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
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