Convert Pixel Array Between Different Conventions
This function provides a simple way to convert arrays of pixel data between different display conventions.
transmat(m, from, to)
- A matrix.
- Specifications of the spatial arrangement of the pixels. See Details.
Pixel images are handled by many different software packages. In virtually all of these, the pixel values are stored in a matrix, and are accessed using the row and column indices of the matrix. However, different pieces of software use different conventions for mapping the matrix indices $[i,j]$ to the spatial coordinates $(x,y)$.
- In theCartesianconvention, the first matrix index$i$is associated with the first Cartesian coordinate$x$,
and$j$is associated with$y$. This convention is used in
- In theEuropean reading orderconvention, a matrix is displayed in the spatial coordinate system as it would be printed in a page of text:$i$is effectively associated with the negative$y$coordinate, and$j$is associated with$x$. This convention is used in some image file formats.
- In the
spatstatconvention,$i$is associated with the increasing$y$coordinate, and$j$is associated with$x$. This is also used in some image file formats.
transmat. If a matrix
mcontains pixel image data that is correctly displayed by software that uses the Cartesian convention, and we wish to convert it to the European reading convention, we can type
mm <- transmat(m, from="Cartesian", to="European"). The transformed matrix
mmwill then be correctly displayed by software that uses the European convention.
Each of the arguments
to can be one of the names
"spatstat" (partially matched)
or it can be a list specifying another convention. For example
to=list(x="-i", y="-j")! specifies that rows of the output matrix
are expected to be displayed as vertical columns in the plot,
starting at the right side of the plot, as in the traditional
Chinese, Japanese and Korean writing order.
- Another matrix obtained by rearranging the entries of
opa <- par(mfrow=c(1,2)) # image in spatstat format Z <- bei.extra$elev plot(Z, main="plot.im", ribbon=FALSE) m <- as.matrix(Z) # convert matrix to format suitable for display by image.default Y <- transmat(m, from="spatstat", to="Cartesian") image(Y, asp=0.5, main="image.default", axes=FALSE) par(opa)