1. Real-time Walkthrough of Virtual Space using Environment Map
CG Lab. ChungAng University
Ryoo, Seung Taek
I am from Computer Graphics Lab. In ChungAng University, Korea
I’m talking about Real-time walkthrough of virtual space using Environment map.

2. Introduction: Real-time Walkthrough
High quality architectural walkthroughs require large complex models with many geometry primitives
Renders real-time walkthrough of virtual space difficult
Visibility Culling
Close objects from viewpoint: use geometry models
Distant objects
Applying the LODs(Level Of Details)
Replacing them with images (Image Based Representation)
High quality architectural walkthroughs require large complex models with many geometry primitives
It is difficult to render them real-time walkthrough
To overcom this problem, Mechanisms such as visibility culling,
using geometry models for close objects from viewpoint
and applying the LODs or replacing them with images for distant objects are used
The thechnique replacing with image is called Image Based Representation
Image-based representation expresses the object using a 2-dimensional image instead of a geometry model to make photorealistic and real-time rendering possible
VSMM 2000

3. Introduction: Image Based Representation
Related Works
Sprite
Imposter
Nailboards
Portal Textures
image caches
Geometry model
LDI
Layered Sprite
Sprite
Texture
mapping
Sprite with depth
Viewpoint
Examples of real-time walkthrough algorithms using IBR techniques are imposters, image caches, nailboards, sprites, portal textures and so forth.
The right figure show how to use image based primitives according to distance between viewpoint and objects
VSMM 2000

4. Realtime Walkthrough System
Realtime walkthrough algorithm
Virtual Space
Modeling
Preprocessing
Cell & Portal Division
Images Capture
Walkthrough
Collision
Detection
I suggests the real-time walkthrough algorithm of virtual space using environment maps
This figure shows Overview of realtime walkthrough algorithm
First, the virual space is modelled into a hierarchical tree structure.
And then, The transformed model is classified into cells and portals
Cells means room, portals means doors or windows in buliding
The scene seen through the portal is captured to express the outler layer of the cell.
Next step is Culling.
The data that is not necessary for rendering is discarded through view-frustum culling and portal culling
After Culling, The models still have a large amount of geometry data.
To resolve this, only a cell with a current viewpoint is expressed as geometry models
and the neighboring cells visible through the portal are replaced with images.
The algorithms replacing geometry visible through portal are texture map, image warping, layer depth image, and environment map method.
I’ll express details about this repacement algorithm next cahpter.
And also, collision detection is carried out through an intersection test using the bounding volume.
The above process is applied repeatedly to create a continuous walkthrough scene.
View-Frustum
Culling
Geometry Model
Cell
Cell +
Portal Texture
morphing
Renderer
Portal Culling
Cell/Portal
Replacement
Portal
Texture map
Image Warping
Layered Depth Image
Environment Map
VSMM 2000

5. Virtual Space Modeling
hierarchical structuring of the models
buildings, rooms(cell), objects in a room, faces of an object, and each point that consists of a face
The current cell : geometry models
the rest : replacement with images
Portal
Cell1
Cell2
Wall 2 7
Wall1 4 5
Wall6
Wall 3
Wall8
Cell1_Obj1
Cell1_Obj2
Cell2_Obj1
Cell2_Obj2
Images
Hierarchical structuring means structuring the virtual space model into a hierarchical tree consisting of layers such as buildings, rooms, objects in a room, faces of an object, and each point that consists of a face
Left tree show whole geometry model.
Right tree shows the appearance the virtual space model has been replaced with the images
The current cell has been expressed using geometry models and the rest has been replaced with images
VSMM 2000

6. Replacement algorithms: current algorithms and its drawback
Replacing geometry visible through portal
Texture Map
Single Texture
Simple and easy to implement
But, appears more like a large painting hanging on the wall than like another room
Multiple Textures
popping effect problem
image warping method
gaps that appear in the warped image
cannot guarantee correct visibility
LDIs method
extra cost of applying the warping algorithm A B
In this section, we will take a look at the characteristcs of the replacement algorithm and its drawbacks.
The most common method is to use a single image for each portal. This method is simple and easy to implement but the single texture appears more like a large painting hanging on the wall than like another room.
Multiple Textures are applied in order to overcome the above problem.
However, this method creates a popping effect problem
The warping method and the LDI method are used to resolve this problem.
But, gaps appear in the warped image such as the right figure
some areas of the scene that should have become visible were not sampled in the reference image
And cannot guarantee correct visibility for the image created from the required viewpoint
Also, the extra cost of applying the warping algorithm is required during walkthrough of virtual space
VSMM 2000

