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1 DH2323 DGI15 INTRODUCTION TO COMPUTER GRAPHICS AND INTERACTION LIGHTING AND SHADING Christopher Peters HPCViz, KTH Royal Institute of Technology, Sweden Based on DGI12 notes by Carl Henrik Ek

2 Image Synthesis In computer graphics, create images based on a model Recall: An underlying process generates observations Can control generation through parameters

3 Nice Results "Distant Shores" by Christoph Gerber "Christmas Baubles" by Jaime Vives Piqueres "Still with Bolts" by Jaime Vives Piqueres

4 Some Constituents I Light Geometry Surface properties Anything else?

5 Some Constituents II Camera Model (pinhole)

6 Row Vs. Column Format Remember this?

7 Row Vs. Column Format

8 Homogeneous Coordinates Allow common operations to be represented as matrices Translation, rotation, projection For positions and vectors, in 3D: (x,y,z,w) T => (x/w, y/w, z/w) T for w!= 0 w = 1.0: position w = 0.0: vector

9 Lighting and Shading In this lecture, you will apply knowledge about: Some applied math, especially vector algebra What is shading? Why is it good? Determining the colour of a pixel Usually determined by a lighting model Provides depth to perception of images Adds a sense of realism

10 Applications Photorealistic

11 Applications Non-photorealistic

12 Lighting Vs. Shading Lighting Interaction between materials and light sources Shading Deciding the colour of a pixel Based usually on a lighting model Other methods possible too though

13 How To Implement? Theory Practice General classifications Lighting fundamentals Lambertian illumination Some shading models Extensions Flat, Gouraud, Phong Maths programming (vector operations, normals, plane, angles, intersections)

14 Some Classifications View Dependent Determine an image by solving the illumination that arrives through the view-port only View Independent Determine the lighting distribution in an entire scene regardless of viewing position. Views are taken after lighting simulation by sampling the full solution to determine the view through the viewport

15 Some Classifications Local Illumination Consider lighting effects only directly from the light sources and ignore effects of other objects in the scene (e.g. reflection off other objects) Global Illumination Account for all modes of light transport

16 Why Go Local? Usually easy to control and express Director's chair: important when you want a scene to look a certain way Fast Easier to obtain real-time performance (or just tractable calculations) Do not require knowledge of the entire scene But Not as accurate or compelling as global models Harder to control than global models

17 How Can It Be Modelled? Use a lighting model as inspiration But real light extremely complicated to simulate Light bounces around the environment Heavy processing required even for coarse approximations Simplifications allow real-time performance Lighting models: Lambertian we will consider this first Phong not to be confused with Phong shading Blinn-Phong and others...

18 Simplifications Simplification #1: use isotropic point light sources Isotropic means that the light source radiates energy equally in all directions Simplifies our light source energy equations that we ll look at When we mention light, we are really talking about energy Simplification #2: simulate only specific surface types Makes it easier to specify materials and calculate reflections But visually limited

19 Radiant Intensity Light is defined by its Radiant Intensity, I Radiant Intensity is measured in Watts/sr sr is the solid angle (in steradians) is the energy leaving the surface per unit time Known as power or flux and measured in Watts But: it s a point light source, so it radiates light equally in all directions So r 2 = 1 (unit sphere) Now know energy leaving light source in any direction

20 Inverse Square Law But we want to know the energy arriving at a surface This irradiance, E, may now be determined: Irradiance is the flux per unit area at a point x, a distance r from the point light source We know the source radiates I Watts in all directions So the power is radiated through a sphere centred at the lightsource At a distance r from the source, the surface area of this sphere is => the power per unit area at x is: This assumes the surface at x is perpendicular to the direction to the light source To handle all angles, we must apply the cosine rule

21 The Cosine Law A surface orientated perpendicular to a light source will receive more energy than a surface orientated at an angle to the light source More energy = brighter appearance The irradiance E is proportional to 1/area As the area increases, the irradiance decreases As increases, the irradiance (thus surface brightness) decreases: 2015

