Fig. 2.

Tile-based computation of local correlation function of the micrograph using a helix reference image. (A) Tiling of the micrograph (left) into overlapping squares of 350–500 Å in dimension (right). (B) Determination of in-plane tile rotation θ with respect to reference helix by angular search between power spectra of reference and tiles (left). The step yields a tile map that contains the assigned rotation value θ as summarized in the vector plot (right). (C) Determination of distance (shift) to helix axis Δ by cross correlating rotated tile and reference helix image (left). The step yields a tile map that contains the shift Δ and cross-correlation (cc) score (right).