Color and Texture

1. Color and Texture

2. Electromagnetic Spectrum

5. Candelas / sq meter Computer Screen 1.8 - 150 ~2 orders of magnitude

6. Physiology: Receptors • Rods • active at low light levels (night vision) • only one wavelength sensitivity function • 100 million rod receptors • Cones • active at normal light levels • three types: sensitivity functions peaks at different wavelengths (“red”, “green”, “blue”) • 6 million cone receptors • Focused in the center of vision (fovea)

7. The basis of color visionand measurementCone Sensitivity Functions

8. Sharp Aquos pixels

9. Gamma g • There is a non linear relationship between the signal given to a monitor and the Luminance that results. L = Vg

10. Acquos Curves

12. Important points 3 Cone types -> Trichromacy. Need only three colors in monitor Saturation is the vividness of a color. We cannot get full saturation Luminance range is limited on a monitor In the real world real world light is additive and linear. Monitors are non-linear – must be corrected for accurate simulation

13. Basic CG lighting (for each vertex) Diffuse = N.L Specular = R.Vk Ambient = Const Specular has color of illumination Ambient and diffuse are influenced by the pigment in the surface

14. Lighting with cast shadows

15. Specular has the color of the illumination

16. Lambertian reflection Amount of light Falling per unit area is smaller as a function Of the angle with the surface cos(q) q

18. Rendering approaches • Light Field • Ray Tracing • Radiosity • Direct polygon (simplification) • + Combinations of above

19. Illumination in openGLglLight, glLightModel float light_position[] = {-10.0,20.0,20.0,1.0}; glLightfv(GL_LIGHT0,GL_POSITION, light_position); float ambient[] = { 0.4f, 0.4f, 0.4f, 1.0f }; glLightfv(GL_LIGHT0, GL_AMBIENT, ambient); float diffuse[] = {0.8f, 0.8f, 0.8f , 1.0f}; glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse);

20. For a light at infinity • Ir = lar*mar + ldr*mdr*(L·N) + lsr*msr*max(0,V·R)a • To get specular use V·Ra • with similar equations for the green and blue components.

21. gLlighting disables glColor, unless • glEnable(GL_COLOR_MATERIAL); is set glMaterialfv(GL_FRONT, GL_SPECULAR, specReflection); glMateriali(GL_FRONT, GL_SHININESS, 20); // note exponent glColorMaterial(GL_FRONT,GL_AMBIENT_AND_DIFFUSE);

22. 28 parameters glMaterialfv(GL_FRONT, GL_AMBIENT, M_ambient); glMaterialfv(GL_FRONT, GL_DIFFUSE, M_diffuse); glMaterialfv(GL_FRONT, GL_SPECULAR, M_spec); glLightfv(GL_LIGHT0, GL_AMBIENT, L_ambient); glLightfv(GL_LIGHT0, GL_DIFFUSE, L_diffuse); glLightfv(GL_LIGHT0, GL_SPECULAR, L_spec); glMateriali(GL_FRONT, GL_SHININESS, k); +Lighting direction

23. More Lighting • Attentuation • float light_position[] = {-10.0,20.0,20.0,0.0}; • glLightfv(GL_LIGHT0,GL_POSITION, light_position); • If last number is zero, light is at infinity. • If non-zero Light is positioned. • flLightf*GL_LIGHT0,GL_CONSTANT_ATTENUATION, const); • flLightf*GL_LIGHT0,GL_LINEAR_ATTENUATION, linear.); • flLightf*GL_LIGHT0,GL_QUADRATIC_ATTENUATION, quad); • attenuation = 1/(const+(linear*dist)+(quad*(dist*dist))) • Spotlights • glLight(GL_LIGHT0,GL_SPOT_CUTOFF, 45.0); // a 45 deg cone • glLight(GL_LIGHT0,GL_SPOT_EXPONENT, 2.0); // light concentration • Can have multiple lights

24. Lets Simplify, A two component model of lighting • Lighting from a source at infinity. • + Ambient light (the rest of our surroundings) • (Note that these can be turned into one) • The surface reflects in two ways • Diffusely, and Specularly

25. OpenGL Lighting • Separate ambient diffuse and specular components of both the light and the surface color. (12) • + Light direction (3) • + shininess (1) • Total 60 parameters. • Easy to end up with summed components >1.0 for r,g,b.

26. Phong Shading • Interpolate surface normals Then apply lighting pixel by pixel

27. Gouraud Shading Calculate lighting at vertices, then interpolate

28. Textures and texture mapping Used for 1) Images (a picture in a 3D scene) 2) For surface properties (wood, stone) 3) Lighting effects. Techniques include procedural textures and texture mapping OpenGL supports texture mapping.

29. Perlin Noise (procedural textures)

30. Cube earth

31. Properties of textures in OpenGL • 1D, 2D, 3D • Must have dimensions defined by a power of two. E.g. 512/256 for a 2D texture. • Have a coordinate system (s,t) from 0-1.

32. glGenTextures(4,texts); // texts is an unsigned int glEnable(GL_TEXTURE_2D); glBindTexture(GL_TEXTURE_2D, texts[1]); // make this the current texture glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); // wrap in S | GL_CLAMP glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT); // wrap in T glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_MODULATE); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 513,512,0, GL_RGB, GL_UNSIGNED_BYTE, stripes); glDisable(GL_TEXTURE_2D);

33. MipMaps • A hierarchy of textures • Helps with aliasing

34. Aliasing and anti-aliasing

35. Short wavelength sensitive cones Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive. Chromatic aberration in the eye is also a problem Blue text on dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive

36. Opponent Process Theory • Cone signals transformed into new channels

37. Color Channel Theory 3:1 recommended 10:1 idea for small text • Luminance contrast needed to see detail

38. Comparing the Channels Some natural philosophers Suppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow Some natural philosophers Suppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow • Spatial Sensitivity • Red/Green and Yellow/Blue each about 1/3 detail of Black/White • Stereoscopic Depth • Pretty much can’t do it with hue alone • Temporal Sensitivity • Moving hue-change patterns seem to move slowly • Form • Shape-from shading works well • Shape-from-hue doesn’t • Information Labeling: Hue works well! Some natural philosophers Suppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow