2.5D Clip-Surfaces for Technical Visualization

2.5D Clip-Surfaces for Technical Visualization
Matthias Trapp & Jürgen Döllner
Hasso-Plattner-Institut, University of Potsdam, Germany
WSCG 2013
21st International Conference in Central Europe on Computer Graphics, Visualization and Computer Vision, 2013

2.5D Clip-Surfaces for Technical Visualization :: Matthias Trapp & Jürgen Döllner2
Clippling-Plane vs. 2.5D Clip-Surface
Original Mesh
w/o Clipping
Clipping using
Half-Space Test
Clipping using a
2.5D Clip-Surface (CS)
h

Motivation

Exemplary Result

Motivation – Wrap Up
2.5 Clip-Surfaces (CS):
 Enables non-planar cut-surfaces
 Create more sophisticated cut-away views
Contributions:
 Extended clipping-plane parameterization
 Automatic cap-surface generation
Implementation Goals:
 Hardware-accelerated rendering
 Interactive configuration
 Material
 Silhouettes
 Lighting & Shading

Outline
Clipping using 2.5D Clip-Surfaces
Creating & Rendering of Cap-Surfaces

2.5D Clip-Surface Parameterization
 O Plane Origin
 U, V Direction Vectors
 S = (sx,sy,sz) Scaling Vector
 OM Offsetmap
VectorScaling
TexturemapOffset
NormalPlane
VectorsDirection,
Origin
,,,,,
3
3
3
RS
OM
VUN
RVU
RO
SOMVUOCS

Clipping Equation
y
N
x
N
N
zsample
s
V
VOP
s
U
UOP
T
NNOPPP
otherwisefalse
sTOMfONNPtrue
PCSclip
,
0,
,

GLSL Implementation
bool clipReliefPlane(in mat4 config, // configuration matrix
in vec4 point, // position in eye-space
in sampler2D reliefSampler) // 2D relief texture
{ // compute plane in eye space...
vec3 O = (gl_ModelViewMatrix * vec4(config[0].xyz, 1.0)).xyz;
vec3 A = normalize( gl_NormalMatrix * normalize(config[1].xyz) );
vec3 B = normalize( gl_NormalMatrix * normalize(config[2].xyz) );
vec3 N = cross(A, B);
// project current fragment coordinate on plane
vec3 pV = point.xyz - dot(point.xyz - O, N) * N;
// compute offset texture coordinates
float s = dot(pV - O, A) / length(config[1].xyz);
float t = dot(pV - O, B) / length(config[2].xyz);
// fetch height... maybe zero
float height = texture2D(reliefSampler, vec2(s,t) * config[3].st).x;
// compute reference plane
float plane = dot(point.xyz, N) - dot(N, O) + (height * config[3].z );
// perform clipping
return (plane < 0.0 && bool(config[3].w));
}

Multiple 2.5D Clip-Surfaces

2.5D Clip-Surfaces for Technical Visualization :: Matthias Trapp & Jürgen Döllner
Capping Openings – Real-World Example

Capping Approach
Goals:
 Convey material / inner structure of the input mesh
 Enable assisted modeling of cap-surface appearance
Approach:
 Compute polygonal cap-surface from 2.5D clip-surface
 Subsequent rendering after the clipping pass
=+
Clipped Mesh Cap-Surface Final Result

Generating the Cap-Surface
A B
CS Derived Surface Clipped Surface
A: Generate Cap-Surface:
 Use Dynamic Mesh Refinement or Tessellation Shader
 Perform displacement, shading, texturing,…
B: Clip Cap-Surface to Opening of the Clipped Mesh:
 Clip “outside” parts of the cap-surface

Clipping the Cap-Surface

Capping – Challenges for 2.5D Surfaces
Cap
Capping Clipping Planes:
Render planar mesh using back-face
information and stencil test
Capping 2.5D Clip-Surfaces:
Back-face information does not
determine non-planar surface
completely
Back Faces CapCap

Decision Problem
Is a point P on the cap-surface inside a solid mesh S ?
Volumetric depth-test approach [Trapp08] :
Additional data structure required:
 Layered Depth Image (LDI) [Shade98]
 Image-based representation of shape volume
 Computed in pre-processing using depth-peeling [Everitt01]
SPOutside
SPInside
SPVDT ,

Example of a Layered Depth Image

Volumetric Depth Test

Volumetric Depth Test – Performance
2 FS 2 BS
2 LS
4 FS 4 BS
4 LS
8 FS 8 BS
8 LS
10 FS 10 BS
10 LS
14 FS 14 BS
14 LS
16 FS 16 BS
16 LS
400
500
600
700
800
900
1000
1100
640x480
800x600
1024x768
1600x1200

Application Examples

Wrap-Up

Conclusions
Summary
 Advanced clipping approach for non-planar surfaces
 Multi-pass real-time rendering technique
 Fully implemented on GPU
Drawbacks
 Requires additional data structure
 Requires high-tessellated cap-surface
Future Work
 Direct manipulation techniques
 Stencil-routed A-Buffer [Myers07] instead of LDI

Thank You ! - Questions ?
Contact:
Matthias Trapp
matthias.trapp@hpi.uni.potsdam.de
Jürgen Döllner
juergen.doellner@hpi.uni.potsdam.de
Computer Graphics Systems Group
http://www.hpi.uni-potsdam.de/doellner/

2.5D Clip-Surfaces for Technical Visualization

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