shrink a bitmap into half size with halide-lang

halfshrink_gen.cpp

#include "Halide.h"

namespace {

    class HalfShrink : public Halide::Generator<HalfShrink> {
    public:
        Input<Buffer<uint8_t>>  input{ "input", 3 };
        Output<Buffer<uint8_t>> halfshrink{ "output", 3 };

        void generate() {
            Var x("x"), y("y"), z("z"), c("c");
            Var x1("x1"), y1("y1");

            Func clamped, input16;
            clamped = Halide::BoundaryConditions::repeat_edge(input,
                    {{input.dim(0).min(), input.dim(0).extent()},
                    {input.dim(1).min(), input.dim(1).extent()}});

            input16(x, y, c) = cast<uint16_t>(clamped(x, y, c));
            Expr px = input16(2*x1, 2*y1, c)
                    + input16(2*x1+1, 2*y1, c)
                    + input16(2*x1, 2*y1+1, c)
                    + input16(2*x1+1, 2*y1+1, c);
            px /= 4;

            halfshrink(x1, y1, c) = cast<uint8_t>(px);
            halfshrink.compute_root().vectorize(x1, 16).parallel(y1);        }

        // void schedule() {
        // }
    };
}  // namespace

HALIDE_REGISTER_GENERATOR(HalfShrink, halfshrink)