im_bandmean - average bands in an image
#include <vips/vips.h> int im_bandmean( in, out ) IMAGE *in, *out;
im_bandmean(3) writes a one-band image where each pixel is the average of the bands for that pixel in the input image. The output band format is the same as the input band format. Any size, any number of bands, any type.
The function returns 0 on success and -1 on error.