A method (700) of determining an image value at a sample position of an output image, is disclosed. The method (700) comprises the steps of determining orientation of an isophote (e.g., 1010) passing through the output sample position and determining a period of intensity variation along the isophote (1010). The method (700) determines the image value at the sample position of the output image based on the period of intensity variation and outputs the determined image value at the sample position of the output image.