Shading. Introduction to Computer Graphics Torsten Möller. Machiraju/Zhang/Möller/Fuhrmann

Transcription

1 Shading Introduction to Computer Graphics Torsten Möller Machiraju/Zhang/Möller/Fuhrmann

2 Reading Chapter Angel Chapter Hughes, van Dam, et al Machiraju/Zhang/Möller/Fuhrmann 2

3 Shading Illumination Model: determine the color of a surface (data) point by simulating some light attributes. Local IM: deals only with isolated surface (data) point and direct light sources. Global IM: takes into account the relationships between all surfaces (points) in the environment. Shading Model: applies the illumination models at a set of points and colors the whole scene. Machiraju/Zhang/Möller/Fuhrmann 3

4 Shading of surfaces We can shade a point on a surface using a local IM How about surfaces? Flat (planar) polygons are computationally attractive Normals, intersections, visibility, projections, etc., are all easy to compute hardware acceleration available Curved surfaces are often tessellated into many small flat polygons in graphics polygonal meshes Machiraju/Zhang/Möller/Fuhrmann 4

5 Polygonal meshes Composed of a set of polygons (often quadrilaterals or triangles) pasted along their edges The dominant surface model for 3D shapes (esp. free-form shapes), e.g., in games 5

6 Shading of surfaces How should polygons be rendered? Machiraju/Zhang/Möller/Fuhrmann 6

7 Shading of flat polygons Flat (constant, faceted) shading compute illumination once per polygon and apply it to whole polygon Interpolated/smooth/Gouraud shading compute illumination at borders (e.g. vertices) and interpolate Accurate shading e.g., Phong shading compute illumination at every point of the polygon Machiraju/Zhang/Möller/Fuhrmann 7

8 Flat shading A single color across the polygon efficient Intensity discontinuity across edges Flat shading would be correct if Light source is at infinity, i.e., light vector l is constant, so n l is constant across polygon, and viewer is at infinity, so r v is constant across polygon, and polygons represent actual surface, not an approximation Also, if there are a very large number of very small polygons, the faceting effects is less obvious spatial integration of our eyes N 1 Machiraju/Zhang/Möller/Fuhrmann 8 N 2

9 Results of flat shading Machiraju/Zhang/Möller/Fuhrmann 9

10 Mach Banding Creates discontinuities in colour easily visible hence we can see the distinct surface patches easily Machbands! caused by lateral inhibition of the receptors in the eye Machiraju/Zhang/Möller/Fuhrmann 10

11 Mach Banding (2) Problems - Machbands! Machiraju/Zhang/Möller/Fuhrmann 11

12 Gouraud shading This shading model is interpolative: Given colors of the polygon vertices, interior points are colored through bilinear interpolation How to compute the normal at a mesh vertex? use prior surface information or take the (normalized) average of the normals of adjacent faces better: area weighted normals Machiraju/Zhang/Möller/Fuhrmann 12

13 Color interpolation Interpolate colors along edges and scanlines Can be done incrementally, i.e., via scanlines Machiraju/Zhang/Möller/Fuhrmann 13

14 Flat Shading vs. Gouraud vs. Phong No Highlight No Highlight Highlight! Gouraud shading does not properly handle specular highlights because only color is interpolated Phong shading accurate shading Interpolates normals at each point instead of colors Apply LIM at each point according to approximated normal Machiraju/Zhang/Möller/Fuhrmann

15 Flat Shading vs. Gouraud vs. Phong Machiraju/Zhang/Möller/Fuhrmann

16 Phong vs. Gouraud shading Phong shading: Handles specular highlights much better Does a better job in handling Mach bands But much more expensive than Gouraud shading Machiraju/Zhang/Möller/Fuhrmann 18

17 Further problems with shading models so far (1) Polygonal silhouette we are quite sensitive in picking these up Solution: subdivide further Exercise: How to determine whether an edge in a mesh is a silhouette edge? Machiraju/Zhang/Möller/Fuhrmann 19

18 Further problems with shading models so far (2) Orientation dependence Note first that interpolation is done along horizontal scanlines When the orientation of the same polygon changes, the same point p may be colored differently Solution: triangulate Machiraju/Zhang/Möller/Fuhrmann 20

19 Further problems with shading models so far (3) Unrepresentative normals As shown, all vertex and interpolated normals are the same Solution: subdivide further Machiraju/Zhang/Möller/Fuhrmann 21

20 Non-Photorealistic Painterly Shading Aims to emulate hand-drawn or painted effects:

21 Recap: An Illumination Model: describes how light interacts with a surface element (e.g. reflection etc.) A Shading Model applies the illumination to a surface, generally to a tessellated representation of a smooth surface Exercise: which OpenGL shader implements what? Machiraju/Zhang/Möller/Fuhrmann 23