CSC345: Advanced Graphics & Virtual Environments

1. CSC345: Advanced Graphics &Virtual Environments Lecture 4: Visual Appearance Patrick Olivier p.l.olivier@ncl.ac.uk 2nd floor in the Devonshire Building Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

2. Objectives • Refresher on simple lighting models • Blending for translucent surfaces • Compositing images • Fog • Gamma correction Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

3. Lighting model (1) • How compute lighting? • We could set colors per vertex manually • For a little more realism, compute lighting from: • Light sources • Material properties • Geometrical relationships Rasterizer light blue Geometry red green Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

4. Diffuse component: idiff • i = iamb + idiff + ispec • Diffuse is Lambert’s law • Photons cattered equally in all directions Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

5. Specular component: ispec • Diffuse is dull (left) • Specular: simulates a highlight • Models: • Phong specular highlight model • Blinn’s highlight formula (variation on Phong) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

6. n r l -l Specular component: Phong Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

7. + + = Ambient component: iamb • Ad-hoc – tries to account for light coming from other surfaces • Just add a constant color: • Sum all components: i = iamb+ idiff+ ispec • This is just a hack! • It has almost nothing to do with reality! Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

8. Additions to the lighting equation • Depends on distance: 1/(a+bt+ct2) • Can have more lights: just sum their respective contributions • Different light types: • directional • point • spot Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

9. What’s lighting and what’s shading? • Lighting: interaction between light & matter • Shading: determine pixel colors from vertex lighting • Three types of shading: • Flat (per polygon) • Gouraud (per vertex) • Phong (per pixel) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

10. Opacity and Transparency • Surfaces: • Opaque: permit no light to pass through • Transparent: permit all light to pass • Translucent: pass some light translucency = 1 – opacity(a) • Translucency in physically correct manner is difficult: • complexity of interactions of light & matter • using a pipeline renderer Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

11. Writing model • Use “A” component RGBA (or RGBa) to store opacity • Can expand our model to use RGBA values blend source component source blending factor destination component color buffer Destination blending factor Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

12. Blending Equation • We can define source and destination blending factors for each RGBA component: s = [sr, sg, sb, sa] d = [dr, dg, db, da] • Suppose source & destination colours are: b = [br, bg, bb, ba] c = [cr, cg, cb, ca] • Blend as: c’ = [br sr+ cr dr, bg sg+ cg dg, bb sb+ cb db , basa+ cada] Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

13. OpenGL Blending and Compositing • Must enable blending and pick source and destination factors: glEnable(GL_BLEND) glBlendFunc(source_factor, destination_factor) • Only certain factors supported: • GL_ZERO, GL_ONE • GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA • GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA • See Redbook for complete list… Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

14. Example… • Suppose that we start with the opaque background colour (R0,G0,B0,1) • This color becomes the initial destination color • We now want to blend in a translucent polygon with colour (R1,G1,B1,a1) • Select GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA as the source and destination blending factors R’1 = a1 R1 +(1- a1) R0, …… • Note this formula is correct if polygon is either opaque or transparent Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

15. Clamping • All the components (RGBA) are clamped and stay in the range (0,1) • However, in a typical (old) system, RGBA values are only stored to 8 bits • Can easily loose accuracy if we add many components together • Example: add together n images • Divide all color components by n to avoid clamping • Blend with source factor = 1, destination factor = 1 • But division by n loses bits Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

16. Order Dependency • Is this image correct? • Probably not… • Polygons are rendered in the order they pass down the pipeline • Blending functions are order dependent Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

17. Opaque and Translucent Polygons • Suppose that we have a group of polygons some of which are opaque and some translucent • How do we use hidden-surface removal? • Opaque polygons block all polygons behind them and affect the depth buffer • Translucent polygons should not affect depth buffer • Render with glDepthMask(GL_FALSE) which makes depth buffer read-only • Sort polygons first to remove order dependency Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

18. Fog • Simple atmospheric effect • A little better realism • Help in determining distances • Color of fog: color of surface: • How to compute f ? • 3 ways: linear, exponential, exponential-squared • Linear: Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

19. Fog example GLfloat fcolor[4] = {……}: glEnable(GL_FOG); glFogf(GL_FOG_MODE, GL_EXP); glFogf(GL_FOG_DENSITY, 0.5); glFOgv(GL_FOG, fcolor); • Often just a matter of: • Choosing fog color • Choosing fog model • Turning it on Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)