Parallax barrier

Last updated

Comparison of parallax-barrier and lenticular autostereoscopic displays. Note: The figure is not to scale. Lenticules can be modified and more pixels can be used to make automultiscopic displays Parallax barrier vs lenticular screen.svg
Comparison of parallax-barrier and lenticular autostereoscopic displays. Note: The figure is not to scale. Lenticules can be modified and more pixels can be used to make automultiscopic displays

A parallax barrier is a device placed in front of an image source, such as a liquid crystal display, to allow it to show a stereoscopic or multiscopic image without the need for the viewer to wear 3D glasses. Placed in front of the normal LCD, it consists of an opaque layer with a series of precisely spaced slits, allowing each eye to see a different set of pixels, so creating a sense of depth through parallax in an effect similar to what lenticular printing produces for printed products [1] [2] and lenticular lenses for other displays. A disadvantage of the method in its simplest form is that the viewer must be positioned in a well-defined spot to experience the 3D effect. However, recent versions of this technology have addressed this issue by using face-tracking to adjust the relative positions of the pixels and barrier slits according to the location of the user's eyes, allowing the user to experience the 3D from a wide range of positions. [3] [4] Another disadvantage is that the horizontal pixel count viewable by each eye is halved, reducing the overall horizontal resolution of the image. [5]

Contents

History

Berthier's diagram: A-B=glass plate, with a-b=opaque lines, P=Picture, O=Eyes, c-n=blocked and allowed views (Le Cosmos 05-1896) 1896-05 A. Berthier - (le cosmos p. 229).jpg
Berthier's diagram: A-B=glass plate, with a-b=opaque lines, P=Picture, O=Eyes, c-n=blocked and allowed views (Le Cosmos 05-1896)

The principle of the parallax barrier was independently invented by Auguste Berthier, who published an article on stereoscopic pictures including his new idea illustrated with a diagram and pictures with purposely exaggerated dimensions of the interlaced image strips, [6] and by Frederic E. Ives, who made and exhibited a functional autostereoscopic image in 1901. [7] About two years later, Ives began selling specimen images as novelties, the first known commercial use.

In the early 2000s, Sharp developed the electronic flat-panel application of this old technology to commercialization, briefly selling two laptops with the world's only 3D LCD screens. [8] These displays are no longer available from Sharp but still being manufactured and further developed from other companies like Tridelity and SpatialView. Similarly, Hitachi has released the first 3D mobile phone for the Japanese market under distribution by KDDI. [9] [10] In 2009, Fujifilm released the Fujifilm FinePix Real 3D W1 digital camera, which features a built-in autostereoscopic LCD measuring 2.8" diagonal. Nintendo has also implemented this technology on its portable gaming consoles, the New Nintendo 3DS and the New Nintendo 3DS XL, after first including it on the previous console, the Nintendo 3DS.

Applications

In addition to films and computer games, the technique has found uses in areas such as molecular modelling [ citation needed ] and airport security. [11] It is also being used for the navigation system in the 2010-model Range Rover, [12] allowing the driver to view (for example) GPS directions, while a passenger watches a movie. It is also used in the Nintendo 3DS hand-held game console [13] and LG's Optimus 3D and Thrill smartphones, [14] HTC's EVO 3D [15] as well as Sharp's Galapagos smartphone series.

The technology is harder to apply for 3D television sets, because of the requirement for a wide range of possible viewing angles. A Toshiba 21-inch 3D display uses parallax barrier technology with 9 pairs of images, to cover a viewing angle of 30 degrees. [16]

Design

The slits in the parallax barrier allow the viewer to see only left image pixels from the position of their left eye, right image pixels from the right eye. When choosing the geometry of the parallax barrier the important parameters that need to be optimised are; the pixel – barrier separation d, the parallax barrier pitch f, the pixel aperture a, and the parallax barrier slit width b. [17]

A cross sectional diagram of a parallax barrier, with all its important dimensions labelled. ParallaxBarrierCrossSection.svg
A cross sectional diagram of a parallax barrier, with all its important dimensions labelled.

Pixel separation

The closer the parallax barrier is to the pixels, the wider the angle of separation between the left and right images. For a stereoscopic display the left and right images must hit the left and right eyes, which means the views must be separated by only a few degrees. The pixel- barrier separation d for this case can be derived as follows.

