15.5.491 regrid

regrid(x, xgrid, ygrid, sx, sy)

Maps a 2D image onto a specified grid and returns the mapped image. Uses nearest-neighbor interpolation.

x

the two-dimensional image to be remapped.

xgrid ygrid

the remapping cell grid: the coordinates of the corners of the remapped image in the coordinate system of the old image. Must be 2D arrays.

sx sy

The interpolation grid dimensions within each grid cell. Must be scalars.

If ngx, ngy are the dimensions of the grid, then the dimensions of the resulting image are equal to (ngx-1)*sx and (ngy-1)*sy, respectively.

Areas in the remapped image for which there is no equivalent data in the original image are set to zero.

Identity mapping, i.e., mappings that return an exact copy of image x, are given by regrid(x,cx,cy,nx,ny) with nx = dimen(x,0)/ngx, ny = dimen(x,1)/ngy, cx = indgen(fltarr(ngx,ngy),0)*dimen(x,0)/(ngx - 1) and cy = indgen(cx,1)*dimen(x,1)/(ngy - 1), with ngx and ngy integers greater than 1. Some examples of linear remappings, with nx, ny, cx, and cy as above for ngx = 2 and ngy = 2, are:

change in scale

regrid(x,cx,cy,nx/2,ny/2) makes the image half the size.

cropping

regrid(x,cx/2,cy/2,nx/2,ny/2) selects the lower half of the image, i.e., x(0:nx/2-1,0:ny/2-1).

translation

regrid(x,cx+100,cy,nx,ny) shifts the image over 100 pixels to the left (in the direction of negative x).

rotation around the origin

regrid(x,cx*cos(phi)+cy*sin(phi),-cx*sin(phi)+cy*cos(phi),nx,ny) rotates the image counterclockwise over angle phi, i.e., the positive x axis rotates over angle phi in the direction of the positive y axis.

If you need to combine some of these operations, then perform them on cx, cy, nx, and ny, and only then apply them to the image. For example, to rotate the image over 1 radian around point (100,200) and expand it by a factor of 3, you could do

 cx2 = cx - 100                ; translate
 cy2 = cy - 200
 cx3 = cx2*cos(1)+cy2*sin(1)   ; rotate
 cy3 = -cx2*sin(1)+cy2*cos(1)
 cx4 = cx3 + 100               ; translate back
 cy4 = cx4 + 200
 y = regrid(x,cx,cy,nx*3,ny*3)

See also: compress, expand, regrid3, regrid3ns