---
title: "Phong shading and Mach-band effect"
subject: "Computer Graphics"
chapter: 8
key: "semester_5/computer_graphics:phong-mach-band-effect"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/phong-mach-band-effect
site: BCT Engineering
---

# Phong shading and Mach-band effect

Computer Graphics · Chapter 8: Illumination and Surface Rendering methods

- Asked in: 2076 Chaitra, 2 marks
- Video: [Shading Model: Flat Shading | Gouraud Shading | Phong Shading Algorithm](https://www.youtube.com/watch?v=jr-PoIsqM7g) — Vinay Mishra
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/phong-mach-band-effect
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-8

## Question

Can we use the Phong shading method to reduce the Mach-band effect?

## Short answer

Yes, Phong shading interpolates normals rather than intensities, resulting in smooth intensity changes across polygon boundaries. This greatly reduces Mach bands, though not completely, and also correctly renders highlights.

## Answer

**Yes.** Phong shading greatly reduces the Mach-band effect. It interpolates the surface normals, so the normal (and therefore the intensity) changes smoothly across each polygon and from one polygon to the next. As a result, the intensity changes gradually at polygon borders, unlike Gouraud shading, which interpolates intensity linearly and has abrupt changes in slope at the edges. Phong shading does not remove the effect completely, but the result is much smoother, and highlights are also shown correctly. The price is a higher computation cost.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Can Phong shading be used to reduce the Mach-band effect?
   A. No, it gives a constant intensity per polygon
   B. No, it produces more Mach bands than Gouraud
   C. Yes, because it interpolates normals so intensity changes more smoothly across polygon borders
   D. Yes, because it computes only one intensity per polygon
   Answer: C. Interpolated normals make the intensity vary gradually between polygons. (Key point: Answer)

2. What does Phong shading interpolate, compared to Gouraud?
   A. Vertex intensities linearly
   B. Vertex depths
   C. Polygon colours only
   D. Surface normals, then it computes the intensity at each pixel
   Answer: D. Gouraud interpolates intensities; Phong interpolates normals. (Key point: What is interpolated)

3. What is the price of Phong shading's better results?
   A. Higher computation cost
   B. Missing specular highlights
   C. A faceted look
   D. Needing a light at infinity
   Answer: A. Illumination is calculated per pixel, which is more expensive. (Key point: Cost)

## Related in Computer Graphics

- [Phong shading method](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aphong-shading-method) — Computer Graphics, chapter 8, asked 9×
- [Compare Gouraud and Phong shading](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Acompare-gouraud-phong) — Computer Graphics, chapter 8, asked 7×
- [Fast Phong shading](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Afast-phong-shading) — Computer Graphics, chapter 8, asked 2×

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