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Computer Graphics Material Colours and Lighting

Computer Graphics Material Colours and Lighting. CO2409 Computer Graphics Week 11. Lecture Contents. Materials Shading Lighting Light Types Light Models Applying Lighting. Materials. A material defines the surface properties of a polygon:

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Computer Graphics Material Colours and Lighting

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  1. Computer GraphicsMaterial Colours and Lighting CO2409 Computer Graphics Week 11

  2. Lecture Contents • Materials • Shading • Lighting • Light Types • Light Models • Applying Lighting

  3. Materials • A material defines the surface properties of a polygon: • Colour, shininess, texture, bumpiness, transparency, etc. • Have looked at material colours already • This lecture considers how material colour is affected by incident light (light hitting surface) • Base material colour can be defined as: • Face colours • Each polygon has a single colour • Vertex colours • Each vertex has a colour • The colour is blended across the polygon using the nearest vertex colours • Like labs so far

  4. Shading • Adjust normal directions at the edges to get these effects • Artists do this to create hard or soft edges as required • When drawing entire polygons / meshes, we can choose whether to blend the colours across the surface: • With hard edges: • All vertex colours in each polygon are the same so each polygon appears flat • Implies vertex duplication • Or use face colours • With soft edges • Vertex colours shared between polygons • Result is smooth • Originally called Gouraud shading

  5. Lighting • Can improve realism of scenes by using lighting • Light colour interacting with any existing vertex / face colours • So far have assumed constant white light everywhere • so everything is perfectly clear • Lights can greatly improve the look of even the simplest model • Several types of light source • Point, directional, spot (see later) • Several light effects on surfaces • Ambient, diffuse, specular (see later) • Don’t confuse these two concepts

  6. Light Types: Directional / Point • Three main types of light: • Directional Lights • Considered to be infinitely far away • All the light comes from the same direction • No attenuation (see later) • Sunlight is the main example • Data: direction + colour • Point Lights • Light emitting in all directions from a single point • Light attenuates with distance • A light bulb is a good example • Data: point + colour

  7. Light Types: Spotlights • Spotlights • Like point lights: • Light emitting from single point • Light attenuates with distance • But also: • Light is constrained to a cone • Only emits in the direction bounded by the cone • Brightest at centre of the cone • Less bright towards the edges • Data: point, direction + colour

  8. Light Attenuation • The light emitted from point lights and spotlights attenuates over distance • The light is diffused (i.e. scattered) by the atmosphere • Light from distant source is weaker than from near source • Physically correct formula: Attenuated Colour =Original Colour / Distance2 • Usually get nicer looking result with: Attenuated Colour =Original Colour / Distance (Effectively gamma correction – advanced point) • So in the following lighting equations, we use the attenuated light colour rather than the actual light colour • Calculation not shown to keep it simple

  9. Light Effects: Diffuse Lighting • Diffuse lighting lights parts of the mesh that point towards the light source • We’ll consider the light hitting a single vertex • The diffuse light hitting a vertex is calculated using a dot product: Diffuse = LightDmax(N • L , 0) LightD is light colour (attenuated) N is the vertex normal L is a normal pointing from the vertex to the light Notes: Diffuse = LightD if normal points at the light Diffuse = 0 if it points away (even a little bit) max used to avoid negative result

  10. Light Effects: Specular Lighting • Another key light effect is specular lighting • Treats the surface as reflective resulting in a reflection of the light source becoming visible • The reflection is called a highlight • [Can extend this technique to create specular mapping – a reflection of the entire scene in a surface] • On a shiny surface highlights are sharp and bright • The surface is smooth, so the reflection is focused • Other surfaces have more spread out highlights • Surface diffuses the reflection more

  11. Specular Lighting - Phong • A couple of mathematical models for specular light • The Phong model is commonly supported in hardware • The Phong specular calculation for a vertex is: Specular = LightSmax(N • H, 0)P LightS is the light colour (attenuated) • Can use different light colours for calculating diffuse and specular to get nicest result N is the vertex normal H is the halfway normal • Described on the diagram • = normalise((L+C) / 2) P is the specular power • The spread of the highlight

  12. Light Effects: Ambient Lighting • A final basic lighting effect is Ambient Light • A background light level, lighting everything evenly • An approximation for indirect light • Light that reaches a surface after reflecting off other surfaces • Without it shadows would be black • Ambient can be a constant colour for an entire scene • Or vary locally depending on lights • Ambient is adjusted for the type of scene (night, day, indoor) • Call the ambient level LightA • Often just added onto the diffuse light equation

  13. Applying Lighting • Total light hitting a vertex is the sum of the three effects: Incident Light = LightA + LightDmax(N • L, 0) + LightS max(N • H, 0)P • We need to combine the result with the colour of the material itself • We use multiplicative blending • Common to define material colour as two components: • Diffuse material colour = basic colour of material • Specular material colour = shininess of material • Gives final result: Colour = MaterialD(LightA + LightDmax(N • L, 0)) + MaterialS LightS max(N • H, 0)P

  14. Blending with Vertex Colours • The final effect on a sphere: Using typical material colours: MaterialD = red • This is the sphere’s colour MaterialA = 1.0 (= white) • The specular light is fully reflected • [The reflection isn’t tinted red] • Note that the calculation is performed separately for the red, green and blue components • If there are several lights then: • Accumulate the incident light for all of them • Then combine with the material colours

  15. Normals & Matrix Transforms • We earlier covered the use of matrices to transform geometry • Start by using each model’s world matrix to transform its vertices from model space into world space (shown) • This process applies to normals too • They need to be put into world space for lighting • Lights are positioned in the world • Scaling a model can cause issues: • Will scale the normals – not normals any longer. Fix by renormalising

  16. Programming Lighting • Equations are calculated in a shader • Can use vertex or pixel shader • Will see difference in a lab later • Need to set up: • Light types, positions, directions, attenuation etc. • Textures and material colours (if used) • The given equations are not physically perfect, just approximations to reality • Other lighting models are available • Can use shader flexibility for alternative light models

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