From Snell’s law:

For small angles: and

Therefore:

For a typical auto-stereoscopic display of pixel pitch 65 micrometers, eye separation 63mm, viewing distance 30 cm, and refractive index 1.52, the pixel-barrier separation needs to be about 470 micrometers.

Pitch

The pitch of a parallax barrier should ideally be roughly two times the pitch of the pixels, but the optimum design should be slightly less than this. This perturbation to the barrier pitch compensates for the fact that the edges of a display are viewed at a different angle to that of the centre, it enables the left and right images target the eyes appropriately from all positions of the screen.

a). If the parallax barrier had exactly twice the pitch of the pixels, it would be aligned in synchronisation with the pixel across whole of the display. The left and right views would be emitted at the same angles all across the display. It can be seen that the viewer's left eye does not receive the left image from all points on the screen. The display does not work well. b). If the barrier pitch is modified, the views can be made to converge, such that the viewer sees the correct images from all points on the screen. c). Shows the calculation which determines the pitch of the barrier that is needed. p is the pixel pitch, d is the pixel barrier separation, f is the barrier pitch. ParallaxBarrierPitchCorrection.png
a). If the parallax barrier had exactly twice the pitch of the pixels, it would be aligned in synchronisation with the pixel across whole of the display. The left and right views would be emitted at the same angles all across the display. It can be seen that the viewer’s left eye does not receive the left image from all points on the screen. The display does not work well. b). If the barrier pitch is modified, the views can be made to converge, such that the viewer sees the correct images from all points on the screen. c). Shows the calculation which determines the pitch of the barrier that is needed. p is the pixel pitch, d is the pixel barrier separation, f is the barrier pitch.

Optimum pixel aperture and barrier slit width

In a parallax barrier system for a high-resolution display, the performance (brightness and crosstalk) can be simulated by Fresnel diffraction theory. [18] From these simulations, the following can be deduced. If the slit width is small, light passing the slits is diffracted heavily causing crosstalk. The brightness of the display is also reduced. If the slit width is large, light passing the slit does not diffract so much, but the wider slits create crosstalk due to geometric ray paths. Therefore, the design suffers more crosstalk. The brightness of the display is increased. Therefore, the best slit width is given by a tradeoff between crosstalk and brightness.

Barrier position

Note that the parallax barrier may also be placed behind the LCD pixels. In this case, light from a slit passes the left image pixel in the left direction, and vice versa. This produces the same basic effect as a front parallax barrier.

Techniques for switching

In a parallax barrier system, the left eye sees only half the pixels (that is to say the left image pixels) and the same is true for the right eye. Therefore, the resolution of the display is reduced, and so it can be advantageous to make a parallax barrier that can be switched on when 3D is needed or off when a 2D image is required. One method of switching the parallax barrier on and off is to form it from a liquid crystal material, the parallax barrier can then be created similar to the way that an image is formed in a liquid crystal display. [19]

An autostereoscopic display that is switchable between 2D and 3D. In 3D mode the parallax barrier is formed with an LC cell, in a similar way to how an image is created on an LCD. In 2D mode the LC cell is switched into a transparent state such that no parallax barrier exists. In this case the light from the LCD pixels can go in any direction and the display acts like a normal 2D LCD. ParallaxBarrierSwitching.svg
An autostereoscopic display that is switchable between 2D and 3D. In 3D mode the parallax barrier is formed with an LC cell, in a similar way to how an image is created on an LCD. In 2D mode the LC cell is switched into a transparent state such that no parallax barrier exists. In this case the light from the LCD pixels can go in any direction and the display acts like a normal 2D LCD.

Time multiplexing to increase resolution

Time multiplexing provides a means of increasing the resolution of a parallax barrier system. [20] In the design shown each eye is able to see the full resolution of the panel.

