SET09115 Intro Graphics Programming Lighting SET09115 Intro Graphics Programming

Breakdown Working with Colour Basics of Lighting Surface Lighting Why Lighting? Ambient Light Surface Lighting Diffuse Light Specular Light Other Lighting Types

Recommended Reading Real Time Rendering Course Text Chapter 5 – Visual Appearance Course Text Chapter 6 – Lighting and Shading

Colours

Colour Representation Modern graphics work with 32 bit colour Typically 4 values of 8 bits used Red, green, blue, alpha (transparency) Other formats exist We could work with values 0..255 Better to work with floats 0.0f ..1.0f 0% to 100%

Working with Colour Treat colour as a value you can work with Add colours together Red + Green = Yellow (1.0, 0.0, 0.0) + (0.0, 1.0, 0.0) = (1.0, 1.0, 0.0) Multiply colours together Magenta * Yellow = Red (1.0, 0.0, 1.0) * (1.0, 1.0, 0.0) = (1.0, 0.0, 0.0)

Vertex Colours Attaching colours to vertices is what we have been doing up until this point Blending occurs over surfaces between colour points OpenGL follows the RGBA format Image formats may not be in RGBA DirectX can support other formats These colours can be used in lighting calculations Typically, we use textured objects to provide more detail

Blending The ability to blend colours in this way is useful Multi-texturing Terrain We will revisit blending colours when working with textures and terrain

Questions? Working with colour

Basics of Lighting

Why Lighting? Lighting adds depth to a scene Unlit objects are flat and lack detail Lighting adds realism to objects Specular detail Light reflection

How Lighting Works Light works in rays from a graphics point of view Light comes from a source Light is reflected off of objects Light eventually makes it into the eye

Materials Light rays are just one part of the equation Light itself can have a colour Objects also have a colour The type of light they reflect into the eye rather than absorb The light that an object reflects is commonly described in its material Material may have other properties

Main Lighting Types Generally, an object has three lighting effects affecting it at any time Ambient Diffuse Specular The colour of an object are these three lighting effects combined Plus any other effectx

What is Ambient Light? Ambient light is the light that affects everything equally in a scene Has no direction Has no distance Doesn’t care about which part of the object is being calculated Outside, daylight is an ambient light effect

Ambient Light Equation Ambient light calculation is the simplest The ambient light of a particular vertex is just the ambient material colour of the object multiplied by the ambient light colour of the scene

Example

Questions? Why lighting? Materials Ambient light

Lighting Surfaces

Lighting a Surface Ambient light is something affecting all geometry equally Still gives a flat appearance to a scene Objects reflect light based on how their surfaces are aligned with the light source Occluded objects / surfaces get no light Surfaces perpendicular to light source get full light

Working with Normals We use surface normals to determine how light should be reflected off a surface Andrew will go into the mathematics of lighting next week Normals are very important for lighting

What is Diffuse Light? Diffuse light is concerned with directionality of the light Geometry facing a light source is lit Geometry facing away from the light source unlit Think sunlight outside

How does Diffuse Light Work? Diffuse light is concerned with the light direction against the surface normal of the piece of geometry in question If angle = 90 then full light If angle 0 to 90, or 90 to 180, then shaded If angle < 0 or > 180 then not lit Think about how this would work if you shone a light on a table

Lambert’s Cosine Law A simple way to calculate the diffuse light of an object is using Lambert’s Cosine Law Cos(0) = 0 Cos(90) = 1 Cos(180) = 0 The cosine graph shows how this works

Diffuse Light Algorithm Using Lambert’s Cosine Law, we can generate the diffuse lighting algorithm Remember, dot product of two vectors is equal to the cosine of the angle between them We would normally have to transform the surface normal if the model is transformed Diffuse alpha should always be 1. We will have to set this

Example

What is Specular Light? Specular light refers to the point on the object where the light strikes Specular reflection This provides a shiny look to objects Also indicates where the light is coming from

How does Specular Light Work? Specular light requires three pieces of information Light direction Surface normal Eye position Specular intensity is based on eye position

Specular Light Algorithm Specular light is the most complicated algorithm

Example

Questions? Surface lighting Diffuse lighting Specular lighting

Other Lighting Types

Other Lighting Types Ambient, Diffuse, and Specular are considered the main environmental lighting types Object materials usually deal with these three types There are of course other lighting types which affect a scene Dynamic lighting Point lights Spot lights

Point Lights A point light is a light that emanates from a particular point in the scene Use the same equations, but calculate light vector based on vertex position and light position Distance to light affects the strength of the light

Spot Lights Spot lights also work using light position and distance The direction of the light is also important Objects behind the spot light aren’t lit Light also has an angle associated with it Beam width at distance

Phong Shading Phong Shading is a popular per-pixel approach to performing lighting calculations Normals are interpolated across the surface of a polygon Provides a smoother shading look Vertex lighting as discussed in this lecture is sometimes referred to as Gourand shading

Questions? Point Lights Spot Lights Phong Shading

To do this week… Investigate some of the different forms of light that can affect a scene In particular, try and find their lighting values Consider the type of lighting you may want in your coursework OpenGL has light functionality built in without the use of shaders Shaders provide more controllable lighting

Summary Lighting is an important concept to provide depth and realism to your scene Lighting is about the colour of the light, and how an object reflects this light Three main types of lighting Ambient Diffuse Specular You should make yourself familiar with how to work these out