int im_affine(in, out, a, b, c, d, dx, dy, x, y, w, h)
IMAGE *in, *out;
double a, b, c, d, dx, dy;
int x, y;
int w, h;
The transformation is described by a, b, c, d, dx, dy. The point (x,y) in the input is mapped onto point (X,Y) in the output by
X = a * x + b * y + dx
Y = c * x + d * y + dy
The area of the output image given by w, h, x, y is generated. (0,0) is the position of the transformed top-left-hand corner of the input image. Function im_affine resamples the transformed image using bilinear interpolation.