1. In a computing system that implements a video encoder, a method comprising: encoding a video frame of a video sequence using intraframe coding to produce encoded video data for the video frame, wherein the encoding includes, for a current block of pixel values in the video frame: performing spatial extrapolation for the current block to produce extrapolated pixel values; determining a residual based upon differences between the current block and the extrapolated pixel values from the spatial extrapolation; and performing a re-oriented frequency transform on pixel values of the residual to produce transform coefficients of the residual, wherein the re-oriented frequency transform addresses non-stationarity in pixel values of the residual due to the spatial extrapolation; for a reconstructed version of the video frame as a reference frame, adaptively filtering one or more boundaries between blocks of pixel values in the reference frame to reduce boundary discontinuities, including: measuring smoothness of neighboring pixel values across a given boundary of the one or more boundaries; and if the smoothness of the neighboring pixel values, compared to a threshold, meets a smoothness test, filtering a pixel value on each side of the given boundary, wherein an adjustment factor for the filtering depends on one or more discontinuity measures that each quantify pixel value discontinuity between a pair of the neighboring pixel values; and outputting, to a bitstream, the encoded video data for the video frame.