Be able to adjust the contrast of an image

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

Common File Formats (Cambridge Syllabus)

Software‑Agnostic Workflow

1. 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).

2. 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).

3. Crop

   • Select the image, choose Crop and drag the black handles to the required area.
   • Press Enter or click outside to confirm.

4. Rotate / Flip

   • Common options: Rotate 90° clockwise, Rotate 180°, Flip Horizontal, Flip Vertical.
   • In Word/PowerPoint: Picture Tools → Format → Rotate.

5. 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 %).

6. 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.

7. 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.

8. 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.

9. Save the edited image

   • Always use File → Save As to keep the original untouched.
   • Adopt a clear naming convention, e.g., img01original.jpg, img01resized.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)

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

1. 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.
2. 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.
3. 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.
4. 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

Mastering these image‑handling skills will give you confidence in both the practical tasks and the written questions of the Cambridge IGCSE ICT examinations.