Improving the SNR During Color Image Processing while Preserving the Appearance of Clipped Pixels Sergio Goma, Milivoje Aleksic AMD, 75 Tiverton Court, Markham, ON, Canada, L3R-9S3 ABSTRACT An image processing path typically involves color correction or white balance resulting in higher than unity color gains. A gain higher than unity increases the noise in that respective channel, and therefore degrades the SNR performance of the input signal. If the input signal does not have enough SNR to accommodate the extra gain, the resultant color image has increased color noise. This is the usual case for color processing in cell phone cameras, which have sensors with limited SNR and high color crosstalk. This phenomenon degrades images more as illuminants differ from D65. In addition, the incomplete information for clipped pixels often results in unsightly artifacts during color processing. To correct this dual problem, we investigate the use of under unity color gains, which, by increasing the exposure of the sensor, would improve the resultant SNR of the color corrected image. The proposed method preserves the appearance of clipped pixels and the overall luminance of the image, while applying the appropriate color gains. Keywords: color processing, improving SNR, reducing noise, clipped pixels, cell-phone camera; 1. INTRODUCTION An increasingly important topic is the SNR response of a color sensor. Color filter design has a trade-off between color separation and overall luminance attenuation: higher color separation implies lower intensity signals at the pixel level, whereas higher pixel signal level implies lower color separation. Also, image quality requirements demands a good color separation which in context of linear color processing and RGB color filters implies high color gains. This paper is structured as follows: Section 2, “Problem Definition” provides a generic block diagram of a color processing pipe that is used to derive SNR degradation using error propagation calculation considering above unity overall color gains. This is used to propose a solution to improve SNR by exposing the sensor longer and use subunit color gains in color processing. The issues raised by this type of processing are examined and some solutions are suggested. This section concludes with a proposed solution to the problem of color processing using under unity overall gains. Section 3, “Implementation”, details a hardware implementation for a 5mpix sensor. Section 4 presents the results. 2. PROBLEM DEFINITION Today’s CMOS sensors used in cell-phone cameras are in range of 5mpix with pixel sizes of 1.75um. They typically deliver a noise performance around 36dB SNR. Also, a significant use case for cell-phone camera is indoor low-light or office light, which both tend to have tungsten as the dominant illuminant (color temperature). For the purpose of this analysis, a generic image processing pipe as shown in Figure 1 is used. It has the following features: Invited Paper Real-Time Image Processing 2008, edited by Nasser Kehtarnavaz, Matthias F. Carlsohn, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 6811, 681102, © 2008 SPIE-IS&T · 0277-786X/08/$18 SPIE-IS&T/ Vol. 6811 681102-1 2008 SPIE Digital Library -- Subscriber Archive Copy