Auto gain control based on look up table from scene·luminance curve in mobile phone camera

Auto-exposure(AE) control automatically calculates and adjusts the exposure of the tone of the subject to the midtone of the photograph. In the case of real-time performance, this is usually controlled by the sensor gain for consecutive input images. However, unsuitable sensor gain control methods invariably cause oscillation of the average luminance values for continuous input images, resulting in flickering. Also, in mobile phone cameras, only simple information, such as the average luminance value, can be utilized to calculate the sensor gain, due to coarse performance and cost. Therefore, this paper presents a new real-time AE control method using a look up table(LUT) based on S(Scene)· L(Luminance) curves to avoid the generation of flickering. Prior to the AE control, a LUT is constructed using S·L curve information, which illustrates the characteristic of the output average luminance for input gray patches with corresponding sensor gains. The AE control is then performed by estimating a current scene as a gray patch using the proposed LUT as the first process. This estimation is feasible because the average luminance is the only factor that can be obtained from an input image in a mobile phone camera. That is, the spatial information of images or patches is meaningless. Therefore, a gray patch and scene with the same average luminance are considered the same in a mobile phone camera. A new sensor gain is then estimated using a transposed LUT with the previously estimated patch. The entire estimation process is performed using linear interpolation to achieve real-time execution. Based on experimental results, the proposed AE control method demonstrated real-time, flicker-free exposure estimation.