7. Replacement algorithms: Environment Map
Replacement algorithm using environment maps
Environment Map
A map that has been created by stitching a multiple number of images acquired by rotating the camera by 360 degrees from a fixed viewpoint
to simplify the computations of specular reflections on a shiny object from a distant scene.
to express new images with a composition of 2D images as an image-based representation mechanism
To resolve the above mentioned problems, we propose a portal replacement algorithm using an environment map.
An environment map is a map that has been created by stiching a multiple number of images acquired by rotating the camera by 360 degrees from a fixed viewpoint.
The environment map was initially used in computer graphics to simplify the computations of specular reflections on a shiny object from a distant scene.
It has been used to express new images with a composition of 2-D images as an image-based representation mechanism.
VSMM 2000

8. Replacement algorithms: Environment Mapping
The objects used for mapping the environment map
Spherical projection
Not suitable for storage on a computer.
those which map onto a plane with consistent neighborhood relationships are generally quite distored.
Cube oriented planar projections
easily stored and accessed by a computer
oversampled in the edges and corners.
Cylindrical projection
easily unrolled into a simple planar map.
without boundaries in the azimuth direction.
the boundary conditions introduced at the top and bottom.
The objects used for mapping the environment map are a sphere, cylinder and a cube.
Spherical projection is due to the lack of representation that is suitable for storage on a computer.
This is particularly difficult if a uniform discrete sampling is required.
Furthermore, those which map onto a plane with consistent neighborhood relationships are generally quite distored.
Cube oriented planar projections can be easily stored and accessed by a computer
but it is oversampled in the edges and corners.
For cylindrical projection, a cylinder can be easily unrolled into a simple planar map.
The surface is without boundaries in the azimuth direction.
One shortcoming of a projection on a finite cylindrical surface is the boundary conditions introduced at the top and bottom
Referenced image from
Plenoptic modeling by Leonard McMillian
VSMM 2000

9. Replacement algorithm using environment maps: Cylindrical Mapping
Hemi-Cylindrical Mapping
the height of the viewpoint is constant
Using a hemi-cylinder
the environment map applied to the portal only requires 180 degrees of the image visible through the portal
The environment map has been mapped onto the hemi-cylinder
hemiCylinder is set in the back of the portal that resides in the current cell
ViewPoint
Env Map
Portal
Current Cell
Neighbor Cell
The height of the viewpoint in the walkthrough of the virtual space is constant and therefore is not greatly effected by the problems caused by cylinder mapping
In this study, a hemi-cylinder has been used for mapping the environment map. The reason for using a hemi-cylinder is that the environment map applied to the portal only requires 180 degrees of the image visible through the portal
The environment map has been mapped onto the hemi-cylinder
HemiCylinder is set in the back of the portal that resides in the current cell, as shown in the right figure
The scene visible through the portal is a hemi-cylinder to which an environment map has been mapped onto
VSMM 2000

10. Creation of Environment Map
Replacement algorithm using environment maps: Creation of Environment Map
Creation of Environment Map
the cylinder which the environment map is to be mapped onto is set to the center of the portal.
The viewpoint and the view volume is set to the boundary of the cylinder to acquire the scene visible through the portal.
The scene from the viewpoint of the boundary of the cylinder is gathered by a certain degree.
The gathered image is combined to create the environment map which is then mapped onto the hemi-cylinder.
The process for creating an environment map is shown in figure below.
First of all, the cylinder to which the environment map is to be mapped onto is set to the center of the portal.
The viewpoint and the view volume is set to the boundary of the cylinder to acquire the scene visible through the portal.
The scene from the viewpoint of the boundary of the cylinder is gathered by a certain degree.
The gathered image is combined to create the environment map which is then mapped onto the hemi-cylinder.
VSMM 2000

