Blind image quality assessment refers to the problem of evaluating the visual quality of an image without any reference. It addresses a fundamental distinction between fidelity and quality, i.e. human vision system usually does not need any reference to determine the subjective quality of a target image. In this paper, we propose to appraise the image quality by three objective measures: edge sharpness level, random noise level and structural noise level. They jointly provide a heuristic approach of characterizing the most important aspects of visual quality. We investigate various mathematical tools (analytical, statistical and PDE-based) for accurately and robustly estimating those three levels. Extensive experiment results are used to justify the validity of our approach.