LCT4 - 2
Sharp Laboratories of Europe Ltd, Oxford Science Park, Oxford, OX4 4GB, UK.
ABSTRACT
A privacy effect for mobile LCD devices: “Colour
Veil View”, has been developed which allows reconfigurable colour side images to be displayed, obscuring the main image content for off-axis viewers. The effect requires image data modification only, so is switchable and causes no image quality reduction in the public mode.
1. INTRODUCTION
Since the development of high information content, wide viewing angle capability LCDs for portable devices such as mobile phones, PDAs and notebook PCs, it has been desirable to provide a switchable privacy function to allow confidential information to be viewed in public places, without compromising the capability to display public information over a wide viewing angle when desired. The objective of this work was to develop an inexpensive, easily implementable, switchable display privacy technology which retained 100% of the base display performance when not in the private mode.
Displays with an electronically switchable privacy function have been previously provided on both mobile phones and notebooks PCs, but these have
Figure 1(a)
Measured VA mode display gamma curves
If the same data series are normalised to their respective maximum values, and the off-axis luminance is shown as a function of the on-axis luminance, for each input data level, as in Figure
1(b), the relative error in off-axis luminance is shown. It can be seen that, although there is minimal contrast inversion, and the black-to-white contrast level off-axis remains high, the relative luminance for all mid-grey values off-axis is excessively high. required either additional LC cells attached to the image panel [1], or modified image panels with a portion of the display sub-pixels used to control the viewing angle, rather than contribute to the image detail and brightness [2-3]. These methods incur both significant cost due to the additional or modified display hardware, and result in degraded display performance when the privacy feature is not in use, due to either absorption or reflection of light from the addition panel, or reduction of resolution or aperture ratio in displays with dedicated portions for view angle control only. The methods presented here use only image data modifications, so require no modification to the LC panel hardware, and therefore allow the display to be operated in the public mode with no image quality degradation
2. METHOD
2.1 VA Mode LC Viewing Angle dependency
Figure 1(a) shows the display luminance against data level for a multidomain vertically aligned (VA)
LC mode display [4-5], measured along the display normal (on-axis) and at a 50° inclination in the horizontal direction (off-axis).
Figure 1(b)
Normalised off-axis against on-axis luminance for a VA mode display
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2.2 Sharing Luminance between Pairs of Pixels
Our approach utilises this inherent viewing angle dependent gamma curve variation to provide a means of varying the average off-axis luminance produced by a pair of pixels in combination, while maintaining the average on-axis luminance. This effect is illustrated in Figure 2, which shows the resulting average off-axis to on-axis luminance values for a range of combinations of two pixel data values.
Figure 2
Normalised off-axis against on-axis average luminance for pixels with all combinations of two data values
The shape of this space of available average on-axis to off-axis luminance points can be understood by considering its extremes. If the two pixels are given the same input data value, the average off-axis to on-axis luminance values are the same as for a single pixel, and the points follow the path of the off-axis measurement of Figure 1(b).
If however, the one pixel has its on-axis luminance increased from the off state to fully on, while the second pixel remains off, the points follow a version of this same curve scaled by half on both axes. The increase of luminance of the second pixel then produces a repeat of this trace, starting at finishing point of the first pixels trace: the
(0.5,0.5) off-axis to on-axis luminance point.
2.3 Image Data Manipulation
As can be seen from Figure 2, if we take a pair of pixels with the same input data level, an off-axis luminance at the upper edge of the available off-axis to on-axis luminance space is produced, indicated by the blue circles in the figure. If the individual data levels are then modified to transfer luminance from one pixel of the pair to the other, the off axis luminance is reduced, by an amount dependent on the amount of luminance transferred, while the combined on-axis luminance of the pair is unchanged. The amount of luminance transfer may be increased until one of the two pixels is at its maximum or minimum level (data level 0 or 255 for an 8 bit/colour display) at which point the off-axis luminance is minimised, and the combined off-axis to on-axis luminance of the pair is at the lower edge of the available space indicated by the red diamonds in Figure 2.
