bitmap ✓
Raster images are also called bitmaps because they are a map of pixels stored as bits.
3. You zoom in a long way on a photo and see blocky squares. What are you seeing?
A. Pixelation: the individual pixels of the raster image ✓
B. A fault where the file has been damaged
Zooming in on a raster image makes each pixel big enough to see, so the picture looks blocky (pixelated).
4. What does each pixel in a raster image store?
A. Numbers that describe its colour ✓
B. A copy of the whole picture
Each pixel holds only its own colour as numbers, such as RGB values. Together the pixels make the whole picture.
5. Which of these is usually stored as a raster image?
A. A photo of your friends at the beach ✓
B. A text message saying 'Hello'
Photos are raster images because a camera records the colour at each tiny point in the scene. Text is stored with character codes instead.

Match the Pairs

raster image → A picture made of a grid of tiny pixels, where each pixel stores its colour as numbers
bitmap → Another name for a raster image, a picture stored pixel by pixel
A photo is a raster image, so every pixel stores its own RGB colour numbers → pixel = (255, 128, 0)
Zooming in on a raster image makes the pixels visible, so it looks blocky (pixelated) → zoom 800% -> blocky squares
A raster image is stored as lines and shapes drawn from maths formulas → This describes a vector image. A raster image is stored as a grid of pixels, each with colour numbers.
A photo stays perfectly smooth no matter how far you zoom in, because it is a bitmap → Bitmaps get pixelated when zoomed in. Only the fixed pixels are enlarged, so the photo looks blocky.

Key Term

raster image
A picture made of a grid of tiny pixels, each storing its colour as numbers. Also called a bitmap. Photos are raster images and look blocky (pixelated) when zoomed in.

Fill in the Blanks

1. <img src="photo.bmp" alt="A raster image">
Hint: Type the file extension that is short for the other name for a raster image.
2. img { image-rendering: pixelated; }
Hint: Type the CSS value that shows blocky square pixels when an image is zoomed in.
3. pixel(2, 3) ← [255, 0, 0]
Hint: Type the highest value the red channel can hold, so this pixel is pure red.
4. FOR each pixel IN image DISPLAY colour of pixel END FOR
Hint: Type the word for each tiny square in a raster image that stores its own colour numbers.
5. width ← 4 height ← 3 total ← width * height
Hint: Type the variable that is multiplied by the width to count all the pixels in the grid.

Spot the Error

Question 1
A kind = "raster"
B width = 4
C height = 3
D pixels = width + height
Line D — The number of pixels in a grid is width multiplied by height (4 x 3 = 12), not added. It should be pixels = width * height.
Question 2
A photo_is_raster = False
B if photo_is_raster:
C print("Made of pixels")
Line A — Photos are raster images, so the value should be True. With False, the message about pixels would never print.
Question 3
A zoom = 8
B pixel_size = 1 / zoom
C blocky = pixel_size > 1
Line B — Zooming in makes each pixel appear bigger, so multiply by zoom. Dividing would shrink the pixels. It should be pixel_size = 1 * zoom.
Question 4
A grid = [[0, 255], [255, 0]
B row = grid[0]
C print(row[1])
Line A — The outer square bracket is never closed, so the grid of pixel values is a syntax error. It needs a final ] at the end.
Question 5
A for row in image:
B for pixel in row:
C print(pixel[3])
Line C — Each pixel stores three colour numbers (red, green, blue) at positions 0, 1 and 2. Position 3 does not exist. It should be pixel[0], pixel[1] or pixel[2].

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