Gouraud shading (/ɡuːˈroʊ/goo-ROH), 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.[1][2][3] However, enhanced hardware support for superior shading models has yielded Gouraud shading largely obsolete in modern rendering.
Gouraud shading works as follows: An estimate to the surface normal of each vertex in a polygonal 3D model is either specified for each vertex or found by averaging the surface normals of the polygons that meet at each vertex. Using these estimates, lighting computations based on a reflection model, e.g. the Phong reflection model, are then performed to produce colour intensities at the vertices. For each screen pixel that is covered by the polygonal mesh, colour intensities can then be interpolated from the colour values calculated at the vertices.
Gouraud shading is considered superior to flat shading and requires significantly less processing than Phong shading, but usually results in a faceted look.
In comparison to Phong shading, Gouraud shading's strength and weakness lies in its interpolation. If a mesh covers more pixels in screen space than it has vertices, interpolating colour values from samples of expensive lighting calculations at vertices is less processor intensive than performing 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 apparent in a Gouraud rendering; conversely, if a highlight occurs at the vertex of a polygon, it will be rendered correctly at this vertex (as this is where the lighting model is applied), but will be spread unnaturally across all neighboring polygons via the interpolation method.
The problem is easily spotted in a rendering which ought to have a specular highlight moving smoothly across the surface of a model as it rotates. Gouraud shading will instead produce a highlight continuously fading in and out across neighboring portions of the model, peaking in intensity when the intended specular highlight aligns with a vertex of the model. While this problem can be fixed by increasing the density of vertices in the object, at some point the diminishing returns of this approach will favour switching to a more detailed shading model.
A Gouraud-shaded sphere-like mesh - note the poor behaviour of the specular highlight.
Another sphere-like mesh rendered with a very high polygon count
Linear vs. hyperbolic interpolationedit
Gouraud's original paper described linear color interpolation.[1] In 1992, Blinn published an efficient algorithm for hyperbolic interpolation[4] that is used in GPUs as a perspective correct alternative to linear interpolation. Both the linear and hyperbolic variants of interpolation of colors from vertices to pixels are commonly called "Gouraud shading".
Mach bandsedit
Any linear interpolation of intensity causes derivative discontinuities which triggers Mach bands, a common visual artifact of Gouraud shading.
^ abGouraud, Henri (1971). Computer Display of Curved Surfaces, Doctoral Thesis (Thesis). University of Utah.
^Gouraud, Henri (1971). "Continuous shading of curved surfaces" (PDF). IEEE Transactions on Computers. C-20 (6): 623–629. doi:10.1109/T-C.1971.223313. S2CID 123827991.
^Gouraud, Henri (1998). "Continuous shading of curved surfaces". In Rosalee Wolfe (ed.). Seminal Graphics: Pioneering efforts that shaped the field. ACM Press. ISBN1-58113-052-X.
^Blinn, James F. (July 1992). "Hyperbolic Interpolation". IEEE Computer Graphics and Applications. 12 (4): 89-94. doi:10.1109/MCG.1992.10028. S2CID 207973430.
May 08, 2024
gouraud, shading, this, article, multiple, issues, please, help, improve, discuss, these, issues, talk, page, learn, when, remove, these, template, messages, this, article, relies, excessively, references, primary, sources, please, improve, this, article, addi. This article has multiple issues Please help improve it or discuss these issues on the talk page Learn how and when to remove these template messages This article relies excessively on references to primary sources Please improve this article by adding secondary or tertiary sources Find sources Gouraud shading news newspapers books scholar JSTOR February 2014 Learn how and when to remove this message This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Gouraud shading news newspapers books scholar JSTOR March 2012 Learn how and when to remove this message Learn how and when to remove this message Gouraud shading ɡ uː ˈ r oʊ goo ROH 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 1 2 3 However enhanced hardware support for superior shading models has yielded Gouraud shading largely obsolete in modern rendering Gouraud shaded triangle mesh using the Phong reflection model Contents 1 Description 2 Comparison with other shading techniques 3 Linear vs hyperbolic interpolation 4 Mach bands 5 See also 6 ReferencesDescription editGouraud shading works as follows An estimate to the surface normal of each vertex in a polygonal 3D model is either specified for each vertex or found by averaging the surface normals of the polygons that meet at each vertex Using these estimates lighting computations based on a reflection model e g the Phong reflection model are then performed to produce colour intensities at the vertices For each screen pixel that is covered by the polygonal mesh colour intensities can then be interpolated from the colour values calculated at the vertices Comparison with other shading techniques edit nbsp Comparison of flat shading and Gouraud shading Gouraud shading is considered superior to flat shading and requires significantly less processing than Phong shading but usually results in a faceted look In comparison to Phong shading Gouraud shading s strength and weakness lies in its interpolation If a mesh covers more pixels in screen space than it has vertices interpolating colour values from samples of expensive lighting calculations at vertices is less processor intensive than performing 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 apparent in a Gouraud rendering conversely if a highlight occurs at the vertex of a polygon it will be rendered correctly at this vertex as this is where the lighting model is applied but will be spread unnaturally across all neighboring polygons via the interpolation method The problem is easily spotted in a rendering which ought to have a specular highlight moving smoothly across the surface of a model as it rotates Gouraud shading will instead produce a highlight continuously fading in and out across neighboring portions of the model peaking in intensity when the intended specular highlight aligns with a vertex of the model While this problem can be fixed by increasing the density of vertices in the object at some point the diminishing returns of this approach will favour switching to a more detailed shading model nbsp A Gouraud shaded sphere like mesh note the poor behaviour of the specular highlight nbsp Another sphere like mesh rendered with a very high polygon countLinear vs hyperbolic interpolation editGouraud s original paper described linear color interpolation 1 In 1992 Blinn published an efficient algorithm for hyperbolic interpolation 4 that is used in GPUs as a perspective correct alternative to linear interpolation Both the linear and hyperbolic variants of interpolation of colors from vertices to pixels are commonly called Gouraud shading Mach bands editAny linear interpolation of intensity causes derivative discontinuities which triggers Mach bands a common visual artifact of Gouraud shading See also editList of common shading algorithms Blinn Phong reflection model Phong shadingReferences edit a b Gouraud Henri 1971 Computer Display of Curved Surfaces Doctoral Thesis Thesis University of Utah Gouraud Henri 1971 Continuous shading of curved surfaces PDF IEEE Transactions on Computers C 20 6 623 629 doi 10 1109 T C 1971 223313 S2CID 123827991 Gouraud Henri 1998 Continuous shading of curved surfaces In Rosalee Wolfe ed Seminal Graphics Pioneering efforts that shaped the field ACM Press ISBN 1 58113 052 X Blinn James F July 1992 Hyperbolic Interpolation IEEE Computer Graphics and Applications 12 4 89 94 doi 10 1109 MCG 1992 10028 S2CID 207973430 Retrieved from https en wikipedia org w index php title Gouraud shading amp oldid 1217201011, wikipedia, wiki, book, books, library,
