Week 13 - Friday Impostors Image processing Lens flare and bloom - - PowerPoint PPT Presentation

Week 13 - Friday

 Impostors  Image processing  Lens flare and bloom  Motion blur  Fog

 Most of the work we've focused on

all semester is doing rendering that in some way mirrors the natural world

 However, a wide area of rendering

is non-photorealistic rendering (NPR)

 Goals:

• Simplified technical drawings
• Simulating artistic styles

 The most common form of NPR in

video games is toon shading

• Also called cel shading

 The goal is to render 3D models as if

they were cartoons

• Shading is often done with either a single

color or a two tone (color and shading) approach

• Then a thick black silhouette is added

around edges

 The color is often determined by the dot product n · l (surface

normal dot light vector)

 If negative, the surface should be darkened  Otherwise, it's some flat color  Or a threshold other than 0 can be used  A more complex system uses a one dimensional texture indexed

into with the dot product

Highlight Normal Shadow

 The more complicated problem is properly

rendering edges with a thick dark line

 A number of different edges are of interest

• Boundary or border edges are edges where one

polygon is not adjacent to any other

▪ Not found in 3D solid objects

• A crease, hard, or feature edge is an edge between

two polygons that is sharper than some threshold angle

• A material edge is an edge between two polygons

with different materials

• A silhouette edge is when two neighboring polygons

face different directions, relative to the eye

 Cel shading focuses on rendering silhouette edges  Many of these techniques rely on manipulating back facing

polygons

• The crossover between front facing and back facing polygons is the

silhouette

• It's easy to determine which is which

 After some manipulation of the backfaces, they are rendered

in black

 If the backfaces are rendered in black without any change, they will be hidden  Before rendering, all backfaces can be translated to be closer to the viewer  Translation can be by

• A fixed amount
• An amount that takes into account non-linear z-depths
• An amount based on the angle of the polygon normal

 None of these techniques give uniform thickness lines

 Another approach is to fatten each backface triangle  The slope of the triangle and the distance from the viewer

determine the expansion of each edge

 It doesn't work well for thin triangles

 A similar approach is to expand backface vertices along their

normals

• The expansion amount is proportional to their z-distance

 This technique fails for situations like a cube, in which faces

with very different normals share vertices

 Rather than using geometry, there is an

image processing approach

 Render all the normal values or depth values

to a buffer and use edge detection algorithms to draw lines where the values change abruptly

• Using normal values can find crease edges

 This technique works for many cases that

failed before

• Even GPU generated surfaces are not a problem

 NPR techniques are very broad  Many approaches try to recreate hand-drawn or hand-painted styles  Silhouette lines can be drawn as paintbrush strokes of varying thickness  Tonal art maps (TAMs) use palettes of hand-drawn textures to do black

and white crosshatch-style shading

 For CAD programs and other 3D tools, we may want to

highlight certain lines or all polygon edges

 Rendering polygon edges on top of existing polygons can be a

pain

• Z-buffer algorithms might hide the lines

 In practice, a small bias is usually added to the polygons

 For drawing simplified models, we may want to include or exclude

the hidden lines

 Wireframe is the easiest, since it is just the lines  Hidden-line uses the z-buffer directly  Obscured-line renders twice, rendering z-fail lines in a lighter color  Haloed lines also use the z-buffer, drawing thick white lines first

and then black lines on top

• Problems happen when lines get too close

 Review up to Exam 1

 Finish Assignment 4

• Due tonight before midnight

 Work on Project 3