22 Lambertian Illumination Model Cosine rule is used to implement Lambertian surfaces Also known as diffuse surfaces Diffuse surfaces reflect light equally in all directions The surface is characterised by a reflectance parameter is the incident power is the reflected power So the reflectance is the ratio of the total incident power to the total reflected power

23 Lambertian Illumination Model To shade a diffuse surface, we need to know The normal to the surface at the point to be shaded The diffuse reflectance of the surface The positions and powers of the light sources in the scene Assuming contribution is from a single isotropic light source: accounts for the reflectance attribute of the surface accounts for the orientation of the surface with respect to the light source

24 Lambertian Illumination Model This is local illumination Only concerned with energy hitting the surface directly from light sources Not concerned with light bouncing off other surfaces and hitting the surface =>Models derived from it are also local

26 Flat Shading Constant shading Very fast Very simple Does not look very smooth Compute the colour of a polygon Use this as the colour for the whole polygon

27 Gouraud Shading Calculates the light intensity at each vertex in a polygon For each interior point in the polygon, we interpolate the light intensity determined at the vertices Given a starting value, and an end value, interpolation can be used to approximate intermediate values Similar idea to the way in which colours are interpolated across the surface of a polygon We only need to do lighting calculations at the vertices Fast! But lighting is only correct at the vertices Unrealistic

29 Limitations Gouraud only calculates the actual lighting at the vertices of the polygon Wikimedia Commons

30 Phong Shading To improve on Gouraud shading, interpolate normals across a surface Lighting model then applied to each interior point in a polygon Must take care to ensure that all interpolated normals are of unit length This is known as Phong Shading Phong shading produces more accurate results than Gouraud Shading But slower!

31 Phong Shading Phong shading can reproduce highlights in the center of a polygon that Gouraud Shading may miss

32 Phong Illumination Model Lambertian Illumination model: only diffuse surfaces Surfaces reflect light in all directions equally What about modelling shiny surfaces too? Reflected radiance depends heavily on the outgoing direction Phong Illumination model consists of: Lambertian Model for diffuse surfaces A function to handle specular reflection Ambient term to approximate all other light

33 Phong Illumination Model NOT the same as Phong Shading

34 Phong Illumination Model Allows us to model many different types of surfaces Easy to control Not a very realistic model But produces good results Each object has material data associated with it: ambient reflectance diffuse reflectance specular reflectance phong exponent (shininess parameter)

35 Materials Parameters: Interaction with light Reflective properties Components m ambient, m diffuse, m specular Proportion of each colour reflected

36 Lambertian Vs Phong Lambertian Surface Phong Illuminated Specular Surface

37 A little bit of OpenGL (1.2 old) Light sources LIGHT0 to LIGHT7 Each light must be enabled Can specify light parameters using Some parameters glenable(gl_light1); gllightf{iv}(gl_light0, param, value); GL_AMBIENT GL_DIFFUSE GL_SPECULAR GL_POSITION

38 Shading in OpenGL 1.2 To enable lighting use: glenable(gl_lighting); OpenGL does not support true Phong shading; it interpolates the intensities across each polygon Gouraud shading glshademodel(gl_smooth);

39 Material Properties We can assign different properties to each side of a polygon To assign material properties: GL_FRONT GL_BACK GL_FRONT_AND_BACK glmaterial{if}v(face, param, value); GL_AMBIENT GL_DIFFUSE GL_AMBIENT_AND_DIFFUSE GL_SPECULAR GL_EMISSION GL_SHININESS RGB Values Phong Exponent

40 Bump Mapping Lots of cool effects possible Bump mapping: modify surface normals for lighting calcs (not actual geometry) Query a heightmap See also: normal mapping

41 Shaders Modern way of implementing rendering techniques Various types: Pixel Vertex Geometry Tessellation Shader languages HLSL, GLSL, CG

42 Next lecture Global illumination Next week (20 th April) 08:00 10:00 L1 Lab support session Thursday 16th April (tomorrow) 10:00 12:00 VIC Studio

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