11. Our Walkthrough System
Applying the environment map to the portal
Eliminate the popping effect
Remove the holes in the image
No extra cost needed to render the virtual space
The creation of the environment map in the preprocessing phase
Problem
cannot guarantee the correct visibility of the image created from the required viewpoint.
re-structuring the environment map using depth information.
Our method using environment map resolve some problem.
The popping effect can be eliminated and the holes caused by the parallax have been eliminated by applying the environment map on replacement algorithm.
Also, because the environment maps create in the preprocessing phase
No the extra cost needed to render the virtual space
The method using environment maps cannot guarantee the correct visibility of the image created from the required viewpoint.
To resolve the problem caused by the parallax, re-structuring the environment map using depth information is needed for future work.
VSMM 2000

12. Wireframe: Geometry vs. Portal Image vs. Environment Map
This figure compares the virtual space model with the geometry model, portal texture and the proposed method of applying an environment map to express the scene.
Geometry Model
Portal Image
VSMM 2000
Env Map

13. Rendering: Geometry vs. Portal Image vs. Environment Map
The image acquired using the environment map method is closer to that of the image expressed using the geometry model than the image acquired using portal texture method.
Geometry Model
Portal Image
VSMM 2000
Env Map

14. Collision Detection
Collision detection of the viewpoint is carried out
objects within the cell
Each Object: Bounding Sphere
walls consisiting cell
Bottom Wall: Bounding Plane
portals that make up the cell
Each Portal: Bounding Sphere
Collision detection of the viewpoint is carried out on the objects within the cell, walls consisiting cell and the portals that make up the cell, as shown in right figure.
Collision detection on the objects within the cell is carried out through the bounding sphere.
For walls, collision test is carried out on the bottom wall through an inside/outside test
A bounding sphere is applied to the portal area for collision detection of the portal.
If the viewpoint exists outside the bounding sphere area of the portal,
the walkthrough scene should be created using geometry models and portal images of the current cell.
if the viewpoint exists inside the bounding sphere area of the portal,
the walkthrough scene is created using geometry models and the portal images of the current cell and the next cell
But, the portals shared by the current cell and the next cell are not replaced with portal images.
VSMM 2000

15. Implementation Implementation Test Model
VRML file as the input data for modeling
A hierarchical structuring of the models
OpenGL for rendering geometry model
Bounding volume for collision detection
Test Model
The number of cells: 13
consists of 36 portals and 180,000 polygons
We have used VRML file as the input data for modeling the virtual space and OpenGL for rendering geometry model
Bounding volume has been used for collision detection.
To implement and test the repalcement algorithm of geometry visible through portal.
The inside of the virtual building has been modelled as shown in table below
The number of cells of the virtual building is 13, and consists of 36 portals and 180,000 polygons.
VSMM 2000

16. Geometry Model vs Portal Images vs Environment Map
Result 1
Geometry Model vs Portal Images vs Environment Map
This figure is the result of the realtime walkthtough algorithm based on the virtual building model explained above.
It shows the resulting images of applying the geometry model, portal texture algorithm and the environment map algorithm respectively.
As you can see from comparing the images, the method of applying the environment map can create a continuous image from a new viewpoint without any extra cost and exposure caused by the lack of information.
VSMM 2000

17. Result 2 VSMM 2000 Geometry Model (Flythrough)
Geometry Model (Walkthrough)
Environment Map(Flythrough)
Environment Map (Walkthrough)
This figure compares the virtual space model with the geometry model and environment map algorithm
Left figure shows the view of the flythrough, Right figure shows the view of the walkthrough
VSMM 2000

18. Demo
This one is the scene that represent the virtual space model with the geometry
It shows the scene using environment maps
It is the scene that show how to use the environment map method
This one compares the virtual space model with the geometry model and environment map algorithm using bird eye’s view
VSMM 2000

19. Conclusion & Future Work
Real-time walkthrough algorithm of virtual space using an environment map
The popping effect and the holes in the image can be eliminated
It eliminates the extra cost needed to render the virtual space.
Future Work
the problem caused by the parallax
re-structuring the environment map using depth information.
Extending the algorithm to the outside of the builing is needed.
Extending the applicable range to the dynamic environment should also be investigated in the future.
I have proposed the real-time walkthrough algorithm of virtual space using an environment map
The popping effect and the holes in the image can be eliminated
It eliminates the extra cost needed to render the virtual space.
The method using an environment map cannot guarantee the correct visibility of the image created from the required viewpoint.
To resolve the problem caused by the parallax, re-structuring the environment map using depth information would be required.
Also, further investigation on extending the algorithm to the outside of the builing is needed.
An algorithm for further extending the applicable range to the dynamic environment should also be investigated in the future.
VSMM 2000