Illumination and Shading

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1 Illumination and Shading

2 Illumination (Lighting)! Model the interaction of light with surface points to determine their final color and brightness! The illumination can be computed either at vertices or fragments illumination

3 Shading! Interpolation from the vertex illumination or apply the lighting model at a set of points across the entire surface Shading

4 Illumination Model! The governing principles for computing the illumination! A illumination model usually considers:! Light attributes (light intensity, color, position, direction, shape)! Object surface attributes (color, reflectivity, transparency, etc)! Interaction among lights and objects (object orientation)! Interaction between objects and eye (viewing dir.)

5 Illumination Calculation! Local illumination: only consider the light, the observer position, and the object material properties θ! Example: OpenGL

6 Illumination Models! Global illumination: take into account the interaction of light from all the surfaces in the scene object 4 object 3 object 2 object 1! Example: Ray Tracing (in CSE 5545)

7 Basic Light Sources sun Point light Directional light Light intensity can be independent or dependent of the distance between object and the light source Spot light

8 Simple local illumination! The model used by OpenGL consider three types of light contribution to compute the final illumination of an object! Ambient! Diffuse! Specular! Final illumination of a point (vertex) = ambient + diffuse + specular

9 Ambient light contribution! Ambient light (background light): the light that is scattered by the environment! A very simple approximation of global illumination object 4 object 3 object 2 object 1! Independent of the light position,object orientation, observer s position or orientation ambient light has no direction

10 Ambient lighting example

11 Ambient light calculation! Each light source has an ambient light contribution (Ia)! Different objects can reflect different amounts of ambient (different ambient reflection coefficient Ka, 0 <= Ka <= 1) Note: both Ia and Ka are vectors for (R,G,B)! So the amount of ambient light that can be seen from an object is: Ambient = Ia x Ka

12 Diffuse light contribution! Diffuse light: The illumination that a surface receives from a light source and reflects equally in all direction It does not matter where the eye is

13 Diffuse lighting example

14 Diffuse light calculation! Need to decide how much light the object point receive from the light source based on Lambert s Law Receive more light Receive less light

15 Diffuse light calculation (2)! Lambert s law: the radiant energy D that a small surface patch receives from a light source is: I: light intensity D = I x cos (θ) θ: angle between the light vector and the surface normal light vector (vector from object to light) θ N : surface normal

16 Diffuse light calculation (3)! Like the ambient light case, different objects can reflect different amount of diffuse light (different diffuse reflection coefficient Kd, 0 <= Kd <= 1))! So, the amount of diffuse light that can be seen is: Diffuse = Kd x Id x cos (θ) θ L θ N cos(θ) = N.L

17 Specular light contribution! The bright spot on the object! The result of total reflection of the incident light in a concentrate region See nothing!

18 Specular light example

19 Specular light calculation! How much reflection you can see depends on where you are The only position the eye can see specular from P if the object has an ideal reflection surface θ? p φ But for a non-perfect surface you will still see specular highlight when you move a little bit away from the idea reflection direction When φ is small, you see more specular highlight

20 Specular light calculation (2)! Phong lighting model specular = Ks x Is x cos(φ) n Ka: specular reflection coefficient (a vector for [R,G,B]) N: surface normal at P Is: specular light intensity (a vector) φ: angle between V and R L N n cos(φ): the larger is n, the smaller is the cos value cos(θ) = R.V θ θ p φ R V

21 Specular light calculation (3)! The effect of n in the phong model n = 10 n = 90 n = 30 n = 270

22 Put it all together! Illumination from a light: Illum = ambient + diffuse + specular = Ka x Ia + Kd x Id x (N.L) + Ks x Is x (R.V)! If there are N lights or n Total illumination for a point P = Σ (Illum)! Some more terms to be added (in OpenGL):! Self emission (N.H)! Global ambient! Light distance attenuation and spot light effect

23 Polygon shading model! Flat shading compute lighting once and assign the color to the whole polygon

24 Flat shading! Only use one vertex (usually the first one) normal and material property to compute the color for the polygon! Benefit: fast to compute! It is used when:! The polygon is small enough! The light source is far away (why?)! The eye is very far away (why?)

25 Mach Band Effect! Flat shading suffers from mach band effect! Mach band effect human eyes accentuate the discontinuity at the boundary perceived intensity Side view of a polygonal surface

26 Smooth shading! Fix the mach band effect remove edge discontinuity! Compute lighting for more points on each face Flat shading smooth shading

27 Smooth shading! Two popular methods:! Gouraud shading (used by OpenGL)! Phong shading (better specular highlight, not supported by OpenGL)

28 Gouraud Shading (1)! The smooth shading algorithm used in OpenGL fixed function pipeline! Lighting is calculated for each of the polygon vertices! Colors are interpolated for interior pixels

29 Gouraud Shading (2)! Per-vertex lighting calculation! Normal is needed for each vertex! Per-vertex normal can be computed by averaging the adjust face normals n1 n n2 n3 n4 n = (n1 + n2 + n3 + n4) / 4.0

30 Gouraud Shading (3)! Compute vertex illumination (color) before the projection transformation! Shade interior pixels: color interpolation (normals are not needed) C1 Ca = lerp(c1, C2) Cb = lerp(c1, C3) for all scanlines C2 C3 Lerp(Ca, Cb) * lerp: linear interpolation

31 Gouraud Shading (4)! Linear interpolation v1 a x b v2 x = a / (a+b) * v2 + b/(a+b) * v1! Interpolate triangle color: use y distance to interpolate the two end points in the scanline, and use x distance to interpolate interior pixel colors

32 Gouraud Shading Problem! Lighting in the polygon interior can be inaccurate

33 Gouraud Shading Problem! Lighting in the polygon interior can be inaccurate

34 Phong Shading! Instead of interpolation, we calculate lighting for each pixel inside the polygon (per pixel lighting)! We need to have normals for all the pixels not provided by the user! Phong shading algorithm interpolates the normals and compute lighting during rasterization (need to map the normal back to world or eye space though)

35 Phong Shading (2)! Normal interpolation n1 na = lerp(n1, n2) n2 lerp(na, nb) nb = lerp(n1, n3) n3! Slow not supported by OpenGL and most of the graphics hardware

36 Lighting in the Graphics Pipeline! Where is lighting performed in the graphics pipeline? v1, m1 modeling and viewing per vertex lighting projection v2, m2 v3, m3 Rasterization texturing shading viewport mapping interpolate vertex colors clipping Display

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