early todo

[mdsumner]

• [x] write cpp versions of ib_index and vb_vertex
• [x] allow ydown in R code ( needs reversing index direction)
• [x] allow ydown in C++ code
• [ ] ~allow more general orientation? (index vs. native coordinates is independent ... so it's all up for grabs)~ let's not go there
• [x] write triangle versions of quad() in R
• [ ] ~write triangle versions of quad() in C++~ needs though, another todo list
• [ ] ~write cpp versions of divide.mesh3d~ see #5

[mdsumner]

Current design is documented in vignette source

but this old copy remains here:

• All coordinates are in x [0, 1] and y [0, 1] and default to "y-up" (begin at 0)
• Quad indexes are counter-clockwise, as required by rgl (see material3d back argument).
• ydown flips the orientation so that y coordinates begin at 1 and count down, and ensures that quad indexes are reversed and correctly counter-clockwise.

R functions using src/

• break_mesh() - removes topology from mesh3d, by expanding every coordinate from vertex array
• quad() generate mesh3d, a rectangular array of quads in the plane in 0,1
• quad_texture() - extension of quad() to include image texture, all geometry and texture has identical x-y plane coordinates in 0,1

Rcpp package using quad.h

Wrap quad_ib and quad_vb callable from R.

IntegerVector quad_index_cpp(IntegerVector nx, IntegerVector ny, LogicalVector ydown) 
NumericVector quad_vertex_cpp(IntegerVector nx, IntegerVector ny, LogicalVector ydown) 

Headers

./inst/include/quad/quad.h namespace quad

// returns nx * ny mesh3d$ib (a 4 * (nx * ny) matrix with a column for each quad
IntegerVector quad_ib(IntegerVector nx, IntegerVector ny, LogicalVector ydown);
// returns nx * ny mesh3d$vb[1:2, ] (a 2 * ((nx+1) * (ny+1)) matrix of xy)
NumericVector quad_vb(IntegerVector nx, IntegerVector ny, LogicalVector ydown);

[mdsumner]

Need to

• [x] ensure unmesh is unused
• [x] test for change in index order in break_mesh() (or break in a way that doesn't do that)
• [x] check that break_mesh doesn't mess up orientation
• [x] test for ydown difference

[mdsumner]

Design above now in master with https://github.com/hypertidy/textures/pull/4