Momocs (version 0.2-03)

pix2chc: Converts lists and matrices of coordinates into .chc (chain-coded) files.

Description

pix2chc converts lists and matrices of coordinates into chain-coded coordinates as used in the SHAPE suite (see Iwata in the References below).

Usage

pix2chc(coo)

Arguments

coo

A list or a 2-col matrix with entire coordinates, see Details.

Value

• Returns a vector of numeric that corresponds to chain-coded outline.

Details

Chain-code is a coding system for describing outlines in numbers from 0 to 7. Given the $i^{th}$ pixel taken on an outline and considering its eight neighbors, the chain-code equivalent for describing the relative position of the $(i+1)^{th}$ pixel is a number from $0$ (the next cell is eastward) to, counting counter clockwise, $7$ (the next cell is south-eastward). See References and Examples below.

References

Freeman H. 1974. Computer processing of line-drawing images. ACM Computing Surveys (CSUR) 6: 57-97. Iwata H, Ukai Y. 2002. SHAPE: a computer program package for quantitative evaluation of biological shapes based on elliptic Fourier descriptors. The Journal of Heredity 93: 384-385. Kuhl FP, Giardina CR. 1982. Elliptic Fourier features of a closed contour. Computer Graphics and Image Processing 18: 236-258. You can also have a look to the SHAPE's manual distributed with the program suite, that gives a description of the .chc format.

See Also

chc2pix for the reverse operation.

Examples

data(bot)
coo <- bot@coo[[1]]
chc <- pix2chc(coo)
coo.plot(chc2pix(chc))

# Illustration of chain coding
plot(NA, xlim=c(0, 3), ylim=c(0, 3), axes=FALSE, ann=FALSE, xaxs="i", yaxs="i")
title("Position of the next pixel and corresponding chain-code")
abline(h=0:3, v=0:3)
rect(1, 1, 2, 2, col="grey80")
text(1.5, 1.5, "Starting
pixel", cex=2)
text(x=c(2.5, 2.5, 1.5, rep(0.5, 3), 1.5, 2.5),
	y=c(1.5, rep(2.5, 3), 1.5, rep(0.5, 3)), labels=0:7, cex=2)