Siril features a powerful layer compositing tool. It was initially created to compose RGB and LRGB images from 3 or 4 one-layer images, but it is now being modified to allow any number of layer of any colour to be composited.
Siril color compositing tool
Images of different size can be composited, which is often required with LRGB when the L image is very detailed and RGB images are binned to acquire faster. To handle the different image sizes, Siril resizes the smaller images to the size of the largest image, using a Lanczos resampling with resize(InputArray src, OutputArray dst, Size dsize, double fx, double fy, int interpolation) OpenCV.