A diagram showing how 3D can be created using time multiplexed parallax barrier. In the first time cycle, the slits in the barrier are arranged in a conventional way for a 3D display, and the left and right eyes see the left and right eye pixels. In the next time cycle, the positions of the slits are changed (possible because each slit is formed with an LC shutter). In the new barrier position, the right eye can see the pixels that were hidden in the previous time cycle. These uncovered pixels are set to show the right image (rather than the left image which they showed in the previous time cycle). The same is true for the left eye. This cycling between the two positions of the barrier, and the interlacing pattern, enables both eyes to see the correct image from half the pixels in the first time cycle, and the correct image from the other half of the pixels in the other time cycle. The cycles repeats every 50th of a second so that the switching is not noticeable to the user, but user has the impression that the appearance each eye is seeing an image from all the pixels. Consequently, the display appears to have full resolution. ParallaxBarrierTimeMultiplexing.svg
A diagram showing how 3D can be created using time multiplexed parallax barrier. In the first time cycle, the slits in the barrier are arranged in a conventional way for a 3D display, and the left and right eyes see the left and right eye pixels. In the next time cycle, the positions of the slits are changed (possible because each slit is formed with an LC shutter). In the new barrier position, the right eye can see the pixels that were hidden in the previous time cycle. These uncovered pixels are set to show the right image (rather than the left image which they showed in the previous time cycle). The same is true for the left eye. This cycling between the two positions of the barrier, and the interlacing pattern, enables both eyes to see the correct image from half the pixels in the first time cycle, and the correct image from the other half of the pixels in the other time cycle. The cycles repeats every 50th of a second so that the switching is not noticeable to the user, but user has the impression that the appearance each eye is seeing an image from all the pixels. Consequently, the display appears to have full resolution.

The design requires a display that can switch fast enough to avoid image flicker as the images swap each frame.

Tracking barriers for increased viewing freedom

In a standard parallax barrier system, the viewer must position themselves in an appropriate location so that the left and right eye views can be seen by their left and right eyes respectively. In a ‘tracked 3D system’, the viewing freedom can be increased considerably by tracking the position of the user and adjusting the parallax barrier so that the left and right views are always directed to the user's eyes correctly. Identification of the user's viewing angle can be done by using a forward-facing camera above the display and image-processing software that can recognise the position of the user's face. Adjustment of the angle at which the left and right views are projected can be done by mechanically or electronically shifting the parallax barrier relative to the pixels. [21] [22] [23]

Crosstalk

Crosstalk is the interference that exists between the left and right views in a 3D display. In a display with high crosstalk, each eye is able to see the image intended for the other eye faintly superimposed. The perception of crosstalk in stereoscopic displays has been studied widely. It is generally acknowledged that the presence of high levels of crosstalk in a stereoscopic display is detrimental. The effects of crosstalk in an image include: ghosting and loss of contrast, loss of 3D effect and depth resolution, and viewer discomfort. The visibility of crosstalk (ghosting) increases with increasing contrast and increasing binocular parallax of the image. For example, a stereoscopic image with high contrast will exhibit more ghosting on a particular stereoscopic display than will an image with low contrast. [24]

Measurement

A technique to quantify the level of crosstalk from a 3D display involves measuring the percentage of light that deviates from one view to the other. [18]

Measurement of crosstalk in 3D displays. Crosstalk is the percentage of light from one view leaking to the other view. The measurements and calculations above show how crosstalk is defined when measuring crosstalk in the left image. Diagrams a) sketch the intensity measurements that need to be made for different outputs from the 3D display. Table b) describe their purpose. Equation c) is used to derive the crosstalk. It is the ratio of the light leakage from the right image into the left image but note that the imperfect black level of the LCD is subtracted out from the result so that it does not change the crosstalk ratio. StereoscopicCrosstalk.png
Measurement of crosstalk in 3D displays. Crosstalk is the percentage of light from one view leaking to the other view. The measurements and calculations above show how crosstalk is defined when measuring crosstalk in the left image. Diagrams a) sketch the intensity measurements that need to be made for different outputs from the 3D display. Table b) describe their purpose. Equation c) is used to derive the crosstalk. It is the ratio of the light leakage from the right image into the left image but note that the imperfect black level of the LCD is subtracted out from the result so that it does not change the crosstalk ratio.

The crosstalk in a typical parallax-barrier-based 3D system at the best eye position might be 3%. Results of subjective tests [25] carried out to determine the image quality of 3D images conclude that for high-quality 3D, crosstalk should be 'no greater than around 1 to 2%'.

