CAP4730: Computational Structures in Computer Graphics 3D Concepts

Outline Basic Idea of 3D Projections What are some things we didn’t have to worry about before? What are some new things we can do?

Right Handed Coordinate System +X +Y +Z +X +Y +Z

Viewing a 3D world We have a model in this world and would like to view it from a new position. We’ll call this new position the camera or eyepoint. Our job is to figure out what the model looks like on the display plane. +X +Y +Z

Parallel Projection +X +Y +Z

Perspective Projection +X +Y +Z

What are some new things to think about? Hidden Surface Removal Visibility Depth Cueing

How to make a 2D image appear as 3D! Output is typically 2D Images Yet we want to show a 3D world! How can we do this? –We can include ‘cues’ in the image that give our brain 3D information about the scene –These cues are visual depth cues

Visual Depth Cues Monoscopic Depth Cues (single 2D image) Stereoscopic Depth Cues (two 2D images) Motion Depth Cues (series of 2D images) Physiological Depth Cues (body cues)

Monoscopic Depth Cues Interposition –An object that occludes another is closer Shading –Shape info. Shadows are included here Size –Usually, the larger object is closer Linear Perspective –parallel lines converge at a single point Surface Texture Gradient –more detail for closer objects Height in the visual field –Higher the object is (vertically), the further it is Atmospheric effects –further away objects are blurrier Brightness –further away objects are dimmer

Stereoscopic Display Issues Stereopsis Stereoscopic Display Technology Computing Stereoscopic Images Stereoscopic Display and HTDs. Works for objects < 5m. Why?

Stereopsis The result of the two slightly different views of the external world that our laterally-displaced eyes receive.

Time-parallel stereoscopic images Image quality may also be affected by –Right and left-eye images do not match in color, size, vertical alignment. –Distortion caused by the optical system –Resolution –HMDs interocular settings –Computational model does not match viewing geometry.

Disparity If an object is closer than the fixation point, the retinal disparity will be a negative value. This is known as crossed disparity because the two eyes must cross to fixate the closer object. If an object is farther than the fixation point, the retinal disparity will be a positive value. This is known as uncrossed disparity because the two eyes must uncross to fixate the farther object. An object located at the fixation point or whose image falls on corresponding points in the two retinae has a zero disparity.

Convergence Angles i  f2 f1 D1 D2  a b c d 11  +a+c+b+d = 180  +c+d = 180  -  = a+(-b) =  1+  2 = Retinal Disparity 22

Stereoscopic Display Stereoscopic images are easy to do badly, hard to do well, and impossible to do correctly.

Stereoscopic Displays Stereoscopic display systems create a three- dimensional image (versus a perspective image) by presenting each eye with a slightly different view of a scene. –Time-parallel –Time-multiplexed

Time Parallel Stereoscopic Display Two Screens Each eye sees a different screen Optical system directs each eye to the correct view. HMD stereo is done this way. Single Screen Two different images projected on the same screen Images are polarized at right angles to each other. User wears polarized glasses (passive glasses).

Passive Polarized Projection Issues Linear Polarization –Ghosting increases when you tilt head –Reduces brightness of image by about ½ –Potential Problems with Multiple Screens (next slide) Circular Polarization –Reduces ghosting but also reduces brightness and crispness of image even more

Problem with Linear Polarization With linear polarization, the separation of the left and right eye images is dependent on the orientation of the glasses with respect to the projected image. The floor image cannot be aligned with both the side screens and the front screens at the same time.

Time Multiplexed Display Left and right-eye views of an image are computed and alternately displayed on the screen. A shuttering system occludes the right eye when the left-eye image is being displayed and occludes the left-eye when the right-eye image is being displayed.

Stereographics Shutter Glasses

Motion Depth Cues Parallax created by relative head position and object being viewed. Objects nearer to the eye move a greater distance

Pulfrich Effect Neat trick Different levels of illumination require additional time (your frame rates differ base of amount of light) What if we darken one image, and brighten another?

Physiological Depth Cues Accommodation – focusing adjustment made by the eye to change the shape of the lens. (up to 3 m) Convergence – movement of the eyes to bring in the an object into the same location on the retina of each eye.

Summary Monoscopic – Interposition is strongest. Stereopsis is very strong. Relative Motion is also very strong (or stronger). Physiological is weakest (we don’t even use them in VR!) Add as needed –ex. shadows and cartoons

What are some new things we can do? Lighting and Shading! Texturing! Stereo Surfaces –Normals –Materials