How do we draw a picture?

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1 1 How do we draw a picture? Define geometry. Now what? We can draw the edges of the faces. Wireframe. We can only draw the edges of faces that are visible. We can fill in the faces. Giving each object a color (constant shading). COSC 342

2 2 Flat Shading Assumes the object is faceted, and light and viewer at infinity. Illumination model is applied only once per polygon.

3 2 Flat Shading Assumes the object is faceted, and light and viewer at infinity. Illumination model is applied only once per polygon. What if the object represents a curved surface? COSC 342

4 3 Flat Shading Works poorly if the model represents a curved surface. Adding more facets helps but... Slows down the rendering. Effectiveness is tempered by Mach banding. COSC 342

5 4 Mach Bands Perceived intensity change at edges are exagerated by receptors in our eyes, making the dark facet look darker and the light facet look lighter. COSC 342

6 5 Is A or B darker? illusion by Ted Adelson COSC 342

7 6 Are you sure? illusion by Ted Adelson COSC 342

8 7 Is A or B darker? illusion by Ted Adelson COSC 342

9 8 Are you sure? illusion by Ted Adelson COSC 342

10 9 Are the diamonds the same colour? illusion by Ted Adelson COSC 342

11 10 Are the diamonds the same colour? illusion by Ted Adelson COSC 342

12 11

13 Are the circles the same colour and size? 12

14 Harmann-grid illusion - See white circles? 13

15 Flat Shading 14

16 Gouraud Shading 15

17 16 Gouraud Shading Also called smooth shading, intensity interpolation shading or color interpolation shading. Discontinuities eliminated by interpolating intensity. Almost removes Mach bands (high curvature can still suffer). OpenGL implements Gouraud shading. Easily implemented in hardware.

18 17 Gouraud Shading: How it works Illumination is calculated per vertex. Average normals of polygons that share vertex. Vertex colors linearly interpolated per pixel, first along the edges, then between those for the scanline. Slower than flat shading, with much better results for curved surfaces.

19 Gouraud Shading: How it works 18

20 Gouraud Shading: Notice any problems? 19

21 20 Gouraud Shading: Problems Highlights are spread along edges. Highlights tend to be larger than they should. Highlights tend to be shaped funny (dependent on number of polys). Misses specular highlights contained within a polygon.

22 Gouraud Shading Problems 21

23 Gouraud Shading Problems 22

24 Gouraud Shading Problems 23

25 Gouraud Shading Problems 24

26 Gouraud Shading Problems 25

27 Gouraud Shading Problems 26

28 Gouraud Shading Problems 27

29 Gouraud Shading Problems 28

30 Gouraud Shading Problems 29

31 Gouraud Shading Problems 30

32 Gouraud Shading Problems 31

33 Gouraud Shading Problems 32

34 Gouraud Shading Problems 33

35 Gouraud Shading Problems 34

36 Gouraud Shading Problems 35

37 36 Phong Shading Don t confuse with Phong illumination (same guy different algorithm). Also called normal-vector interpolation shading. Interpolates the surface normals instead of the intensity values. Calculate illumination at every pixel, as well as a vector normalization. Much slower than Gouraud. Gives better results, especially for highlights. Can handle highlights within a polygon.

38 Phong Shading: How it works 37

39 38 Gouraud Shading Patrick Coleman

40 39 Phong Shading Patrick Coleman

41 40 Shading Comparison from Watt, 1989

42 41 Problems with Interpolation Shading Must store normals at each vertex. Silhouette edges not smoothed. Interpolation can mask regular changes. Interpolation in image space ignores perspective distortion. Crease edges should not have smooth shading. Orientation dependence. from Watt, 1989

43 42 Interpolation Shading Problems from Watt, 1989

44 43 Interpolation Shading Problems from Watt, 1989

45 44 Interpolation Shading Problems from Watt, 1989

46 45 Interpolation Shading Problems A D p B C from Watt, 1989

47 46 Interpolation Shading Problems B A p C D from Watt, 1989

48 47 Interpolation Shading Problems A B D p B A p C C D from Watt, 1989

49 48 Fixes for Interpolation Shading Use triangles. Use more triangles. Use multiple normals (to get sharp edges) from Watt, 1989

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