# Backlight Unit Excessive Dimming Based on Perceptual Image Analysis
## 2 Background
### Contrast Response Function  
Figure 4a indicates the degree of the perceived contrast difference degraded by the excessive dimming for each contrast level. The contrast response function indicates that if the contrast is large or small, the sensitivity to contrast change decreases [20]. Figure 4a shows the degree of deterioration of the perceived contrast if contrast decreases by the same amount, where the same amount of dimming occurs at regions A, B, and C. As shown in the figure, regions A, B, and C correspond to high, middle, and low contrast region, respectively. In high contrast region, the image quality degradation caused by pixel saturation from excessive dimming is not perceived much. Further, there is little difference in the actual perceived contrast change during excessive dimming if the contrast is low. To mathematically model such a phenomenon, the contrast of the input image where pixel saturation occurs is evaluated using the image slope. The image slope is computed by dividing the difference in pixel values by the distance between the pixels. To calculate the characteristics of each image block, the average image slope of the corresponding block is used. In addition, since human eyes are sensitive to the difference in adjacent pixel values, the image slope is based on a pixel block with a moderately small size, which is homogeneous, too. The level of the homogeneity of an image block can be enhanced with smaller image block sizes, which may allow more excessive dimming. The block sizes of \(8\times 8\) or \(16\times 16\) that are commonly used as the image compression standards can be used for the homogeneity. However, it increases the number of blocks for large display panels and processing complexities (power dissipation) for portable display panels. Therefore, larger block sizes are usually used for the dimming calculations. The adjacent pixels used in the image slope calculation may be included in all, partially, or not included at all in the saturated pixel area, depending on whether the target pixel is in the bright area, the middle area, or the dark area, respectively. The degree of pixel saturation depends on where it belongs to among the above three cases, and the degree of image quality deterioration due to the contrast decrease varies correspondingly to determine which case it belongs to, mean luminance was used, which is the average of pixel values in a pixel block to maximize the excessive dimming using the brightness sensitivity. Figure 3: Brightness sensitivity. (**a**) Pixel value vs. Luminance; (**b**) Brightness sensitivity. However, if the contrast is very high and there is no homogeneity with several peaks in histogram, even though the mean luminance is in the middle, a significant portion of the pixels may exist in the region with high luminance, so that the image quality deterioration due to excessive dimming may be significant. This case is safely classified as significant image quality deterioration in Figure 4b. Figure 4b represents the degree of image quality deterioration caused by pixel saturation coming from excessive dimming with respect to mean luminance and contrast. There are nine circles, which represent each nine different areas that are defined by three different levels of pixel value mean luminance and contrast each. The darker a circle is, the greater the loss of image quality becomes due to the excessive dimming. For each of the above nine cases of Figure 4b, the pixel value range that is not in the saturated region so not affected by excessive dimming is indicated as thin lines, whereas the pixel value range that is saturated by excessive dimming is indicated as the dotted line and the pixel value range that exists by the mean luminance is indicated as the bold line. Figure 4: Contrast response function. (**a**) Contrast response function; (**b**) Nine cases of mean luminance and contrast combinations.