Gouraud Shading: Gouraud Shading: Illuminating Computer Vision

By Fouad Sabry

What is Gouraud Shading

Gouraud shading, named after Henri Gouraud, is an interpolation method used in computer graphics to produce continuous shading of surfaces represented by polygon meshes. In practice, Gouraud shading is most often used to achieve continuous lighting on triangle meshes by computing the lighting at the corners of each triangle and linearly interpolating the resulting colours for each pixel covered by the triangle. Gouraud first published the technique in 1971. However, enhanced hardware support for superior shading models has yielded Gouraud shading largely obsolete in modern rendering.

How you will benefit

(I) Insights, and validations about the following topics:

Chapter 1: Gouraud shading

Chapter 2: Phong shading

Chapter 3: Phong reflection model

Chapter 4: Per-pixel lighting

Chapter 5: Vertex normal

Chapter 6: Rendering (computer graphics)

Chapter 7: Shader

Chapter 8: Computer graphics lighting

Chapter 9: Polygonal modeling

Chapter 10: Vertex (computer graphics)

(II) Answering the public top questions about gouraud shading.

(III) Real world examples for the usage of gouraud shading in many fields.

Who this book is for

Professionals, undergraduate and graduate students, enthusiasts, hobbyists, and those who want to go beyond basic knowledge or information for any kind of Gouraud Shading.

• Language: English
• Publisher: One Billion Knowledgeable
• Release date: May 4, 2024

Book preview

Chapter 1: Gouraud shading

Gouraud shading, named after Henri Gouraud, is an interpolation technique employed in computer graphics to generate continuous shading of surfaces represented by polygon meshes. In practice, Gouraud shading is utilized most frequently to generate continuous lighting on triangle meshes by computing the illumination at the corners of each triangle and linearly interpolating the resulting colors for each pixel covered by the triangle. The approach was initially published by Gouraud in 1971.

An estimate of the surface normal of each vertex in a polygonal 3D model is either supplied for each vertex or calculated by averaging the surface normals of the polygons that converge at each vertex. Using these estimations, lighting computations based on a reflection model, such as the Phong reflection model, are then done to determine the vertex color intensities. The color intensities for each pixel covered by the polygonal mesh can then be interpolated from the color values derived at the vertices.

Gouraud shading is regarded as superior than flat shading and takes much less processing than Phong shading, however it typically produces a faceted appearance.

In comparison to Phong shading, the strength and weakness of Gouraud shading lay in its interpolation. If a mesh occupies more pixels in screen space than it has vertices, interpolating colour values from samples of costly lighting calculations at vertices is less processor intensive than running the lighting calculation for each pixel, as in Phong shading. However, highly localized lighting effects (such as specular highlights, e.g. the glint of reflected light on the surface of an apple) will not be rendered correctly, and if a highlight lies in the middle of a polygon, but does not spread to the polygon's vertex, it will not be visible in a Gouraud rendering.

A rendering that should have a specular highlight traveling smoothly across the surface of a rotating model reveals the error immediately. Gouraud shading will provide a highlight that fades in and out constantly across nearby areas of the model, reaching its maximum intensity when the intended specular highlight aligns with a model vertex. This issue can be resolved by increasing the vertex density of the object, but at some point the decreasing returns of this strategy will make it preferable to convert to a more sophisticated shading model.

A geometry with a Gouraud shading resembling a spherical; note the bad behavior of the specular highlight.

Another sphere-shaped mesh with an extremely large polygon count.

The original Gouraud work described linear color interpolation, a perspective-correct alternative to linear interpolation utilized in GPUs. The linear and hyperbolic variations of color interpolation from vertices to pixels are collectively referred to as Gouraud shading..

Any linear interpolation of intensity results in discontinuous derivatives, which generates Mach bands, a frequent visual artifact of Gouraud shading.

{End Chapter 1}

Chapter 2: Phong shading

Phong shading, or Phong interpolation, is an interpolation technique for surface shading in 3D computer graphics that was developed by computer graphics pioneer Bui Tuong Phong. Phong shading interpolates surface normals over rasterized polygons and calculates pixel colors in accordance with the interpolated normals and a reflection model. Phong shading can also refer to a particular combination of Phong interpolation and the Phong reflection model.

Bui Tuong Phong invented Phong shading and the Phong reflection model at the University of Utah and published them in his 1973 Ph.D. dissertation. At the time of their release, Phong's approaches were deemed revolutionary, but they have subsequently become the standard shading technique for many rendering applications. Due to their typically effective use of computing time per produced pixel, Phong's techniques have gained widespread acceptance.

Phong shading provides a more accurate approximation of the shading of a smooth surface than Gouraud shading. Phong shading presupposes a continuously fluctuating normal vector on the surface. When used to a reflection model with modest specular highlights, such as the Phong reflection model, the