Color and Texture

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