A means of merging a main and side image to provide a combined multi-view image therefore exists, by way of modifying the data values of pairs of neighbouring pixels in the input main image data by an amount inversely proportional to the side image data at the corresponding image region. If the data modifications are made so as to maintain the average on-axis luminance of the pair, and the resolution of the display is sufficiently high to cause the eye to perceive the average luminance of the pair rather than their individual values, then the side image will be visible only to off-axis viewers without disrupting the main image appearance on-axis.
3. IMPLEMENTATION
3.1 LUT based method
For the purposes of simplified processing, the required amounts of luminance transfer were pre-calculated and stored in a Lookup Table in the display control ASIC. This LUT consists of a pair of output data values for each combination of main and side image input data levels. In order to reproduce the side image as clearly as possible to the off-axis viewer, the output data values stored were those that produced equally spaced off-axis luminances for each main image input data value.
A plot of example LUT values for a 4 level side image is shown in Figure 3 (a), and the resultant off-axis to on-axis luminance values for each side image value are shown in Figure 3(b).
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Figure 3(a)
A plot of example LUT values for a 4 level side image effect. Note each side image value has two output values per main image value, and the difference in these values decreases with increasing side image value.
Figure 3(b)
A plot of the resulting off-axis to on-axis luminance responses produced by the LUT of Figure3(a).
Note that generally, due to small differences in the gamma curve for each of the display primaries, a unique LUT is required for each colour channel, so a four side image level LUT for each of the R, and B inputs would allow a 64 colour side image.
When in the privacy mode, the pixel data values comprising the main image are replaced with the data values corresponding to the main image data and side image data at each point in the image from the LUT. The two data values provided by the
LUT for each main image/side age data combination are assigned to pixels alternately, so that on average the intended on-axis and off-axis luminance is generated by each neighbouring pair of pixels.
3.2 Main image pre-conditioning
As can be seen from Figure 3(b), the available contrast of the side image is strongly main image value dependent. The greatest side image contrast is available for main image regions of luminance equal to half the maximum. For this reason, it is effective to pre-compress the main image to increase privacy strength at the expense of contrast. Also, due to the fact that pairs of pixels are required to produce an average on-axis luminance with a degree of control over the simultaneous off-axis luminance, there is some perceived resolution loss incurred operating the display in the private mode. A content adaptive image filter was developed to mitigate this resolution loss [6]. This filter has sufficiently low resource requirement to allow inclusion on the modified driver IC along with the processing for retrieving the LUT output values based on the input main and side image data. This allowed main images of the native display resolution to still be displayed in the private mode, with only a minor blurring effect apparent to prevent noticeable image artefacts.
4 . RESULTS
4.1 Side Image Appearance
Despite a maximum side image contrast ratio of only 1.9:1 at a viewing angle of 50° to the display normal, and reduced main image resolution and brightness (which may be offset by an increase in backlight power during private mode use), a satisfactorily strong multiview effect for privacy purposes was obtained using the method outlined above. 64 side image colours were found to be sufficient to accurately reproduce the side image.
Functionality was also included to allow animation of the side image and the use of user generated content as the side image. In order to optimise the privacy effect for the type of main image content which was most likely to require privacy protection
(fine text in SMS messaging, email and web browsing) the main image was pre-compressed to half maximum luminance.
Figure 4(a) shows a microscope image of a portion of the display showing both bright and dark side image regions, over a uniform main image region.
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Figure 4(a)
Micrograph of the pixel appearance in the private mode
Although the coarsened sub-pixel appearance in the dark side image region is clear, the effect is not strongly noticeable on the image as a whole, demonstrated by the photographs of the front and side view of such an image in Figure 4(b).
Figure 4(b)
Appearance of the processed image to the viewer on-axis and off-axis.
4.2 Market Acceptance
The “Colour Veil View” multiview image processing method described above has been commercialised on all Sharp mobile phone models in Japan since 2008, replacing the additional LC cell based “Veil View” technology. “Colour Veil
View” provides users of mobile LCDs with effective privacy, with a fully reconfigurable masking side image, without any loss of image quality in the public display mode.
ACKNOWLEDGMENTS
The authors would like to thank the members of the Platform technology Development Centre
(PTDC) of Sharp Communications System Group
(CSG) in Hiroshima for their support in development of this work.
REFERENCES
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