Patent ID: 7711199

Claim:
A method for estimating quantization factor of a compressed digital input image encoded by an encoder, comprising: employing an image processor for: detecting flat regions of an input compressed image by finding and excluding the strong edges or texture regions from the input compressed to generate essentially homogeneous regions, wherein finding and excluding the strong edges or texture regions further includes: performing low-pass filtering on the input compressed image signal Y IN using a n×m window at each pixel location (x, y) to generate a filtered signal Y L ; applying four directional sobel filtering operators to the filtered image Y L to find image edges Y H , each directional filter providing an edge location along the direction; obtaining a binary map Y B based on the image edges Y H using a threshold by applying a threshold value τ; removing spots caused by noisy information of the input compressed image and refinishing the edges to obtain refined edge regions Y M ; generating a flat binary map Y B as a function of the refined edge regions Y M ; generating flat region data Y 1 by multiplying the input compressed image signal Y IN and the flat region binary map Y B (x, y) indicating only flat areas in the input compressed image signal Y IN ; calculating an estimated noise power of the detected flat regions; comparing the estimated noise power with a statistical model to determine a quantization parameter used in an encoder when the input compressed image was compressed; providing the quantization parameter as a quality measure of the input compressed image.