---
title: "Compare Gouraud and Phong shading"
subject: "Computer Graphics"
chapter: 8
key: "semester_5/computer_graphics:compare-gouraud-phong"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/compare-gouraud-phong
site: BCT Engineering
---

# Compare Gouraud and Phong shading

Computer Graphics · Chapter 8: Illumination and Surface Rendering methods

- Asked in: 2073 Chaitra, 9 marks; 2078 Kartik, 8 marks; 2070 Chaitra, 8 marks; 2075 Asoj, 6 marks; 2068 Chaitra, 2 marks; 2068 Bhadra (old course), 3 marks; 2067 Mangsir (old course), 3 marks (asked 7 times)
- Video: [Difference between Gouraud and Phong shading | Computer Graphics](https://www.youtube.com/watch?v=d8daQT2lUEA) — Blazengg Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/compare-gouraud-phong
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-8

## Question

Compare and contrast Gouraud shading and Phong shading (mention the advantages of Phong shading over Gouraud shading).

## Short answer

Gouraud shading interpolates vertex intensities, while Phong shading interpolates normals and computes illumination per pixel. Phong correctly captures specular highlights inside polygons, reduces Mach bands, and gives more realistic results for shiny surfaces, but is slower. Gouraud is faster but may miss highlights and show Mach bands.

## Answer

Both are polygon-rendering (smooth shading) methods that remove the faceted look of a polygon mesh.

- **Gouraud shading** computes the illumination at the polygon **vertices** and **interpolates the intensity** linearly across the polygon.
- **Phong shading** interpolates the **normal vectors** across the polygon and computes the illumination at **every pixel**.

| Point | Gouraud shading | Phong shading |
|---|---|---|
| Quantity interpolated | Intensity (colour) | Surface normal |
| Illumination model applied | At vertices only | At every pixel |
| Computation | Less, fast | More, slow |
| Specular highlights | Missed or distorted, especially inside a polygon | Rendered correctly |
| Mach-band effect | Present (intensity changes slope at edges) | Greatly reduced |
| Image quality | Good | Better, more realistic |
| Memory/time | Low | High |
| Hardware | Common, easy | Needs more powerful hardware |
| Also called | Intensity-interpolation | Normal-vector interpolation |

**Advantages of Phong over Gouraud**
1. Highlights are correct, even if they fall inside a polygon (Gouraud may miss them).
2. Smoother shading and far fewer Mach bands.
3. More accurate for shiny (high specular) surfaces.
4. Better results with fewer polygons.

The extra cost is its disadvantage; Gouraud is preferred when speed matters.

## Check yourself

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

1. What do Gouraud and Phong interpolate respectively?
   A. Intensity and normal
   B. Normal and intensity
   C. Depth and colour
   D. Colour and depth
   Answer: A. Gouraud interpolates intensity; Phong interpolates normals. (Key point: Interpolated quantity)

2. Where is the illumination model applied in Gouraud shading?
   A. At every pixel
   B. At vertices only
   C. At polygon centres only
   D. At edges only
   Answer: B. Intensities are then interpolated. (Key point: Illumination applied)

3. Why can Gouraud miss specular highlights inside a polygon?
   A. Normals are averaged
   B. Pixels are too small
   C. Only vertex intensities are used
   D. The polygon is flat
   Answer: C. A highlight inside a polygon never appears in vertex values. (Key point: Highlights)

4. Which is an advantage of Phong over Gouraud?
   A. Faster computation
   B. Less memory
   C. Simpler hardware
   D. Correct highlights and far fewer Mach bands
   Answer: D. The cost is higher, but quality is better. (Key point: Advantage of Phong)

## Related in Computer Graphics

- [Fast Phong shading](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Afast-phong-shading) — Computer Graphics, chapter 8, asked 2×
- [Gouraud shading for polygon rendering](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Agouraud-shading-intensity-interpolation) — Computer Graphics, chapter 8, asked 6×
- [Phong shading method](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aphong-shading-method) — Computer Graphics, chapter 8, asked 9×
- [Phong shading and Mach-band effect](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aphong-mach-band-effect) — Computer Graphics, chapter 8
- [Gouraud intensity at midpoint P](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Agouraud-intensity-midpoint-scan-line) — Computer Graphics, chapter 8

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