LED backlight power saving dimming method based on histogram transformation

Abstract: The power consumption of the backlight can account for more than 50% of the total power consumption of the liquid crystal display module. This paper proposes a backlight power-saving dimming method based on histogram transformation, which is cut and moved by the histogram on the ARM platform. And stretching to process the image, and then using pulse width modulation to reduce the brightness of the LED backlight to achieve energy saving. Experiments show that the proposed method can achieve backlight power saving of about 35% with a distortion of 5%.

introduction

Energy-saving and environmental protection technology is the focus of the current world. In the liquid crystal display module, the power consumption of the backlight can account for more than 50% of the total power consumption. Especially in portable devices such as mobile phones, PDAs, and MP3s that are displayed below 10 inches, battery power is basically used, and the power consumption problem is particularly prominent. In order to effectively reduce the brightness of the backlight of the liquid crystal display to achieve the purpose of power saving, this paper implements a backlight dimming method based on histogram transformation on the ARM development platform. Experiments show that the proposed method has a distortion of 5%. In the case of the backlight can save about 35%.

1 backlight dimming solution

Taking the TFT liquid crystal panel structure as an example, including a backlight, a polarizer, a liquid crystal array, a color filter and the like, the display image perceived by the human eye is a comprehensive effect of the above various parts. Assume that the backlight brightness is normalized and set to b (the real number in the [0,1] interval), 0 corresponds to the backlight off condition, and 1 corresponds to the maximum brightness of the backlight. If the light source is an LED, the adjustment of b may include current pulse width modulation, current amplitude adjustment, and the like.

Suppose the image is discussed as an 8-bit grayscale image. f(x, y) represents the grayscale value of a point (x, y) in the image, and x and y are the coordinates of the point. Then, the liquid crystal transmittance at the position of the point can be expressed as:

t (x,y)=f(x,y)/255 (1)

The brightness L(x, y) that can be observed by the image point is a combined effect of backlight illumination and liquid crystal transmittance, and can be expressed as an approximately linear combination of backlight luminance b and liquid crystal transmittance t (x, y):

L(x,y) =b·t (x,y) =b·f(x,y)/255 (2)

According to the requirements of visual light efficiency consistency, it is necessary to find a way to make the pixel gray value increase as much as possible (to compensate for the backlight b becoming smaller). In order to achieve this goal, the paper first limits the gray value range of the image to a certain interval, and then stretches the histogram to achieve the increase of the pixel gray value. The image histogram is cropped by the thresholds fgl and fgh so that some points in the image are clamped to fgl and fgh, the grayscale interval of the image is limited to [fgl, fgh], and the backlight dimming problem is converted to this histogram. How the cropped image should be adjusted so that its backlight can minimize brightness.

Obviously, for fixed distortion, fgl and fgh can have multiple values, and the value should be in the formula (3):

Min (fgh- fgl ) (3)

The purpose of this processing is to limit the grayscale interval of the image to a minimum range.

After the formula (3) is realized, the next step is to linearly move the histogram, so that the gray scale is shifted to the dark region by the whole fgl. Thus the image gray region is compressed by the original distribution in the interval [0, 255] at [0, fgh - fgl] interval.

Assuming that the backlight is dark, the image should be grayscale stretched to compensate for the brightness loss caused by the backlight. If the linear stretching method is adopted, it is obvious that the maximum multiple of stretching is 255/(fgh-fgl). At this time, the pixel gray level is not saturated, and the backlight brightness can be reduced from 1 to (fgh-fgl)/255. According to the formula (2) ), the processed image has the same visual effect before and after backlight adjustment.

2 hardware and software implementation

This experiment uses the mini2440 development board for verification. The mini2440 is a low-cost and practical ARM9 development board. The processor is Samsung S3C2440 (ARM920T, the highest frequency can reach 532MHz). The LCD screen supports black and white, 4-level grayscale, 16-level grayscale, 256-color, 4,096-color STN LCD display. The size ranges from 3.5 to 12.1 in. The screen resolution can reach 1,024 & TImes; 768 pixels. The experiment uses 3.5 in. LED backlight TFT LCD screen.

In the experiment, the gray value of the image is calculated according to the image algorithm, and the histogram of the displayed image is cropped to be limited to a certain range, and then the histogram is stretched, and then calculated by the calculation formula (fgh-fgl)/255. The display brightness of the backlight is controlled, and the duty ratio of the backlight pulse width modulation output pulse is controlled to realize the brightness adjustment of the backlight LED. The hardware block diagram is shown in Figure 1.

Figure 1 Image processing and backlight control block diagram based on S3C2440

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