Function heif_context_encode_image 

pub unsafe extern "C" fn heif_context_encode_image(
    arg1: *mut heif_context,
    image: *const heif_image,
    encoder: *mut heif_encoder,
    options: *const heif_encoding_options,
    out_image_handle: *mut *mut heif_image_handle,
) -> heif_error

Compress the input image. Returns a handle to the coded image in ‘out_image_handle’ unless out_image_handle = NULL. ‘options’ should be NULL for now. The first image added to the context is also automatically set the primary image, but you can change the primary image later with heif_context_set_primary_image().