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Refined Dynamic Luminance Scaling Algorithm to Mitigate Flicker

IP.com Disclosure Number: IPCOM000176314D
Publication Date: 2008-Nov-12
Document File: 6 page(s) / 153K

Publishing Venue

The IP.com Prior Art Database

Abstract

Currently, in order to reduce LCD power consumption, portable devices are using dynamic luminance scaling (DLS). DLS achieves the power reduction goal by boosting image luminance and dimming LCD backlight simultaneously while keeping the aggregate brightness unchanged. However, inaccurately synchronized luminance and backlight change often cause flicker issues, which degrade the video experience greatly. This article introduces a new method that mitigates the flicker issue significantly without compromising the power saving efficiency of DLS. Keywords: LCD Backlight Dimming, Dynamic Luminance Scaling, Scene Change Detection

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Refined Dynamic Luminance Scaling Algorithm to Mitigate Flicker

Abstract

Currently, in order to reduce LCD power consumption, portable devices are using dynamic luminance scaling (DLS).  DLS achieves the power reduction goal by boosting image luminance and dimming LCD backlight simultaneously while keeping the aggregate brightness unchanged.  However, inaccurately synchronized luminance and backlight change often cause flicker issues, which degrade the video experience greatly.  This article introduces a new method that mitigates the flicker issue significantly without compromising the power saving efficiency of DLS.

Keywords: LCD Backlight Dimming, Dynamic Luminance Scaling, Scene Change Detection

1.      How DLS Works

LCD backlight plays a significant role in portable device power consumption.  LCDs often consume 60% or more system power during video playback.  To reduce such power consumption, dynamic luminance scaling was developed.  DLS boosts image luminance and backlight dimming simultaneously while keeping the aggregate brightness unchanged.  The aggregate brightness of a picture perceived by human eyes is determined by the transmissivity of the panel and the backlight level.  While the transmissivity is a function of the picture luminance (Y component in YUV format), boosting the luminance of a picture will increase the transmissivity of the panel.  In turn, the backlight level can be dimmed proportionally without impacting the aggregate brightness.  The formulas below show the theory of DLS.

l(i) = t(i)*b = (t(i)*β)*(b/ β)

t(i) = Y(i)

 

here l(i) means the aggregate brightness, t(i) means transmissivity if picture i and b is the backlight. The typical relationship between transmissivity and Y is shown in figure 1.

Figure 1, Transmissivity versus Y

        The typical way to boost the Y of a picture is to adjust the coefficients of the color space conversion matrix. The original CSC formula is as below.

R=Y + 1.138*V

G=Y – 0.395*U – 0.580*V

B=Y + 2.032*U

        If DLS is enabled, the new CSC formula is:

R=α*Y + 1.138*V

G=α*Y – 0.395*U – 0.580*V

B=α*Y + 2.032*U

        The effect of DLS is shown in figure 2.

(a) original picture                                (b) Y boosted                                        (c) backlight dimmed

Figure 2, effect of DLS

2.      Flicker Issue Coming with DLS

       

Although DLS saves significant amounts of LCD power (30% or more), it also raises some issues.  Two main issues are color shifting and flicker.  Since luminance boosting is achieved by scaling up the Y component in YUV format, color will no doubt be slightly different after the picture is converted to RGB space.  Currently, two ways are used to mitigate this issue:  limit the Y boost factor...