Causes and countermeasures

Diffraction can be a major cause of crosstalk. [18] Theoretical simulations of diffraction have been found to be a good predictor of experimental crosstalk measurements in emulsion parallax barrier systems. These simulations predict that the amount of crosstalk caused by the parallax barrier will be highly dependent on the sharpness of the edges of the slits. For example, if the transmission of the barrier goes from opaque to transparent sharply as it moves from barrier to slit then this produces a wide diffraction pattern and consequently more crosstalk. If the transition is smoother then the diffraction will not spread so widely and less crosstalk will be produced. This prediction is consistent with experimental results for a slightly soft-edged barrier (whose pitch was 182 micrometers, slit width was 48 micrometers, and transition between opaque and transmissive occurred over a region of about 3 micrometers). The slightly soft-edged barrier has a crosstalk of 2.3%, which is slightly lower than the crosstalk from a harder-edged barrier which was about 2.7%. The diffraction simulations also suggest that if the parallax barrier slit edges had a transmission that decreases over a 10 micrometers region, then crosstalk could become as 0.1. Image processing is an alternative crosstalk countermeasure. The figure shows the principle behind crosstalk correction. [26]

The principle of crosstalk correction. CrosstalkCorrection.svg
The principle of crosstalk correction.

There are three main types of autostereoscopic displays with a parallax barrier:

See also

Related Research Articles

<span class="mw-page-title-main">Stereoscopy</span> Technique for creating or enhancing the illusion of depth in an image

Stereoscopy is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word stereoscopy derives from Greek στερεός (stereos) 'firm, solid', and σκοπέω (skopeō) 'to look, to see'. Any stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope.

Flicker is a visible change in brightness between cycles displayed on video displays. It applies to the refresh interval on cathode ray tube (CRT) televisions and computer monitors, as well as plasma computer displays and televisions.

<span class="mw-page-title-main">Lenticular printing</span> Technology for creating optical illusions

Lenticular printing is a technology in which lenticular lenses are used to produce printed images with an illusion of depth, or the ability to change or move as they are viewed from different angles.

<span class="mw-page-title-main">3D display</span> Display device

A 3D display is a display device capable of conveying depth to the viewer. Many 3D displays are stereoscopic displays, which produce a basic 3D effect by means of stereopsis, but can cause eye strain and visual fatigue. Newer 3D displays such as holographic and light field displays produce a more realistic 3D effect by combining stereopsis and accurate focal length for the displayed content. Newer 3D displays in this manner cause less visual fatigue than classical stereoscopic displays.

<span class="mw-page-title-main">Active shutter 3D system</span> Method of displaying stereoscopic 3D images

An active shutter 3D system is a technique of displaying stereoscopic 3D images. It works by only presenting the image intended for the left eye while blocking the right eye's view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image.

<span class="mw-page-title-main">Polarized 3D system</span> Using polarized light to create a 3D image

A polarized 3D system uses polarization glasses to create the illusion of three-dimensional images by restricting the light that reaches each eye.

<span class="mw-page-title-main">Anaglyph 3D</span> Method of representing images in 3D

Anaglyph 3D is the stereoscopic 3D effect achieved by means of encoding each eye's image using filters of different colors, typically red and cyan. Anaglyph 3D images contain two differently filtered colored images, one for each eye. When viewed through the "color-coded" "anaglyph glasses", each of the two images reaches the eye it's intended for, revealing an integrated stereoscopic image. The visual cortex of the brain fuses this into the perception of a three-dimensional scene or composition.

<span class="mw-page-title-main">Autostereoscopy</span> Any method of displaying stereoscopic images without the use of special headgear or glasses

Autostereoscopy is any method of displaying stereoscopic images without the use of special headgear, glasses, something that affects vision, or anything for eyes on the part of the viewer. Because headgear is not required, it is also called "glasses-free 3D" or "glassesless 3D". There are two broad approaches currently used to accommodate motion parallax and wider viewing angles: eye-tracking, and multiple views so that the display does not need to sense where the viewer's eyes are located. Examples of autostereoscopic displays technology include lenticular lens, parallax barrier, and may include Integral imaging, but notably do not include volumetric display or holographic displays.

