Cosc 6326/Psych6750X

1. Cosc 6326/Psych6750X Vision and Visual Displays

2. Depth perception • retinal images are 2-D but world is 3-D. Suggests loss of information and ambiguity. • how do we (or do we) reconstruct 3-D environments? We need additional information • assumptions/models of the structure of the environment • multiple views accumulated over time (structure from motion) and/or space (binocular vision) • metric, qualitative, task specific

3. This and many of these figures from Howard and Rogers, 2002

4. Pictorial Cues to Depth • Static monocular depth perception • ambiguous interpretation, no general solutions • rely on additional information • unconscious inference • invariants • hypothesis testing • observer motion/active vision • regularization • constraints • …

5. Perspective • Variety of related effects due to perspective projection • linear perspective • foreshortening • texture gradients • image size change • familiar size • height in field

6. A Virtual image Image planes q P B B' D f A’ A’

7. http://zeus.rutgers.edu/~ikovacs/SandP

11. relative size can be a cue

13. Familiar size theoretically can be used to judge distance • controversial • confusion between judgements of angular and linear size

14. Height in the field • rectangle looks further away than triangle • assumed to lie on ground plane • nearness to horizon signals relative distance

15. texture gradients • scaling • density • aspect ratio/compression Todd and Reichel 1989

17. lots of basic support in three-dimensional computer graphics for perspective cues (texture mapping, view/projective transformations, …) http://www.cs.virginia.edu/~gfx/Courses/2001/Intro.fall.01/

18. Calibration and alignment very important • focal length should be precisely calibrated (especially in AR) • distortion causes misjudgement of 2D contour and 3D depth from perspective • agreement among pictorial cues and with other depth cues

19. Occlusion cues • interposition • transparency • specifies depth order only

20. support in computer graphics • back face culling • z buffering

21. Shading and Shadows • Shape from shading is ambiguous • illumination from above • viewing from above • global convexity • complicated effects with specular surfaces Langer and Bulthoff 2001

23. Computer graphics support for shading • texture and lighting of polygons (shading) • bump mapping/pixel shaders (hardware) • ray tracing (not real time yet) • little/no hardware support for shadows

24. Blur and accomodation cues • lens accommodates to bring fixated object into focus • accommodation is correlated with distance, some evidence that accommodation can contribute to distance perception • change in accommodation may signal relative depth • pupil size specifies certain depth of focus • nearer and further objects are defocused but sign is ambiguous (near vs. far)

25. changes in defocus with accommodation can signal depth (depth sign) and help distinguish blur from variations in the sharpness of markings on surfaces • occlusion edge blur may be cue on its own • blur, vignetting • which surface has blur may specify depth order

26. in computer graphics can simulate with image blur (in hardware with NVIDIA type shaders) • but does not change with accommodation

27. Aerial Perspective • Effects of haze and atmospheric scatter - contrast of images decreases with increased distance • Distance and size of objects tend to be overestimate • Favourite in games: fog, dust