In this paper, we discuss the visibility problem of the digital watermarking from the point of unobtrusiveness. A new approach to fully automatic watermark visibility computation is presented. From the view of spatial domain, the visibility can be explained from two aspects, contrast and luminance. Two criterions for contrast and luminance are also proposed. Applying the criteria to local windows, two kinds of images are obtained called T-image and L-image. Experiments show that these two images provide good measurement for unobtrusiveness.