Parallax scanning depth enhancing imaging methods rely on discrete parallax differences between depth planes in a scene. The differences are caused by a parallax scan. When properly balanced (tuned) and displayed, the discrete parallax differences are perceived by the brain as depth.

Display motion blur, also called HDTV blur and LCD motion blur, refers to several visual artifacts that are frequently found on modern consumer high-definition television sets and flat panel displays for computers.

Digital 3D is a non-specific 3D standard in which films, television shows, and video games are presented and shot in digital 3D technology or later processed in digital post-production to add a 3D effect.

<span class="mw-page-title-main">3D stereo view</span> Enables viewing of objects through any stereo pattern

A 3D stereo view is the viewing of objects through any stereo pattern.

<span class="mw-page-title-main">Fujifilm FinePix Real 3D</span>

The Fujifilm FinePix Real 3D W series is a line of consumer-grade digital cameras designed to capture stereoscopic images that recreate the perception of 3D depth, having both still and video formats while retaining standard 2D still image and video modes. The cameras feature a pair of lenses, and an autostereoscopic display which directs pixels of the two offset images to the user's left and right eyes simultaneously. Methods are included for extending or contracting the stereoscopic baseline, albeit with an asynchronous timer or manually depressing the shutter twice. The dual-lens architecture also enables novel modes such as simultaneous near and far zoom capture of a 2D image. The remainder of the camera is similar to other compact digital cameras.

A 3D camcorder can record 3D video.

<span class="mw-page-title-main">Famicom 3D System</span> Accessory for the Nintendo Family Computer

The Famicom 3D System is a Japan-exclusive accessory for the Nintendo Family Computer released in 1987.

<span class="mw-page-title-main">Wiggle stereoscopy</span> 3-D image display method

Wiggle stereoscopy is an example of stereoscopy in which left and right images of a stereogram are animated. This technique is also called wiggle 3-D, wobble 3-D, or sometimes Piku-Piku.

FPR is a technology promoted by LG that is employed in its line of 3D televisions based on circular polarization. It shows left and right images through different patterns in a circular polarizer. Left/right polarized glasses allow the left and right images to then be seen by the left and right eyes separately. Both images are combined in the brain and generate the 3D effect. The FPR technology uses the precise film which polarizes different pixels differently to show a different image for each eye. FPR 3D tech is said to deliver a brighter screen with less cross talk, less ghosting, and no flickering.

A 3D display is multiscopic if it projects more than two images out into the world, unlike conventional 3D stereoscopy, which simulates a 3D scene by displaying only two different views of it, each visible to only one of the viewer's eyes. Multiscopic displays can represent the subject as viewed from a series of locations, and allow each image to be visible only from a range of eye locations narrower than the average human interocular distance of 63 mm. As a result, not only does each eye see a different image, but different pairs of images are seen from different viewing locations.

<span class="mw-page-title-main">Stereoscopic video game</span>

A stereoscopic video game is a video game which uses stereoscopic technologies to create depth perception for the player by any form of stereo display. Such games should not be confused with video games that use 3D game graphics on a mono screen, which give the illusion of depth only by monocular cues but lack binocular depth information.

<span class="mw-page-title-main">Barrier-grid animation and stereography</span> Animation method

Barrier-grid animation or picket-fence animation is an animation effect created by moving a striped transparent overlay across an interlaced image. The barrier-grid technique originated in the late 1890s, overlapping with the development of parallax stereography (Relièphographie) for 3D autostereograms. The technique has also been used for color-changing pictures, but to a much lesser extent.

