A method and apparatus are provided for determining a weighted average measured reflectance parameter Rm for pixels in an image for use in integrated cavity effect correction of the image. For each pixel of interest Pi,j in the image, an approximate spatial dependent average Ai,j, Bi,j of video values in a region of W pixels by H scan lines surrounding the pixel of interest Pi,j is computed by convolving video values Vi,j of the image in the region with a uniform filter. For each pixel of interest Pi,j a result of the convolving step is used as the reflectance parameter Rm. The apparatus includes a video buffer for storing the pixels of the original scanned image, and first and second stage average buffers for storing the computed approximate spatial dependent averages Ai,j, Bi,j. First and second stage processing circuits respectively generate the first and second stage average values Ai,j, Bi,j by convolving the video values of the image in a preselected region with a uniform filter.