Topic 12 – Images (Cambridge IGCSE ICT 0417)
Learning Objectives
- Insert an image into a document, presentation or spreadsheet and position it accurately.
- Resize, crop, rotate/flip and group images while preserving the required aspect ratio.
- Adjust brightness and contrast – including the underlying mathematics.
- Reduce file size by changing resolution or colour‑depth.
- Save the edited image correctly and retain the original file.
- Choose the appropriate file format (PNG, JPEG, GIF, BMP, TIFF) for the task.
Why Work with Images?
- Enhance the visual impact of reports, presentations and web pages.
- Make scanned text or diagrams clearer and more readable.
- Prepare graphics for printing or online publishing where file‑size limits apply.
- Demonstrate competence in the practical tasks required in Papers 2 & 3.
Key Concepts & Terminology
| Term | Definition / ICT Use |
|---|
| Resolution | Number of pixels per inch (ppi) or total pixel dimensions (e.g., 1024 × 768). Determines image clarity and file size. |
| Aspect ratio | Proportion of width to height (e.g., 4:3, 16:9). Maintaining it prevents distortion when resizing. |
| Crop | Removing unwanted outer areas of an image. |
| Rotate / Flip | Changing the orientation of an image (90°, 180°, horizontal or vertical mirror). |
| Brightness | Overall lightness of an image; each pixel value p is shifted up or down: p' = p + b, where b is the brightness offset. |
| Contrast | Difference between the lightest and darkest parts. For an 8‑bit image (0–255):
p' = 128 + c × (p − 128)
- c > 1 → increase contrast
- c = 1 → no change
- 0 < c < 1 → decrease contrast
Results are clipped to the 0–255 range. |
| Colour depth | Number of bits used for each pixel (e.g., 24‑bit = 16 777 216 colours, 8‑bit = 256 colours). Reducing depth lowers file size roughly in proportion to the bit reduction. |
| Layer / Group | Stacking images so they can be moved or formatted together. |
| File‑size reduction | Lowering resolution, reducing colour depth, or saving in a compressed format (JPEG, PNG, GIF, BMP, TIFF). |
| File format | Container that defines how pixel data is stored. See the “Common File Formats” table below. |
Common File Formats (Cambridge Syllabus)
| Format | Typical Use | Compression | Transparency |
|---|
| JPEG | Photographs, complex colour images | Lossy (adjustable quality) | No |
| PNG | Web graphics, screenshots, images needing loss‑less quality | Lossless (deflate) | Yes (alpha channel) |
| GIF | Simple graphics, animations (max 256 colours) | Lossless (LZW) | Yes (binary transparency) |
| BMP | Uncompressed raster images (rarely used in exams) | None (large files) | No |
| TIFF | High‑quality scans, printing | Lossless or LZW‑compressed | Yes (optional) |
Software‑Agnostic Workflow
- Insert the image
- Use Insert → Picture → From File (Word, PowerPoint, LibreOffice) or File → Open (stand‑alone editors).
- Turn on gridlines/rulers and open the Position dialog to set exact X‑ and Y‑coordinates (e.g., 2 cm from the left margin).
- Resize
- Drag a corner handle while holding Shift (or enable “Maintain aspect ratio”).
- Or enter exact dimensions in the Size box (e.g., 150 mm × 100 mm).
- Crop
- Select the image, choose Crop and drag the black handles to the required area.
- Press Enter or click outside to confirm.
- Rotate / Flip
- Common options: Rotate 90° clockwise, Rotate 180°, Flip Horizontal, Flip Vertical.
- In Word/PowerPoint: Picture Tools → Format → Rotate.
- Adjust Brightness
- Mathematical model:
p' = p + b (positive → lighter, negative → darker).
- In most editors use a slider or type a numeric offset (e.g., +20 % or ‑15 %).
- Adjust Contrast
- Use the formula
p' = 128 + c × (p − 128) (see “Contrast Adjustment – Quick Reference”).
- Enter a contrast factor c or move the contrast slider.
- Group / Layer images
- Select multiple pictures (Ctrl‑click) → Group → Group. The group behaves as a single object.
- In GIMP/Photoshop use the Layers panel – keep layers separate for later edits or Merge Down to flatten.
- Reduce file size (optional)
- Resolution: Image → Image Size → lower pixel dimensions (e.g., 1920 × 1080 → 800 × 450). Keep the aspect ratio locked.
- Colour depth: Convert 24‑bit → 8‑bit via Image → Mode → Indexed (GIMP) or “Save As → PNG‑8”. Approximate size reduction ≈ ⅓.
- Compression format: Save as JPEG (quality 70 %–80 % for photos) or PNG for loss‑less graphics.
- Quick size estimate:
size (bytes) ≈ (pixels × colour‑depth) / 8. Example: 800 × 600 px @ 8‑bit ≈ 0.38 MB.
- Save the edited image
- Always use File → Save As to keep the original untouched.
- Adopt a clear naming convention, e.g.,
img01_original.jpg, img01_resized.png, img01_contrastAdjusted.tif.
- Choose the appropriate format (see “Common File Formats”).
Contrast Adjustment – Quick Reference
For an 8‑bit image (pixel values 0–255):
p' = 128 + c × (p – 128)
- Calculate the new value for each pixel.
- Clip results below 0 to 0 and above 255 to 255.
- Typical contrast factors:
- c = 0.5 – softening.
- c = 1.2 – moderate increase (exam‑style default).
