color and texture
DESCRIPTION
Color and Texture. Electromagnetic Spectrum. Candelas / sq meter. Computer Screen 1.8 - 150 ~2 orders of magnitude. Physiology: Receptors. Rods active at low light levels (night vision) only one wavelength sensitivity function 100 million rod receptors Cones - PowerPoint PPT PresentationTRANSCRIPT
Color and Texture
Electromagnetic Spectrum
Candelas / sq meter
Computer Screen
1.8 - 150
~2 orders of magnitude
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)
The basis of color visionand measurement
Cone Sensitivity Functions
Sharp Aquos pixels
Gamma g
• There is a non linear relationship between the signal given to a monitor and the Luminance that results.
L = Vg
Acquos Curves
Important points
3 Cone types -> Trichromacy. Need only three colors in monitorSaturation is the vividness of a color. We cannot get
full saturationLuminance range is limited on a monitorIn the real world real world light is additive and linear.
Monitors are non-linear – must be corrected for accurate simulation
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
Lighting with cast shadows
Specular has the color of the illumination
Lambertian reflectionAmount of lightFalling per unit area issmaller as a functionOf the angle with the surface cos(q)
q
Rendering approaches
• Light Field• Ray Tracing• Radiosity• Direct polygon (simplification)
• + Combinations of above
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);
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.
• 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);
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
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
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
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.
Phong Shading
• Interpolate surface normalsThen apply lighting pixel by pixel
Gouraud Shading
Calculate lighting at vertices, then interpolate
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.
Perlin Noise (procedural textures)
Cube earth
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.
glGenTextures(4,texts); // texts is an unsigned int
glEnable(GL_TEXTURE_2D);glBindTexture(GL_TEXTURE_2D, texts[1]); // make this the current textureglTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); // wrap in S | GL_CLAMPglTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT); // wrap in TglTexParameteri(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);
MipMaps
• A hierarchy of textures• Helps with aliasing
Aliasing and anti-aliasing
Short wavelength sensitive cones
Blue text on a dark backgroundis to be avoided. We have very fewshort-wavelength sensitive cones in the retina and they are not very sensitive
Blue text on dark backgroundis to be avoided. We have very fewshort-wavelength sensitive cones in the retina and they are not very sensitive
Blue text on a dark backgroundis to be avoided. We have very fewshort-wavelength sensitive cones in the retina and they are not very sensitive.Chromatic aberration in the eye is also a problem
Blue text on a dark backgroundis to be avoided. We have very fewshort-wavelength sensitive cones in the retina and they are not very sensitive
Opponent Process Theory• Cone signals transformed into new channels
Color Channel Theory
• Luminance contrast needed to see detail
3:1 recommended10:1 idea for small text
Comparing the Channels• 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 philosophersSuppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow
Some natural philosophersSuppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow
Some natural philosophersSuppose that these colors arise from the accidental vapours diffused in the air, which communicates their own hues to the shadow