00   Introduction
01   Getting started with Funcs, Vars, and Exprs
02   Processing images
03   Inspecting the generated code
04   Debugging with tracing, print, and print_when
05   Vectorize, parallelize, unroll and tile your code
06   Realizing Funcs over arbitrary domains
07   Multi-stage pipelines
08   Scheduling multi-stage pipelines
09   Multi-pass Funcs, update definitions, and reductions
10   AOT compilation part 1
10   AOT compilation part 2
11   Cross-compilation
12   Using the GPU
13   Tuples
14   The Halide type system
15   Generators part 1
15   Generators part 2
16   RGB images and memory layouts part 1
16   RGB images and memory layouts part 2
// Halide tutorial lesson 2: Processing images

// This lesson demonstrates how to pass in input images and manipulate
// them.

// On linux, you can compile and run it like so:
// g++ lesson_02*.cpp -g -I ../include -I ../tools -L ../bin -lHalide `libpng-config --cflags --ldflags` -lpthread -ldl -o lesson_02 -std=c++11
// LD_LIBRARY_PATH=../bin ./lesson_02

// On os x:
// g++ lesson_02*.cpp -g -I ../include -I ../tools -L ../bin -lHalide `libpng-config --cflags --ldflags` -o lesson_02 -std=c++11
// DYLD_LIBRARY_PATH=../bin ./lesson_02

// If you have the entire Halide source tree, you can also build it by
// running:
//    make tutorial_lesson_02_input_image
// in a shell with the current directory at the top of the halide
// source tree.

// The only Halide header file you need is Halide.h. It includes all of Halide.
#include "Halide.h"

// Include some support code for loading pngs.
#include "halide_image_io.h"
using namespace Halide::Tools;

int main(int argc, char **argv) {

    // This program defines a single-stage imaging pipeline that
    // brightens an image.

    // First we'll load the input image we wish to brighten.
    Halide::Image<uint8_t> input = load_image("images/rgb.png");

    // See below for a smaller version.

     

    // Next we define our Func object that represents our one pipeline
    // stage.
    Halide::Func brighter;

    // Our Func will have three arguments, representing the position
    // in the image and the color channel. Halide treats color
    // channels as an extra dimension of the image.
    Halide::Var x, y, c;

    // Normally we'd probably write the whole function definition on
    // one line. Here we'll break it apart so we can explain what
    // we're doing at every step.

    // For each pixel of the input image.
    Halide::Expr value = input(x, y, c);

    // Cast it to a floating point value.
    value = Halide::cast<float>(value);

    // Multiply it by 1.5 to brighten it. Halide represents real
    // numbers as floats, not doubles, so we stick an 'f' on the end
    // of our constant.
    value = value * 1.5f;

    // Clamp it to be less than 255, so we don't get overflow when we
    // cast it back to an 8-bit unsigned int.
    value = Halide::min(value, 255.0f);

    // Cast it back to an 8-bit unsigned integer.
    value = Halide::cast<uint8_t>(value);

    // Define the function.
    brighter(x, y, c) = value;

    // The equivalent one-liner to all of the above is:
    //
    // brighter(x, y, c) = Halide::cast<uint8_t>(min(input(x, y, c) * 1.5f, 255));
    //
    // In the shorter version:
    // - I skipped the cast to float, because multiplying by 1.5f does
    //   that automatically.
    // - I also used an integer constant as the second argument in the
    //   call to min, because it gets cast to float to be compatible
    //   with the first argument.
    // - I left the Halide:: off the call to min. It's unnecessary due
    //   to Koenig lookup.

    // Remember, all we've done so far is build a representation of a
    // Halide program in memory. We haven't actually processed any
    // pixels yet. We haven't even compiled that Halide program yet.

    // So now we'll realize the Func. The size of the output image
    // should match the size of the input image. If we just wanted to
    // brighten a portion of the input image we could request a
    // smaller size. If we request a larger size Halide will throw an
    // error at runtime telling us we're trying to read out of bounds
    // on the input image.
    Halide::Image<uint8_t> output =
        brighter.realize(input.width(), input.height(), input.channels());

    // Save the output for inspection. It should look like a bright parrot.
    save_image(output, "brighter.png");

    // See below for a small version of the output.

     

    printf("Success!\n");
    return 0;
}