English: Part of a series of images to compare and contrast the levels of spatial aliasing that can result when an image that contains some strong spatial frequencies is digitally rescaled. An image of an 1801 engraving of King's College Chapel, Cambridge was used as a test object, and resized to 50% using different filters and different programs.

The image on this page was resized to 50% using Image Magick 6.7.5 with the "point" option -- i.e. merely retaining every other pixel (decimation). The result is very similar to the GIMP "no interpolation" option: interaction between spatially repeated structures in the engraving and the finite resolution has resulted in a highly visible Moiré banding pattern becoming apparent in the sky, the clouds and the building on the right, the result of spatial aliasing. Compared to the GIMP result, the phase of the Moiré fringes appears to be shifted by 180 degrees -- presumably because IM chooses to retain a different point of the four possible candidates.

Other images in the series look at the results of using various available lowpass filters to try to minimse or suppress such artefacts.