We have studied a picture coding system that can directly process compressed picture data, and have proposed a pixel value interpolation method using B-spline surfaces. We showed that its compression performance exceeds that of the conventional DCT picture coding system. A problem with this system, however, is that the coding frequency range that can yield high compression performance is limited because the applied polygon size is fixed. This paper proposes a solution to the problem, improves the performance of the proposed coding system, and uses it as a compression tool for B-spline surfaces. We propose an adaptive and automatic selection of polygon size to synthesize a surface depending on the fineness of a picture, and show a stable high compression performance for a broader range of coding frequencies. Furthermore, we study additional performance improvement achieved by increasing the kinds of polygon vertexes used, applying a weighting process to reduce coding error at block boundaries when we calculate polygon vertex value. We show the reduction of block distortions, seen at low coding frequencies in a reproduced picture. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(7): 55–65, 1998