// Copyright (C) 2019 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!

\title Using Image-Based Lighting
\page quick3d-asset-conditioning-ibl howto

\section1 Introduction

Qt Quick 3D supports IBL (Image-Based Lighting) to illuminate scenes or individual materials.

IBL is a lighting technique that allows scenes to be illuminated with images. This is especially
useful when you want to create realistic lighting and reflections in indoor and outdoor scenes.

You can use any image file for IBL, but it is recommended to use 360º HDR (High Dynamic Range)
images. HDR images have a much higher dynamic range than for example JPEG or PNG images. A higher
dynamic range provides more realistic lighting through a great range of luminance levels from
very bright to very dark.

The following example demonstrates the lighting effect on an object using an HDR image vs a single
directional light:

\table
\header
  \li Light
  \li Smooth Dielectric material
  \li Rough Dielectric material
  \li Smooth Metallic material
  \li Rough Metallic material
\row
  \li \b{Single directional light}
  \li \image IBL-sphere-smooth-directional-light.png
  \li \image IBL-sphere-rough-directional-light.png
  \li \image IBL-sphere-metallic-smooth-directional-light.png
  \li \image IBL-sphere-metallic-rough-directional-light.png
\row
  \li \b{Image-based light}
  \li \image IBL-sphere-smooth-environment-light.png
  \li \image IBL-sphere-rough-environment-light.png
  \li \image IBL-sphere-metallic-smooth-environment-light.png
  \li \image IBL-sphere-metallic-rough-environment-light.png
\endtable

\section1 Scene Lighting

To illuminate a \l {SceneEnvironment}{scene} using an image you'll add the image as a \l Texture to
the \l {SceneEnvironment::lightProbe}{lightProbe} property.

\qml
lightProbe: Texture {
    source: "maps/OpenfootageNET_garage-1024.hdr"
}
\endqml

Once you have selected an image, IBL is set up for your scene. All models in the scene are
illuminated by the light probe by default.

\note You can also combine IBL with any other light source to compliment the lighting effect on an
object.

Now that you have IBL set up for your scene, let us have a look at the different properties for
the probe. In many cases the default values provide a satisfying result, but you can tweak the
following property values depending on the image and desired result:

\list
  \li \l {SceneEnvironment::probeExposure}{Exposure}
        The amount of light emitted by the light probe.
  \li \l {SceneEnvironment::probeHorizon}{Horizon Cut-Off}
        Increasing the value adds darkness (black) to the bottom half of the environment,
        forcing the lighting to come predominantly from the top of the image
        (and removing specific reflections from the lower half).
  \li \l {SceneEnvironment::probeOrientation}{Orientation}
        This property when defines the orientation of the light probe.
        Orientation is defined in terms of euler angles in degrees over the
        x, y, and z axes.
\endlist

\table
\header
  \li Property
  \li Metallic material
  \li Dielectric material
\row
  \li \e{\b{Default settings}}
  \li \image IBL-sphere-metallic-smooth-environment-light.png
  \li \image IBL-sphere-smooth-environment-light.png
\row
  \li \e{\b{Exposure}}
  \li \image IBL-sphere-metallic-smooth-environment-light-exposure.png
  \li \image IBL-sphere-smooth-environment-light-exposure.png
\row
  \li \e{\b{Horizon Cut-off}}
  \li \image IBL-sphere-metallic-smooth-environment-light-horizon.png
  \li \image IBL-sphere-smooth-environment-light-horizon.png
\row
  \li \e{\b{Orientation}}
  \li \image IBL-sphere-metallic-smooth-environment-light-orientation.png
  \li \image IBL-sphere-smooth-environment-light-orientation.png
\endtable

\section1 Material Lighting

To use image-based lighting only on one material instead of a whole scene, or use a separate light
probe for a model already illuminated by image-based lighting, set the image as the
\l {Material::lightProbe}{light probe} for the material.

Once you have followed the steps above, you have a separate light probe set for the material.
This light probe overrides the scene light probe if there is one specified.

\section1 Pre-generating IBL cubemap

When IBL is used, a cubemap for the IBL image needs to be generated by the
application. By default this happens during application startup and can be quite
slow, especially on embedded and mobile devices. It is therefore possible to
pre-generate this cubemap using \l {Balsam Asset Import Tool}{Balsam}. Simply
run \l {Balsam Asset Import Tool}{Balsam} with the .hdr file as input and it
will output a cubemap file with the same name as the input but with a ktx file
extension. One can then reference this file in the lightProbe property's
associated \l Texture, and Qt will then load the pregenerated cubemap without
any costly processing at run time.

\section2 Manual baking

As an example, let's assume the application uses a .hdr image for its light
probes or the skybox:

\badcode
    View3D {
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.SkyBox
            lightProbe: Texture {
                source: "environment.hdr"
            }
            probeOrientation: Qt.vector3d(0, -90, 0)
        }
        // ...
    }
\endcode

This is fully functional, assuming environment.hdr is available at run
time. However, loading the .hdr image involves expensive pre-processing. This
can be avoided by running:

\badcode
    balsam environment.hdr
\endcode

The result is a new file \c{environment.ktx}. Shipping this instead of the .hdr file
and changing the Texture source provides significantly faster loading times.

\badcode
    lightProbe: Texture {
        source: "environment.ktx"
    }
\endcode

\section2 Build time baking via CMake

Manually running balsam on assets is not always ideal. Therefore, applications
are recommended to rely on CMake to automatically perform the same task at
application build time.

This is done by using the qt6_add_lightprobe_images CMake function provided by
the Quick3D component of the Qt6 package:

\badcode
    ...
    find_package(Qt6 COMPONENTS Quick3D)
    ...
    qt6_add_lightprobe_images(application_target "ibl_assets"
        PREFIX
            "/ibl"
        FILES
            "environment.hdr"
    )
\endcode

Replace \c application_target with the appropriate target. Here, there is no
need to run balsam manually on environment.hdr anymore, and the .hdr file does
not need to be shipped with the application. Rather, during the build balsam
will be invoked automatically, and an environment.ktx will be added to the
application resources at \c{:/ibl/environment.ktx}. The lightProbe's \l Texture
needs to then reference this file.

\badcode
    lightProbe: Texture {
        source: "qrc:/ibl/environment.ktx"
    }
\endcode

\note Setting PREFIX so that the final name in the resource system has a path
matching the .qml file's location allows using a relative source path instead of
having to provide an absolute path with the qrc scheme.

In addition to PREFIX, the keyword BASE is also available. The behavior follows
that of qt6_add_resources. For example, the following leads to generating
\c{:/ibl/maps/environment.ktx}:

\badcode
    qt6_add_lightprobe_images(application_target "ibl_assets"
        PREFIX
            "/ibl"
        BASE
            "../data/shared"
        FILES
            "../data/shared/maps/environment.hdr"
    )
\endcode

Like in qt6_add_shaders, the OUTPUTS keyword is available to allow specifying a
completely custom name for the file in the resource system. For example, the
following also generates \c{:/ibl/maps/environment.ktx}:

\badcode
    qt6_add_lightprobe_images(application_target "ibl_assets"
        PREFIX
            "/ibl"
        FILES
            "../data/shared/maps/environment.hdr"
        OUTPUTS
            "maps/environment.ktx"
    )
\endcode

\note For each entry in the FILES list, there must be a corresponding entry in
OUTPUTS.

*/