- c = 2.0 – strong; may cause clipping.
Practical Examples (Software Snapshots)
| Task | MS Word / PowerPoint | LibreOffice Writer / Impress | GIMP / Photoshop |
|---|
| Insert & position |
Insert → Pictures → This Device → Layout Options → In Front of Text → Position tab for X/Y. |
Insert → Image → From File → Right‑click → Position and Size. |
File → Open → Image → Tools → Transform Tools → Move (use toolbox for exact coordinates). |
| Resize (keep aspect ratio) |
Drag corner handle + hold Shift OR Format → Size → lock aspect ratio. |
Right‑click → Position and Size → Keep ratio. |
Scale Tool → Set Width/Height → lock chain icon. |
| Crop |
Picture Format → Crop → drag handles. |
Right‑click → Crop Image. |
Tools → Transform Tools → Crop → drag rectangle. |
| Rotate 90° clockwise |
Picture Format → Rotate → Rotate Right 90°. |
Right‑click → Rotate → 90° Right. |
Image → Transform → Rotate 90° CW. |
| Adjust brightness & contrast |
Picture Format → Corrections → Brightness/Contrast (sliders). |
Picture → Adjust → Brightness/Contrast. |
Colors → Brightness‑Contrast (enter numeric values). |
| Group two pictures |
Select both → Picture Tools → Group → Group. |
Select both → Right‑click → Group. |
Layers panel → right‑click → Merge Down (or keep separate). |
| Reduce file size |
File → Save As → JPEG → set quality 75 %. |
File → Export → JPEG → quality slider. |
Image → Scale Image → lower dimensions; Export As → JPEG → quality. |
Suggested Workflow Diagram (textual outline)
[Open / Insert Image] → [Position Precisely] → [Resize (maintain ratio)] → [Crop] →
[Rotate / Flip] → [Adjust Brightness] → [Adjust Contrast] → [Group / Layer (if needed)] →
[Reduce File Size (optional)] → [Save As (new file, proper name)] → [Check final output]
Assessment Checklist (Exam‑style)
- Can you insert an image and set its exact X/Y position?
- Can you resize an image without distorting it?
- Can you crop, rotate and flip an image as required?
- Can you explain the effect of the contrast factor c and the brightness offset b on pixel values?
- Can you adjust brightness and contrast while avoiding clipping or colour shift?
- Can you group several images and move them as a single object?
- Can you reduce the file size by changing resolution or colour depth, and choose the correct file format for the task?
- Can you save the edited image without overwriting the original, using a clear naming convention?
Exam‑style Practice Questions
- Insert a photograph into a Word document, position it 3 cm from the left margin and 2 cm from the top of the page, and resize it to a width of 120 mm while keeping the aspect ratio.
- Using any image‑editing software, increase the contrast of a scanned diagram by a factor of 1.3 and reduce the brightness by 10 %. Explain how the pixel values change mathematically.
- Crop a screenshot so that only the central 800 × 600 pixel area remains, then rotate it 180° and save it as a PNG file under a new name.
- Two icons need to appear side‑by‑side in a PowerPoint slide. Group them, then reduce the combined file size so the slide file does not exceed 500 KB. Describe the steps you would take, including any resolution or colour‑depth changes.
Common Pitfalls & How to Avoid Them
- Clipping of details: After contrast or brightness changes, zoom to 100 % and check shadows/highlights. Reduce the factor if details disappear.
- Colour shift when adjusting contrast: Use “Luminance” or “Gray‑scale” adjustments where available; otherwise keep the contrast moderate.
- Distorted resizing: Always lock the aspect ratio or use a corner handle; entering only one dimension (width or height) is safer than both.
- Over‑compression of JPEGs: Quality below 60 % produces visible artefacts. Aim for 70 %–80 % for photographs, 90 %+ for text‑heavy images.
- Saving over the original: Use “Save As” and add a suffix (e.g.,
_edited) to preserve the source file for the exam.
- Mis‑choosing file format: Remember:
- JPEG – photographs, lossy.
- PNG – lossless, supports transparency.
- GIF – ≤ 256 colours, simple graphics or animation.
- BMP – uncompressed, large; rarely needed.
- TIFF – high‑quality scans, printing.
Quick‑Check Summary Table
| Operation | Key Menu Path (generic) | Typical Shortcut |
|---|
| Insert image | Insert → Picture → From File | Ctrl + V (paste after copy) |
| Position precisely | Format → Position → X/Y coordinates | Ctrl + Shift + P (varies) |
| Resize (keep ratio) | Format → Size → Lock aspect ratio | Shift + drag corner |
| Crop | Picture → Crop | Ctrl + Shift + X |
| Rotate 90° CW | Picture → Rotate → 90° Right | Alt + R |
| Flip Horizontal | Picture → Rotate → Flip Horizontal | Alt + H |
| Adjust Brightness | Picture → Corrections → Brightness | Ctrl + B |
| Adjust Contrast | Picture → Corrections → Contrast | Ctrl + C |
| Group images | Select → Group | Ctrl + G |
| Reduce resolution | Image → Image Size → Change dimensions | Ctrl + Alt + I |
| Reduce colour depth | Image → Mode → Indexed (or similar) | Ctrl + Shift + D |
| Save As (new file) | File → Save As | F12 |
Mastering these image‑handling skills will give you confidence in both the practical tasks and the written questions of the Cambridge IGCSE ICT examinations.