References

  1. ↑ Howard, Bill (2003). "Reviews by PC Magazine - Sharp Actius RD3D". www.pcmag.com. Archived from the original on 20 April 2008. Retrieved 25 January 2008.
  2. ↑ "The Register - Sharp's 3D LCD: how's that work, then?". www.theregister.co.uk. 2004. Retrieved 25 January 2008.
  3. ↑ no specified authors . (2015). New Nintendo 3ds. December 28, 2016, by Nintendo of America Inc Website: https://www.nintendo.com/3ds/new-nintendo-3ds/
  4. ↑ Norris, Ashley (6 December 2002). "Guardian Unlimited - Special reports - The return of 3D". London: www.guardian.co.uk. Retrieved 25 January 2008.
  5. ↑ "Better glasses-free 3-D" . Retrieved 1 July 2011. A fundamentally new approach to glasses-free 3-D displays could save power, widen the viewing angle and make 3-D illusions more realistic.
  6. ↑ Berthier, Auguste. (May 16 and 23, 1896). "Images stéréoscopiques de grand format" (in French). Cosmos34 (590, 591): 205–210, 227-233 (see 229-231)
  7. ↑ Ives, Frederic E. (1902). "A novel stereogram". Journal of the Franklin Institute. 153: 51–52. doi:10.1016/S0016-0032(02)90195-X. Reprinted in Benton "Selected Papers n Three-Dimensional Displays".
  8. ↑ "2D/3D Switchable Displays" (PDF). Sharp white paper. Archived (PDF) from the original on 30 May 2008. Retrieved 19 June 2008.
  9. ↑ "Woooケータイ H001 | 2009年 | 製品アーカイブ | au by KDDI". Au.kddi.com. Archived from the original on 4 May 2010. Retrieved 15 June 2010.
  10. ↑ "Hitachi Comes Up with 3.1-Inch 3D IPS Display". News.softpedia.com. 12 April 2010. Retrieved 15 June 2010.
  11. ↑ Twist, Jo (9 June 2004). "BBC NEWS - Technology - Easy 3D X-rays for air security". news.bbc.co.uk. Retrieved 25 January 2008.
  12. ↑ "Land Rover e-brochure PDF (page 19)" (PDF). www.landrover.com. 2011. Retrieved 29 December 2011.
  13. ↑ "Nintendo unveils 3DS handheld games console". www.bbc.co.uk. 15 June 2010. Retrieved 17 June 2010.
  14. ↑ "LG unveils world's first 3-D smartphone". www.cnn.com. 2011. Retrieved 15 February 2011.
  15. ↑ HTC EVO 3D, from GSMArena
  16. ↑ "Toshiba Mobile Display touts 21-inch glasses-free 3D HDTV, raises a few eyebrows". Engadget. 27 April 2010.
  17. ↑ Yamamoto, Hirotsugu (October 2000). "Optimum parameters and viewing areas of stereoscopic full colour LED display using parallax barrier". IEICE Trans Electron. E83-c no 10.
  18. 1 2 3 Montgomery, David J. (2001). "Performance of a flat-panel display system convertible between 2D and autostereoscopic 3D modes". In Woods, Andrew J; Bolas, Mark T; Merritt, John O; Benton, Stephen A (eds.). Stereoscopic Displays and Virtual Reality Systems VIII. Vol. 4297. pp. 148–159. CiteSeerX   10.1.1.197.3858 . doi:10.1117/12.430813. S2CID   122846572.
  19. ↑ "2D/3D Switchable Displays" (PDF). Sharp white paper. Archived (PDF) from the original on 30 May 2008. Retrieved 19 June 2008.
  20. ↑ USpatent US6476850,Kenneth Erbey,"Apparatus for the generation of a stereoscopic display"
  21. ↑ USpatent 5808792,Graham John Woodgate, David Ezra, Nicolas Steven Holliman, Basil Arthur Omar, Richard Robert Moseley, Jonathan Harrold,"Autostereoscopic display and method of controlling an autostereoscopic display",issued 1995-February-9
  22. ↑ Mather, Jonathan (June 2011). "3D TV without glasses". Physics World. 24 (6): 33–36. Bibcode:2011PhyW...24f..33M. doi:10.1088/2058-7058/24/06/34.
  23. ↑ "Archived copy". Archived from the original on 16 June 2011. Retrieved 11 October 2012.{{cite web}}: CS1 maint: archived copy as title (link)
  24. ↑ Andrew Woods (2010), Understanding Crosstalk in Stereoscopic Displays (PDF), retrieved 21 September 2012
  25. ↑ Atsuo Hanazato; et al. (2000), "Subjective evaluation of crosstalk disturbance in stereoscopic displays", SID
  26. ↑ USpatent 8144079,Jonathan Mather, David J. Montgomery, Graham R. Jones, Diana U. Kean,"Multiple-viewer multiple-view display and display controller",issued 2005-January-26

Commons-logo.svg Media related to Parallax barrier at Wikimedia Commons

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.